ENGINEERING CHEMISTRY

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1 ENGINEERING CHEMISTRY NOTE MATERIAL Admitted batch (R13 Regulations) M V G R COLLEGE OF ENGINEERING, VIZIANAGARAM

2 UNIT I Water Technology WATER TECHNOLOGY Syllabus: Hard Water Estimation of hardness by EDTA method Potable water- Sterilization and Disinfection Boiler feed water Boiler troubles Printing and foaming, scale formation, corrosion, caustic embrittlement, turbine deposits Softening of water Lime soda, Zeolite processes Reverse osmosis Electro Dialysis, Icon exchange process Objectives: For prospective engineers knowledge about water used in industries (boilers etc.) and for drinking purposes is useful; hence chemistry of hard water, boiler troubles and modern methods of softening hard water is introduced. OUTLINES Introduction Hardness of water Estimation of total hardness of water by EDTA method Scale and sludge formation in boilers Caustic embrittlement Boiler corrosion Priming and foaming Softening methods Potable water Desalination of brackish water Engineering Chemistry Page 2

3 UNIT I Water Technology INTRODUCTION Water is the nature s most wonderful, abundant and useful compound. Of the many essential elements for the existence of human beings, animals and plants, water is rated to be of greatest importance. Without food human being can survive for a number of days, but water is such an essential thing without it one cannot survive. Water is not only essential for the lives of animals and plants but also occupies unique position in industries. Probably its most important use as an engineering material is in the steam generation. Water is also used as a coolant, in power, and chemical plants. In addition to it, water can also be used in the production of steel, rayon, paper, textiles, chemicals, irrigation, drinking fire fighting, etc. 2. Hardness of water Hardness in water is that character, which prevents the lathering with soap. This is due to the presence of certain salts of calcium, magnesium and other heavy metals dissolved in water. A sample of hard water when treated with soap, (sodium or potassium salt of higher fatty acid such as oleic, palmatic or stearic) does not produce lather, but on the other hand forms a white scum or precipitate. This precipitate is resulted due to the formation of insoluble salts of calcium and magnesium. Typical reactions of soap with calcium chloride and magnesium sulphate are depicted below. 2C 17 H 35 COONa + CaCl 2 Sodium stearate (Hardness) (Sodium soap) (C 17 H 35 COO) 2 Ca + 2NaCl Calcium stearate (Insoluble) 2C 17 H 35 COONa + MgSO 4 (C 17 H 35 COO) 2 Mg + Na 2 SO 4 Magnesium stearate (Insoluble) Thus, water which does not form lather readily with a solution of soap, but forms a white scum, is hard water. On the other hand water which lathers easily on shaking with soap solution is called soft water. Such water, consequently, does not contain dissolved calcium and magnesium salts in it. 2.1 Temporary or carbonate hardness Temporary hardness is caused due to the presence of dissolved bicarbonates of calcium, magnesium and other heavy metals and the carbonate of iron. It is eliminated merely by boiling,when bicarbonates are decomposed yielding insoluble carbonates or hydroxides, which are deposited as a crust at the bottom and sides of the vessel. Heat Ca(HCO 3 ) 2 CaCO 3 + H 2 O + CO 2 Calcium Bicarbonate Calcium Carbonate Engineering Chemistry Page 3

4 UNIT I Water Technology Heat Mg(HCO 3 ) 2 Mg(OH) 2 + H 2 O + 2CO 2 Magnesium Magnesium Bicarbonate Hydroxide 2.2 Permanent or non-carbonate hardness This is due to the presence of chlorides and sulphates of calcium, magnesium iron and other heavy metals. Unlike temporary hardness, permanent hardness is not eliminated by simple boiling. 2.3 EQUIVALENTS OF CALCIUM CARBONATE The concentration of hardness as well as non-hardness causing ions is usually expressed in terms of equivalents of CaCO 3, since this mode of representation permits the multiplication and division of concentration, when required. The choice of CaCO 3 in particular is due to its molecular weight which is 100 and moreover, it is most insoluble salt that can be precipitated in water treatment. Dissolved salt solution Ca(HCO 3 ) 2 Mg(HCO 3 ) 2 CaSO 4 CaCl 2 MgSO 4 MgCl 2 CaCO 3 MgCO 3 CO 2 Ca(NO 3 ) 2 Mg(NO 3 ) 2 HCO 3 - OH - 2- CO 3 NaAlO 2 Al 2 (SO 4 ) 3 FeSO 4.7H 2 O H + HCl Table.1 Calculation of equivalents of calcium carbonate Molar mass The equivalents of CaCO 3 = Chemical equivalent Mass of hardness producing substance Multiplication factor converting into equivalents of CaCO 3 100/ / / / / /95 100/ /84 100/44 100/ / / /34 100/60 100/ / / /2 100/73 Equivalent Weight of CaCO 3 Equivalent Weight of hardness producing substance x = Mass of hardness producing substance x 50 Equivalent Weight of hardness producing substance Engineering Chemistry Page 4

5 UNIT I Water Technology UNITS OF HARDNESS 1. Parts per million (ppm) is the parts of calcium carbonate equivalent hardness per 10 6 parts of water i.e., 1 ppm = 1 part of CaCO 3 eq. hardness in 10 6 parts of water. 2. Milligram per liter (mg/l) is the number of milligrams of CaCO 3 equivalent hardness present per liter of water, Thus; 1mg/L= 1mg of CaCO 3 eq. hardness of 1 L of water But I liter of water weighs 10 6 mg 1mg/L= 1mg of CaCO 3 eq. hardness in 10 6 parts of water =1 part of CaCO 3 eq. hardness in 10 6 parts of water 1mg/L = 1 ppm 3. Clarke s degree is the number of grains (1/7000lb) of CaCO 3 equivalent hardness per gallon (10lb) of water or its parts of CaCO 3 equivalent hardness per 70,000 parts of water. Thus: 1 o Clarke = 1 grain of CaCO 3 eq. hardness in gallon of water 1 o Cl = 1 part of CaCO 3 eq. hardness in 70,000 parts of water 4. Degree French is the parts of CaCO 3 equivalent hardness per 10 5 parts of water. Thus: 1 o Fr = 1 part of CaCO 3 eq. hardness in 10 5 parts of water 5. Milli equivalents per liter (meq/l) is the number of Milliequivalents of hardness present per liter Thus: 1 meq/l = 1 meq of CaCO 3 per liter of water = 10-3 x 50g of CaCO 3 eq. per liter of water = 50 mg of CaCO 3 eq. per liter = 50 mg/l of CaCO 3 = 50ppm 6. Relation between various units of hardness: 1 ppm = 1 mg/l = 0.1 o Fr = 0.07 o Cl = 0.02 meq/l 1 mg/l = 1 ppm = 0.1 o Fr = 0.07 o Cl = 0.02 meq/l 1 o Cl = o Fr = 14.3 ppm = 14.3 mg/l = meq/l 0.1 o Fr = 10 ppm = 10 mg/l = 0.7 o Cl = 0.2 meq/l 1 meq/l = 50mg/L = 50 ppm = 5 o Fr = 0.35 o Cl 3. Estimation of total hardness of water by EDTA method This is a complexometric method. Ethylene diamine tetra acetic acid (EDTA) HOOC.CH 2 HOOC.CH 2 N-CH 2 -CH 2 -N CH 2.COOH CH 2.COOH Engineering Chemistry Page 5

6 UNIT I Water Technology In the form of its salt yields the anion; - OOC.CH 2 - OOC.CH 2 N-CH 2 -CH 2 -N CH 2.COO - CH 2.COO - This forms complex ions with Ca 2+ and Mg 2+ O=C-O O-C=O M H 2 C N N CH 2 - OOC.CH 2 CH 2 CH 3 CH 2. COO - Where M = Ca or Mg. It may be pointed that the EDTA is employed as its disodium salt, NaOOC.CH 2 HOOC.CH 2 N-CH 2 -CH 2 -N CH 2.COONa CH 2.COOH In order to determine the equivalence point, EBT (Eriochrome Black- T) indicator is employed, which forms unstable wine red complex with Ca 2+ and Mg 2+ ions. However, this indicator is effective at a ph of about 10. When EBT is added to hard water buffered to a ph of about 10, a wine red unstable complex is formed. Thus: ph = 10 M 2+ + EBT M-EBT Complex (Ca 2+ or Mg 2+ of hard water) wine-red So initially a wine red coloured is obtained. During the course of titration against EDTA solution, EDTA combines with Ca 2+ and Mg 2+ ions and form stable complex M-EDTA and releasing free EBT, which instantaneously combines with M 2+ ions still present in the solution, thereby wine red colour is retained thus: [M-EBT] Complex wine-red + EDTA Titration [M-EDTA] Complex ph = 10 M 2+ + EBT M-EBT Complex (Ca 2+ or Mg 2+ still present) (blue) wine-red + EBT (blue) Engineering Chemistry Page 6

7 UNIT I Water Technology However, When nearly all M 2+ (Ca 2+ or Mg 2+ ions)ions have formed[m-edta] complex, the next drop of EDTA added displaces the EBT indicator form [M-EBT] complex and the wine red colour changes to blue colour. Thus, at the equivalence point, [M-EBT] Complex wine-red + EDTA Titration [M-EDTA] Complex + EBT (blue) Thus change of wine red colour to a distinct blue marks the end of the titration. 1. Standard hard water: 1 gm of dry CaCO 3 is dissolved in minimum quantity of HCl and evaporate the solution to dryness on a water bath, and then diluted to 1 lit with water. Each ml of this solution then contains 1 mg of CaCO 3 hardness. 2. EDTA solution : 4 gm of EDTA crystals gm MgCl 2 in 1lit 3. Indicator : 0.5 gm of EBT in100 ml of alcohol. 4. Buffer solution : 67.5 gm NH 4 Cl ml of Con. Ammonia solution diluted with distilled water to 1 lit. 5. Titration of permanent hardness of water: Take 250 ml of the water sample in a large beaker.boil till the volume is reduced to 50 ml. Filter, wash the precipitate with distilled water collecting filtrate and. Finally make the volume to 250 ml with distilled water. Then titrate 50 ml of the boiled water sample just as in step (5). Let volume used by V 3 ml Calculations: 50 ml of standard hard water = V 1 ml of EDTA :. 50 x1 mg of CaCO 3 = V 1 ml of EDTA :.1 ml of EDTA = 50/V 1 mg of CaCO 3 eq. Now 50 ml. of given hard water = V 2 ml EDTA = V 2 x 50/V 1 mg of CaCO 3 eq. :. 1 L (1,000 ml) of given hard water = 1000 V 2 /V 1 mg of CaCO 3 eq. :. Total hardness of water = 1000 V 2 /V 1 mg/l = 1000 V 2 /V 1 ppm Now 50 ml of boiled water = V 3 ml of EDTA = V 3 x 50 V 1 mg of CaCO 3 eq 1000 ml (= 1 L) of boiled water = 1000 V 3 /V 1 mg of CaCO 3 eq Permenent hardness = 1000 V 3 /V 1 ppm And Temporary hardness = Total hardness Permanent hardness Engineering Chemistry Page 7

8 UNIT I Water Technology = = 1000 V 2 V 1 V 3 V (V 2 - V 3 ) V 1 ppm ppm 3.1 Advantages of EDTA method: This method is definitely preferable to the other methods, because of the (i) Larger accuracy; (ii) Convenience; (iii) Rapid procedure 3.2 Problems : ml of sample consumed 15 ml of 0.01 M-EDTA before boiling and 5 ml of the same EDTA after boiling.calculate the degree of total hardness, permanent hardness and temporary hardness. Solution. 50ml of water sample = 15 ml of 0.01M-EDTA = 15 X 1000 ml of 0.01 M-EDTA = 300ml of 0.01 M-EDTA 50 Hence, total hardness Now 50 ml of boiled water = 2 X 300 ml of 0.01 N-EDTA (Since Molarity of EDTA = 2 x Normality of EDTA) = 600ml (or 0.6 L) of 0.01 eq. CaCO 3 = 0.6 x 0.01 x 50 g eq. CaCO 3 = 0.30 g or 300 mg of CaCO 3 eq. = 300 mg/l or 300 ppm. = 5 ml of 0.01 M-EDTA ml of boiled water = 5 X = 100 ml of 0.01M-EDTA = 200 ml (or 0.2 L) of 0.01 N-EDTA = 0.2 x 0.01 x 50 g of CaCO 3 eq. = 0.1 g or 100mg of CaCO 3 eq. Hence, permanent hardness = 100 mg/l or ppm ml of 0.01 M-EDTA Temporary hardness = ( ) ppm = 200 ppm. 2. Calculate the total hardness of a sample of water containing Mg(HCO 3 ) 2 = 73 mg/l; Ca(HCO 3 ) 2 = 162 mg/l, MgCl 2 = 95 mg/l., CaSO 4 = 136 mg/l. Ans. Total hardness of water: 350 ppm 4. SCALE AND SLUDGE FORMATION IN BOILERS In boilers, water evaporates continuously and the concentration of the dissolved salts increases progressively. When their concentrations reaches saturation point, they are thrown out of water in the form of precipitates which stick to the inner walls of the boiler. If the precipitation takes place in the Engineering Chemistry Page 8

9 UNIT I Water Technology form of loose or slimy precipitate it is called sludge. On the other hand, if the precipitated matter forms a hard adhering crust/ coating on the inner walls of the boiler, it is a scale. Sludge is a soft, loose and slimy precipitate formed within the boiler. Sludge can be easily scrapped off with a wire brush. It is formed at comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow at bends. Sludges are formed by substances which have greater solubilities in hot water than in cold water. Examples are MgCO 3, MgCl 2, CaCl 2, MgSO 4 etc. Fig.1. Scale and sludge in boilers. 4.1 Disadvantages of sludge formation: 1. Sludges are poor conductors of heat, so they tend to waste a portion of heat used. 2. If sludges are formed along with scales, the former get entrapped in the later and both get deposited as scales. 3. Excessive sludge formation disturbs the working of the boiler. It settles in the regions of poor water circulation such as pipe connection, plug opening, gauge glass connection thereby causing even chocking of the pipes. 4.2 Prevention of sludge formation: 1. By using well softened water. 2. By a frequent blow down operation, i.e., drawing off a portion of the concentrated water Scales are hard deposits, which stick very firmly to the inner surface of the boiler. Scales are very difficult to remove even with the help of hammer and chisel. Scales are the main source of boiler troubles. Formation of scales may be due to 1. Decomposition of calcium bicarbonate Ca(HCO 3 ) 2 CaCO 3 + H 2 O + CO 2 However, scale composed chiefly of calcium carbonate is soft and is the main cause of scale formation in low pressure boilers. But in high pressure boilers CaCO 3 is soluble. CaCO 3 + H 2 O Ca(OH) 2 + CO 2 Engineering Chemistry Page 9

10 UNIT I Water Technology Decomposition of calcium sulphate: The solubility of calcium sulphate in water decreases with increase in temperature. Thus, solubility of calcium sulphate is 3,200 ppm at 15 o C and it reduces to 55 ppm at 230 o C and 27 ppm at 320 o C. In other words, calcium sulphate is soluble in cold water, but almost completely insoluble in superheated water. Consequently calcium sulphate gets precipitated as hard scale on the heated portions of the boiler. This is the main cause in the high pressure boilers. 3. Hydrolysis of magnesium salts: Dissolved magnesium salts undergo hydrolysis at prevailing high temperatures in the boiler forming magnesium hydroxide precipitate, which forms a soft type of scale. MgCl 2 + 2H 2 O Mg(OH) 2 + 2HCl 4. Presence of silica: (SiO 2 ), even if present in small quantities, deposits as calcium silicate (CaSiO 3 ) and/ or magnesium silicate (MgSiO 3 ). These deposits stick very firmly to the inner walls of the boiler surface and are very difficult for removal. One important source of silica in water is the sand filter used. 4.3 Disadvantages of scale formation: 1. Wastage of fuels: Scales have a low thermal conductivity, so the rate of transfer of heat from boiler to inside water is largely decreased. In order to provide a steady supply of heat to water, excessive or over heating is done which causes unnecessary increase in fuel consumption. Thickness of the scale (mm) Wastage of fuel 10% 15% 50% 80% 150% 2. Lowering of boiler safety: Due to scale formation, over-heating of the boiler has to be done in order to maintain a constant supply of steam. The over-heating of the boiler tube makes the boiler material softer and weaker and this causes distortion of the boiler tube and makes the boiler tube unsafe to bear the pressure of the steam especially in high-pressure boilers. 3. Decrease in efficiency: Scales may sometimes get deposited in the valves and condensers of the boiler and choke them partially or totally. This results in decrease the efficiency of the boiler. 4. Danger of explosion: When thick scales crack due to uneven expansion, the water comes in contact with the overheated iron plates. This causes a release of a large amount of steam suddenly, developing a high pressure, which may cause explosion in the boiler. 4.4 Removal of scales: 1. With the help of scraper or piece of wood or wire brush, if they are loosely adhering. 2. By giving thermal shocks like heating the boiler and suddenly cooling it with cold water. Engineering Chemistry Page 10

11 UNIT I Water Technology Dissolving scales by adding suitable chemicals, if they are adherent and hard. Thus calcium carbonate scales can be dissolved by the addition of 5% HCl. Calcium sulphate scales can be dissolved by the addition of EDTA ( ethylene diamine tetra acetic acid), with which they form complexes. 4. By frequent blow down operation, if the scales are loosely adhering. 5. CAUSTIC EMBRITTELMENT Caustic embrittlement is a type of boiler corrosion, caused by using highly alkaline water in the boiler. During softening process by lime- soda process, free sodium carbonate is usually present in small proportion in the softened water. In high pressure boilers, sodium carbonate decomposes to give sodium hydroxide and carbon dioxide, and their presence makes the boiler water caustic. Na 2 CO 3 + H 2 O NaOH + CO 2 The water containing sodium hydroxide flows into the minute hair cracks always present, by capillary action in to the inner sides of the boiler. Here as water evaporates the dissolved caustic soda concentration increases progressively. This concentrated caustic soda attacks the surrounding area dissolving inner iron side of the boiler by forming sodium ferroate. This causes the embrittlement of the boiler parts, particularly stressed parts such as bends, joints, rivets etc., causing even failure of the boiler operations. Caustic cracking can be explained by the following concentration cell Iron at + Concentrated Bends, rivets and joints NaOH solution Dilute NaOH solution - Iron at plane surfaces The iron surrounded by the dilute NaOH becomes the cathodic surface and the iron present with the high concentration of NaOH becomes anodic which is consequently dissolved or corroded. 5.1 Caustic embrittlement can be avoided by 1. By using sodium phosphate as a softening agent instead of sodium carbonate. 2. By adding tannin or lignin to the boiler water, since these substances block the hair cracks, thereby preventing the infiltration of the caustic soda solution in to these. 3. By adding sodium sulphate to boiler water: Sodium sulphate blocks the hair cracks preventing the infiltration of caustic soda solution in to these. It has been observed that caustic cracking can be prevented, if sodium sulphate is added to the boiler in the ratio of Na 2 SO 4 : NaOH as 1:1; 2:1; 3:1 in boilers working respectively at pressures up to 10, 20 and above 20 atmospheres. 6. BOILER CORROSION Boiler corrosion is the decay of boiler material (iron) either by chemical or electro chemical attack of its environment. Main reasons for the boiler corrosion are: Engineering Chemistry Page 11

12 UNIT I Water Technology Dissolved oxygen: Water usually contains 8 mg of dissolved oxygen per liter at room temperature. Dissolved oxygen in water in the presence of prevailing high temperature of the boiler, attacks the boiler material as 2Fe + 2 H 2 O + O 2 2 Fe(OH) 2 4 Fe(OH) 2 + O 2 2 [Fe 2 O 3.2 H 2 O] Removal of the dissolved oxygen: a. By adding calculated amount of sodium sulphite or hydrazine or sodium sulphide. 2Na 2 SO 3 + O 2 2 Na 2 SO 4 N 2 H 4 + O 2 N H 2 O Na 2 S + O 2 Na 2 SO 4 b. By b. Mechanical deaeration: In this process water is sprayed in to a tower fitted with perforated plates (Fig), heated from sides and connected to vacuum pump. High temperature, low pressure and large exposed surface area reduce the dissolved oxygen in water. Perforated paltes Tower Water feed To vacuum pump Steam jacket Perforated paltes Dearated water Fig 2. Mechnical De-aeration of water 6.2 Dissolved carbon dioxide: Carbon dioxide dissolved in water forming carbonic acid, has a slow corrosive effect on the boiler material. Carbon dioxide is also released inside the boiler, if water, containing bicarbonates is used for steam generation CO 2 + H 2 O H 2 CO 3 Mg(HCO 3 ) 2 MgCO 3 + CO 2 + H 2 O Removal of dissolved carbon dioxide: a. By adding calculated amount of ammonia 2NH 4 OH + CO 2 (NH 4 ) 2 CO 3 b. By mechanical de-aeration process along with oxygen (described above) Engineering Chemistry Page 12

13 UNIT I Water Technology Acids from dissolved salts: Water containing dissolved salts of magnesium liberates acids on hydrolysis. MgCl 2 + H 2 O Mg(OH) HCl The liberated acid reacts with the iron material of the boiler in chain like processes, producing HCl again and again. Thus: Fe + 2HCl FeCl 2 + H 2 FeCl 2 + 2H 2 O Fe(OH) HCl Consequently, presence of even small amount of magnesium chloride will cause corrosion to a large extent and may cause damage to the boiler material. Removal of acids: a) Softening boiler water to remove magnesium chloride, if any. b) By frequent blow down operation of removal of concentrated water with fresh soft water. c) Addition of inhibitors as sodium silicate/sodium phosphate/sodium chromate, which protect the boiler material against acid attack. 7. PRIMING AND FOAMING When a boiler is producing steam rapidly, some particles of the condensed liquid water are carried along with the steam. The process of wet steam formation is called priming. Priming is caused by 1. The presence of large amounts of dissolved solids 2. High steam velocities 3. Sudden boiling 4. Improper boiler design 5. Sudden increase in the steam production rate. Foaming is the production of persistent foam or bubbles in boilers, which do not break easily. Foaming is due to the presence of substances like oils in water, which reduce the surface tension of water. Priming and foaming usually occur together. They have to be eliminated because a. Dissolved salts in boiler water are carried by the wet steam to super heater and turbine blade, where they get deposited as water evaporates. This deposit reduces the efficiency of the boiler. b. Dissolved salts may enter the other parts of the machinery, where steam is being used, thereby decreasing the life of the machinery c. Actual height of the water column cannot be judged properly making the maintenance of the boiler pressure difficult. Engineering Chemistry Page 13

14 UNIT I Water Technology Priming can be avoided by fitting mechanical steam purifiers, avoiding the rapid change in steaming rate, maintaining low water levels in boilers, efficient softening and filtration of the boiler feed water. Foaming can be avoided by adding anti foaming chemicals like castor oil, or removing oil from boiler water by adding compounds like sodium aluminate. 8. SOFTENING METHODS Water used for industrial purposes (such as for steam generation) should be sufficiently pure. it should, therefore, be freed from hardness- producing salts before it is being put to use. The process of removing hardness-producing salts from water is known as softening of water. In industry three methods are mainly employed for softening of water. 8.1 Lime soda process: In this method, the soluble calcium and magnesium salts in water are chemically converted into insoluble compounds, by adding calculated amounts of lime [Ca(OH) 2 ] and soda [Na 2 CO 3 ]. Calcium carbonate [CaCO 3 ] and magnesium hydroxide [Mg(OH) 2 ] are precipitated and removed. Exact amounts as required are calculated as below. Table 2.Calculation of lime soda requirement Constituent Reaction Need Ca 2+ (Perm.Ca) Mg 2+ (Perm.Mg) Ca 2+ + Na 2 CO 3 CaCO Na + S Mg 2+ + Ca(OH) 2 Mg(OH) Ca 2+ L+S Ca 2+ + Na 2 CO 3 CaCO Na + HCO 3 - (e.g.,nahco 3 ) 2HCO Ca(OH) 2 CaCO 3 + 2H 2 O + CO 3 2- L-S Ca(HCO 3 ) 2 (Temp.Ca) Mg(HCO 3 ) 2 (Temp.Mg) Ca(HCO 3 ) 2 + Ca(OH) 2 2CaCO 3 + 2H 2 O L Mg(HCO 3 ) 2 + 2Ca(OH) 2 2CaCO 3 + Mg(OH) 2 + 2H 2 O 2L CO 2 CO 2 + Ca(OH) 2 CaCO 3 + 2H 2 O L H + (Free acids, HCl,H 2 SO 4,etc.) 2H + + Ca(OH) 2 Ca H 2 O L+S Ca 2+ + Na 2 CO 3 CaCO Na + Coagulants : FeSO 4 Fe 2+ + Ca(OH) 2 Ca 2+ + Fe(OH) 2 2Fe(OH) 2 + H 2 O + O 2 2Fe(OH) 3 Ca 2+ + Na 2 CO 3 CaCO Na + L+S Al 2 (SO 4 ) 3 2Al Ca(OH) 2 3Ca Al(OH) 3 L+S 3Ca Na 2 CO 3 3CaCO 3 + 6Na + NaAlO 2 NaAlO 2 + H 2 O NaOH + Al(OH) 3 2NaOH is eq to Ca(OH) 2 -L Engineering Chemistry Page 14

15 UNIT I Water Technology Now 100 parts by mass of CaCO 3 are equivalent to: (i) 74 parts of Ca(OH) 2, and (ii) 106 parts of Na 2 CO Lime requirment for softening = Temp. Ca x Temp.Mg 2+ + Perm.(Mg 2+ + Fe 2+ + Al 3+ ) + CO 2 + H + (HCl or H 2 SO 4 ) + HCO NaAlO 2 all in terms of CaCO 3 eq... Soda requirment for softening = Perm.(Mg 2+ + Fe 2+ + Al 3+ + Ca 2+ ) + H + (HCl or H 2 SO 4 ) all in terms of CaCO 3 eq 1. Calculate the amount of lime required for softening 50,000 liter of hard water containing CaCO 3 = 25 ppm, MgCO 3 = 144, CaCl 2 = 111ppm, MgCl 2 = 95 ppm, Na 2 SO 4 = 15 ppm, Fe2O 3 = 25 ppm. Constituent Multiplication factor CaCO 3 equivalent CaCO 3 = 25 MgCO 3 = 144 CaCl 2 = 111 MgCl 2 = / /84 100/ /95 25 X 100 X 100 = 25.0 mg/l 144 X 100 X 84 = mg/l 111 X 100 X 111 = mg/l 95 X 100 X 95 = mg/l Lime required for softening 50,000 L of water = 74/100 [CaCO x MgCO 3 + MgCl 2 as CaCO 3 eq] x Vol. of water = 74/100 [ x ]mg/L x 50,000 L = 74/100 [ mg/l] x 50,000 L = 1, 73,10,820 mg = (1.73, 10,820 x 106) kg = kg 2. Calculate the quantity of lime and soda required for softening 50,000 liters of water containing the following salts per liter: Ca(HCO 3 ) = 8.1mg ; Mg(HCO 3 ) 2 = 7.5mg ; CaSO 4 = 13.6 mg; MgSO 4 = 12.0 mg; MgCl 2 = 2.0 mg; and NaCl = 4.7 mg. Ans kg (Lime) ; kg(soda). 3. A water sample contains the following impurities: Ca 2+ = 20ppm. Mg 2+ = 18 ppm, HCO - 3 = 183 ppm and SO 2-4 = 24 ppm. Calculate the amount of lime and soda needed for softening. Ans ppm or mg/l (Lime): Negative or nil (Soda). 4. Explain with chemical and the amount of lime and soda needed for softening 1,00,000 liters of water containing the following : HCl = 7.3 mg/l ; Al 2 (SO 4 ) 3 = 34.2 mg/l MgCl 2 = 9.5 mg/l ; NaCl = mg/l Purity of lime is 90% and that of soda is 98%. 10% of chemicals are to be used excess in order to complete the reaction quickly. Ans kg. (Lime): kg (Soda). Engineering Chemistry Page 15

16 UNIT I Water Technology Cold Lime Soda process: In this method, calculated quantity of chemicals (lime and soda) are mixed with water at room temperature. At room temperature, the precipitates formed are finely divided, so they do not settle down easily and cannot be filtered easily. Consequently, it is essential to add small amounts of coagulants (like alum, aluminium sulphate, sodium aluminates, etc.), which hydrolyse to flocculent, gelatinous precipitate of aluminium hydroxide, and are entrapped in the fine precipitates. Use of sodium aluminate as coagulant also helps the removal of silica as well as oil, if present in water. Cold L-S process gives water containing a residual hardness of 50 to 60 ppm. NaAlO 2 + 2H 2 O NaOH + Al(OH) 3 Sodim aluminate Al 2 (SO 4 ) 3 + 3Ca(HCO 3 ) 2 2Al(OH) 3 + 3CaSO 4 + 6CO 2 Coagulent Calcium. Bicarbonate (Hardness in water) The details of the process are given below: Method: Raw water and calculated quantities of chemicals (Lime + soda + coagulant) are fed from the top into the inner vertical circular chamber, fitted with a vertical rotating shaft carrying a number of paddles. As the mixture of raw water and chemicals flows down, there is a vigorous stirring and continuous mixing, whereby softening of water takes place. As the softened water comes into the outer co-axial chamber, it rises upwards to reach the filter. Fig. 3. Continuous cold lime soda softener. The heavy sludge (or precipitated floc) settles down in the outer chamber by the time the softened water reaches up. The softened water then passes through a filtering media (usually made of wood fibers) to Engineering Chemistry Page 16

17 UNIT I Water Technology ensure complete removal of sludge. Filtered soft water finally flow out continuously through the out let at the top (fig 3).Sludge setting at the bottom of the outer chamber is drawn off occasionally Hot Lime- Soda process: It involves in treating water with softening chemicals at a temperature of 80 to 150 o C. Since hot process is operated at a temperature close to the boiling point of the solution, (a) the reaction proceeds faster, (b) the softening capacity of hot process is increased to many fold and (c) the precipitate and sludge formed settle down rapidly and hence, no coagulants are needed. (d) Much of the dissolved gases (such as CO 2 and air) are driven out of the water. (e) Viscosity of softened water is lower, so filtration of water becomes easier. This in-turn increases the filtering capacity of filters, and (f) hot lime-soda process produces water of comparatively lower residual hardness of 15 to 30 ppm. Hot lime- soda plant consists essentially ( fig 4) of three parts: (a) a reaction tank in which raw water, chemicals and steam are thoroughly mixed: (b) a conical sedimentation vessel in which sludge settles down and (c) a sand filter which ensures complete removal of sludge from the softened water. Fig. 4. Continuous hot lime-soda softener. 8.2 Zeolite or permutit process: Chemical structure of sodium zeolite may be represented as: Na 2 O.Al 2 O 3.xSiO 2.y,H 2 O where x = 2-10 and y = 2-6. Zeolite is hydrated sodium aluminosilicate, capable of exchanging reversibly its sodium ions for hardness-producing ions in water. Engineering Chemistry Page 17

18 UNIT I Water Technology Zeolites are also known as permutits. Zeolites are of two types: (i) Natural zeolites are non- porous. For example, natrolite, Na 2 O.Al 3 O 3.4SiO 2.2H 2 O. (ii) Synthetic zeolites are porous and possess gel structure. They are prepared by heating together china clay, feldspar and soda ash. Such zeolites possess higher exchange capacity per unit weight than natural zeolites. Process: For softening of water by zeolite process, hard water is percolated at a specified rate through a bed of Zeolite, kept in a cylinder ( Fig. 5). The hardness- causing ions (Ca 2+, Mg 2+, etc.) are retained by the Zeolite as CaZe and MgZe; while the outgoing water contains sodium salts. Reactions taking place during the softening process are: Na 2 Ze + Ca(HCO 3 ) 2 CaZe + 2NaHCO 3 Na 2 Ze + Mg(HCO 3 ) 2 MgZe + 2NaHCO 3 Na 2 Ze + CaCl 2 (or CaSO 4 ) CaZe + 2NaCl (or Na 2 SO 4 ) Na 2 Ze + MgCl 2 (or MgSO 4 ) MgZe + 2NaCl (or Na 2 SO 4 ) (Zeolite) (Hardeness causing species) Regeneration: After some time, the zeolite is completely converted into calcium and magnesium zeolites and it ceases to soften water, i.e., it gets exhausted. At this stage, the supply of hard water is stopped and the exhausted zeolite is reclaimed by treating the bed with a concentrated (10%) brine (NaCl). Fig. 5. Zeolite softener CaZe (or MgZe) + 2 NaCl Na 2 Ze + CaCl 2 (or MgCl 2 ) ( Exhausted zeolite) (Brine) (ReclimedZeolite) (Washings) < Limitations of zeolite process: (1) If the supply of water is turbid, the suspended matter must be removed before the water is admitted into the zeolite bed; otherwise the turbidity will clog the pores of zeolite bed, making it inactive. (2) If water contains large quantities of coloured ions such as Mn 2+ and Fe 2+, they must be removed first, because these ions produce manganese and iron zeolites, which will not allow easy regeneration of the zeolite. Engineering Chemistry Page 18

19 , UNIT I Water Technology (3) Mineral acids, if present in water, destroy the zeolite bed and therefore they must be neutralized with soda, before admitting the water into the zeolite softening plant Advantages of zeolite process: (1) It removes the hardness almost completely and water of about 10 ppm hardness is obtained. (2) The equipment used is compact, occupying a small space (3) No impurities are precipitated, so there is no danger of sludge formation in the treated-water at a later stage. (4) The process automatically adjusts itself for variation in hardness of incoming water. (5) It is quite clean. (6) It requires less time for softening. (7) It requires less skill for maintenance as well as operation Disadvantages of zeolite process: (1) The treated-water contains more sodium salts than in the lime-soda process. (2) The method only replaces Ca 2+ and Mg 2+ ions, but leaves all the acidic ions in the soft water. (3) Water having high turbidity cannot be treated efficiently by this method. 8.3 Ion exchange or de-ionization or de-mineralization process: Ion-exchange resins are insoluble, cross-linked, long chain organic polymers with a microporous structure, and the functional groups attached to the chains are responsible for the ion-exchanging properties. Resins containing acidic functional groups (-COOH, -SO 3 H etc.) are capable of exchanging their H + ions with other cations, which come into their contact; whereas those containing basic functional groups (-NH 2 =NH as hydrochloric acid) are capable of exchanging their anions with other anions, which come into their contact. The ion-exchange resins may be classified as: (i) Cation exchange resins (RH + ) are mainly styrene-divinyl benzene copolymers, which on sulphonation or carboxylation become capable to exchange their hydrogen ions with the cations present in the raw water. H 2 C HC CH 2 CH CH 2 CH CH 2 SO 3 - H + SO 3 - H + H 2 C CH 2 H 2 C CH HC CH 2 SO 3 - H + SO 3 - H + Acidic or cation exchange resin( sulphonate form) Engineering Chemistry Page 19

20 UNIT I Water Technology (ii) Anion exchange resins (R 1 OH - ) are H 2 C HC CH 2 CH CH 2 CH CH 2 styrene-divinyl benzene amineformaldehyde copolymers, which contains amino or quaternary ammonium or quaternary phosphonium or tertiary sulphonium groups as an integral part of the resin matrixs. These, after treatment with dil. NaOH solution, become capable to exchange their OH - anions with anions present in the raw water. CH 2 NMe + 2 OH - CH 2 NMe + 2 OH - H 2 C CH 2 H 2 C CH HC CH 2 - HO + Me 2 NH 2 C CH 2 NMe + 2 OH - Basic or anion exchange resin( hydroxide form) Process: The hard water is passed first through cation exchange column, which removes all the cations like Ca 2+.Mg 2+, etc., from it and an equivalent amount of H + ions are released from this column to water. Thus: 2RH + + Ca 2+ R 2 Ca H + 2RH + + Mg 2+ R 2 Mg H + The water which is now free from cations, is passed through anion exchange column, which removes all the anions like SO 2-4, Cl - etc., present in the water and equivalent amount OH - ions are released from this column to water. Thus: R'OH - + Cl - R'Cl + OH - 2R'OH - + SO 4 2-2R'OH - + CO 3 2- R' 2 SO OH - R' 2 CO OH - H + and OH - ions (released from cation exchange and anion exchange columns respectively) combine to produce water. H + + OH - H 2 O Thus the water coming out from the exchanger is free from all cations as well as anions. Ion-free water is known as deionised or demineralised water. Regeneration: When capacities of cation and anion exchangers to exchange H + and OH - ions respectively are lost, they are then said to be exhausted. Engineering Chemistry Page 20

21 UNIT I Water Technology The exhausted cation exchange column is regenerated by passing a solution of dil. HCl or H 2 SO 4. The regeneration can be represented as: R 2 Ca H + 2RH + + Ca 2+ (Washing) The column is washed with deionised water and such washing (which containing Ca 2+, Mg 2+, etc. and Cl - or SO 2-4 ) is passed into sink or drain. The exhausted anion exchange column is regenerated by passing a solution of dil. NaOH. The regeneration can be represented as: 2- R' 2 SO 4 + 2OH - 2R'OH - + SO 2-4 (Washing) The column is washed with deionised water and such washing (which contains Na + and SO 2-4 or Cl - ions) is passed into sink or drain. The regenerated ion exchange resins can be used again. Fig. 6. Demineralization of water Advantages (1) The process can be used to soften highly acidic or alkaline waters. (2) It produces water of very low hardness (2 ppm). Disadvantages (1) The equipment is costly and expensive chemicals are needed. (2) If water contains turbidity, then the output of the process is reduced. Engineering Chemistry Page 21

22 UNIT I Water Technology (3) The turbidity must be below 10 ppm. If it is more, it has to be removed first by coagulation followed by filtration. 9. Potable water The water which is fit for human consumption is known as potable water Municipalities have to supply potable water, i.e., water which is safe to human consumption should satisfy the following essential requirements 1. It should be sparkling clear and odourless. 2. It should be pleasant in taste 3. It should be perfectly cool 4. Its turbidity should not exceed 10 ppm 5. It should be free from objectionable dissolved gases like hydrogen sulphide. 6. It should be free from objectionable minerals such as lead, arsenic, chromium and manganese salts. 7. Its alkalinity should not be high. Its ph should not be above It should be reasonably soft 9. Its total dissolved solids should be less than 500 ppm 10. It should be free from disease- producing micro- organisms. Purification of domestic water for domestic use: For removing various types of impurities in the natural water from various sources, the following treatment process is employed; 9.1 Removal of suspended impurities : a. Screening: The raw water is passed through screens, having large number of holes, when floating matter are retained by them. b. Sedimentation: It is the process of allowing water to stand undisturbed in big tanks, about 5 m deep, when most of the suspended particles settle down to the bottom, due to gravity. The clear supernatant water is then drawn from the tank with the help of pumps. The retention period in a sedimentation tank ranges from 2-6 hours. c. Filtration: It is the process of removing colloidal matter by passing water through a bed of fine sand and other proper sized granular materials. Filtration is carried out by using sand filter. 9.2 Removal of micro-organisms: The process of destroying /killing the disease producing bacteria, micro-organisms, etc., from the water and making it safe for the use, is called disinfestation. Engineering Chemistry Page 22

23 UNIT I Water Technology a. Boiling: By boiling water for minutes, all the disease producing bacteria is killed and the water becomes safe for use. b. Adding bleaching powder: In small water works, about 1 kg of bleaching powered per 1000 kiloliter of water is mixed and allowed to standing undisturbed for several hours. The chemical action produces hypochlorous acid (a powerful germicide) CaOCl 2 + H 2 O Ca(OH) 2 +Cl 2 Cl 2 + H 2 O HCl+ HOCl Germs+ HOCl Germs are killed c. Chlorination: Chlorination (either gas or in concentrated solution from) produces hypochlorous acid, which is a powerful germicide. Cl 2 + H 2 O HCl+ HOCl Bacteria+ HOCl Bacteria are destroyed 9.3. Break point chlorination (or) or free residual chlorination : It involves addition of sufficient amount of chlorine to oxidize: (a) organic matter (b)reducing substance and (c) free ammonia in raw water; leaving behind mainly free chlorine, which possesses disinfecting action against disease- producing bacteria. Oxidation of reducing componds by chlorine a Formation of Chloro Organiccompounds and chloroamines Destrution of chloroorganic and chloroamines b Applied chlorine dose Fig 8. Break-point chlorination curve c Free resdual chlorine d Break point The addition of chlorine at the dip or break is called break point chlorination. This indicates the point at which free residual chlorine begins to appear. Engineering Chemistry Page 23

24 UNIT I Water Technology Advantages: (1) It oxides completely organic compounds, ammonia and other reducing compounds. (2) It removes color, odour and taste of water. (3) It removes completely all the disease causing bacteria/micro-organism (4) It prevents the growth of any weeds in water Using Chloramine (ClNH 2 ): When chlorine and ammonia are mixed in the ratio of 2:1 by volume, chloramine is formed. Cl 2 +NH 3 ClNH 2 + HCl Chloramine is a better bactericidal than chlorine Disinfection by Ozone: Ozone gas is an excellent disinfectant, which is produced by passing silent electric discharge through cold and dry oxygen. 3O 2 2O Desalination of brackish water O 3 O 2 + [O] The process of removing common salt (NaCl) from the water is known as desalination. Water containing high concentration of dissolved salts with a peculiar salty taste is called brackish water. Sea water is an example containing 3.5% of dissolved salts. The common methods for the desalination of brackish water are; 10.1 Eletctrodialysis: It is a method in which the ions are pulled out of the salt water by passing direct current, using electrodes and thin rigid plastic membrane pair. An Eletctrodialysis cell (Fig. 9.) consists of a large number of paired sets of rigid plastic membranes. Hard water is passed between the membrane pairs and an electric field is applied perpendicular to the direction of water flow. Positively charged membrane and negatively charged membrane repel positively charged ions and negatively charged ions respectively to pass through. So, in one compartment of the cell, the salt concentration decreases while in the adjacent compartment it increases. Thus, we get alternative stream of pure water and concentrated brine. Advantages: 1. It is most compact unit. Fig. 9. Eletctrodialysis cell Engineering Chemistry Page 24

25 UNIT I Water Technology The coast of the plant and its operation is economical Reverse osmosis: When two solutions of unequal concentrations are separated by a semi permeable membrane, flow of solvent takes place from dilute to concentrate sides, due to osmosis. If, however a hydrostatic pressure in excess to osmotic pressure is applied on the concentrated side, the solvent flow is reversed, i.e, solvent is forced to move from concentrated side to dilute side across the membrane. This is the principle of reverse osmosis.(ro) Fig. 10. Reverse Osmosis cell Thus in reverse osmosis method, pure solvent is separated from its contaminants, rather than removing contaminants from the water. The membrane filtration is sometimes also called super-filtration or hyperfiltration. Method: In this process, pressure is applied to the sea water or impure water to force the pure water content of it out the semi-permeable membrane, leaving behind the dissolve solids. The principle of reverse osmosis as applied for treating saline/sea water as illustrated in fig 10. The membrane consists of very thin film of cellulose acetate, affixed to either side of a perforated tube. However, more recently superior membranes made of polymethacrylate and polyamide polymers have come into use. Advantages 1. Reverse osmosis possesses distinct advantages of removing ionic as well as non-ionic, colloidal and high molecular weight organic matter. 2. It removes colloidal silica, which is not removed by demineralization. 3. The maintenance cost is almost entirely on the replacement of the semi permeable membrane. 4. The life time of membrane is quite high, about 2 years, 5. The membrane can be replaced within a few minutes, thereby providing nearly uninterrupted water supply. 6. Due to low capital cost, simplicity, low operating cost and high reliability, the reverse osmosis is gaining grounds at present for converting sea water into drinking water and for obtaining water for very high pressure boilers. Engineering Chemistry Page 25

26 UNIT I Water Technology Assignment Questions 1. a) Discuss the impurities in water and their effects b) Describe the demineralization of water by ion-exchange method c) Mention the disadvantages of using hard water for any two industries. 2. a) What is the principle of EDTA method? Explain the estimation of hardness of water by Complexometric method b) Write a short note on priming and foaming, phosphate conditioning and caustic Embrittlement c) Write short note on break-point chlorination and dissolved oxygen, Hardness of water, sedimentation and coagulation. 3. a) Explain scale and sludge formation in boilers. What are their disadvantages? How are they removed? b) Describe the method to determine various alkalinities in a given sample of water. What do you mean by hardness of water? How is it classified? Mention the disadvantages of using hard water for domestic purpose. 4. a) What are the specifications of potable water? Discuss the various steps involved in the treatment of water for domestic purpose. 5. b) What are the chemical reactions involved in the conditioning water by lime soda process? Explain the hot lime soda process with a neat diagram g of calcium carbonate was dissolved in dilute HCl and diluted to 1000ml. 50 ml of this solution requires 48ml of EDTA solution for titration. 50 ml of hard water samples require 15 ml of EDTA solution for titration. 50 ml of same water sample on boiling, filtering, require 10ml EDTA solution. Calculate the different kinds of hardness in ppm. 7. Water having following composition to be softened by lime soda process Calcium bicarbonate = 220ppm; magnesium bicarbonate = 56 ppm, magnesium chloride =130ppm, magnesium sulphate =84ppm, calcium sulphate = 98 ppm. Calculate the amount of lime and soda required to soften million liters of water. 8. Write a note on mechanical deareation? What are the requirements of boiler feed water? 9. What are ion exchange resins? Discuss their applications in water softening. Engineering Chemistry Page 26

27 UNIT II ELECTROCHEMISTRY ELECTROCHEMISTRY Syllabus: Concept of ionic mobilities Applications of Kohlrausch s law Conductometric titrations Galvanic cells Electrode potentials Nernst equation Electrochemical series Potentiometric titrations Concentration cells Ion selective electrode: Batteries and Fuel cells Objectives: Knowledge of galvanic cells, electrode potentials, and concentration cells is necessary for engineers to understand corrosion problem and its control; also this knowledge helps in understanding modern bio-sensors, fuel cells and improve them. OUTLINES Introduction Arrhenius theory & Debye- Huckel s theory Kohlrausch s law of independent migration of ions Applications of Kohlrausch s law Conductometric titrations Galvanic cells electrode potentials Electrochemical series Electromotive force (Nernst equation) Potentiometric titrations Concentration cells Ion selective electrodes Batteries Fuel cells Engineering Chemistry Page 26

28 UNIT II ELECTROCHEMISTRY INTRODUCTION Electric current is the flow of electrons generated by a battery, when the circuit is completed. A substance which allows electric current to pass through it is called a conductor, e.g., all metals, graphite, fused salts, aqueous solutions of acids, bases and salts; while insulator or non-conductor is a substance which does not conduct electric current i.e., which does not allow the passage of electric current through it. The conductors are of two types a. Metallic conductors are the substances which conduct electricity, but are not decomposed by it, e.g., all metals, graphite etc. b. Electrolyte is a substance which in aqueous solution or in molten state liberates ions and allows electric current to pass through, thereby resulting in its chemical decomposition, e.g., acids, bases and some salts. 1.1 IONIC MOBILITIES- Arrhenius theory The main postulates of the Arrhenius theory are.. 1. In solutions, all electrolytes are spontaneously dissociated, to some extent, into charged particles, called ions. The ions carrying positive charge are called cations; while those carrying negative charge are called anions. 2. Cations are generally, metallic radicals obtained by loss of electrons from the metal atoms; while anions are non-metallic atoms or radicals (a group of atoms of two or more elements) obtained by gain of electrons: M M n+ + n e - Metal cation electrons A + n e - A n- Non-metal anion 3. The total positive charge on the cations present in a solution is equal, but opposite to the total negative charge present on the anions. Thus, the solution of an electrolyte is neutral as a whole. 4. The cations and anions present in a solution are constantly reuniting to form un dissociated, electrically neutral parent molecules and a state of dynamic equilibrium exists between the ionized and the unionized molecules AB Unionized molecule A + + B - cation Thus, the process of electrolytic dissociation is a reversible process. 5. The ions are free to move under the influence of electric current, they are directed towards anion oppositely charged electrodes. Cations move towards cathode, while anions move toward the anode, when electricity is passed through the solution. Engineering Chemistry Page 27

29 UNIT II ELECTROCHEMISTRY The properties of electrolytes in solution are the properties of the ions produced. 7. The electrolyte at a given dilution may not be completely ionized and the fraction of the total molecules ionized, is termed as degree of ionization, i.e. No. of molecules dissociated into ions Degree of ionization = Total number molecules taken Electrolytes like mineral acids, alkalis and many salts, which give solutions with high conductivities, are called strong electrolytes. This is because strong electrolytes are more or less completely ionized, when in solution, e.g., HCl H + + Cl - NaCl Na + + Cl - KOH K + + OH - In other words, their degree of ionization is 1 or 100%. 8. Electrolytes like many organic acids, bases, water and ammonium hydroxide, which are poor conductors in solutions, are weak electrolytes. This is due to the fact that only a small fraction of their molecules dissociate into ions in solutions. Their solutions contain smaller number of ions in equilibrium with unionized molecules, e.g., H 2 O H + + OH - CH 3 COOH H + + CH 3 COO 9. The conductivity of an electrolytic solution generally increases with the increase in temperature, because the average kinetic energy of the ions increases as temperature increases. 1.2 DEBYE-HUCKEL THEORY The Arrhenius theory is found to be valid only in case of weak electrolytes. The failure of the theory, in case of strong electrolytes, has been satisfactorily explained by Debye-Huckel and Onsager (1923). According to them: 1. All strong electrolytes are completely ionized even in solid state, i.e., their degree of dissociation is 1 or 100%. However, the ions are not free to move in solid state and, therefore, cannot conduct electricity. 2. On melting or dissolution, the ions become mobile and mobility of these ions depends upon: viscosity of the medium, the extent of solvation (the number of solvent molecules attached to each ion). 3. The ratio of ʌ eq c / ʌ eq for strong electrolytes does not represent the degree of ionization, but it is only a conductivity ratio. 4. The increase in conductivity (ʌ eq c ) of strong electrolyte solution on dilution is due to increase in the mobility of ions. 5. The lower values of mobility of ions in concentrated solutions, in spite of complete dissociation is due to the following ionic interferences: Engineering Chemistry Page 28

30 UNIT II ELECTROCHEMISTRY c ʌ eq = ʌ eq (A + B ʌ eq ) 1.4 CONDUCTIVITY OF ELECTROLYTES According to ohm s law, the resistance of a conductor(r) is directly proportional to its length (L) and inversely proportional to its cross sectional area(a) R = ρ (L/A), where ρ is a constant of proportionality, termed as specific resistance Thus, if L = 1cm and A = 1 cm 2 R = ρ, i.e., resistivity or specific resistance of a conductor is the resistance between two opposite faces of centimeter cube of that substance. The units of resistivity are ohm-cm a. Conductance : It is the tendency of a material to allow the flow of current through it. It is the reciprocal of resistance. Conductance (C) = 1/R b. Specific conductivity (κ) is the reciprocal of specific resistance of an electrolyte solution i.e., κ(ka-pa) = (1/ρ) = (L/AR) The usual unit of specific conductivity is ohm -1 cm -1 or S cm -1 If, L = 1cm and A = 1 cm 2, then κ = 1/R = Specific conductance Hence, specific conductivity (κ) is the conductivity of 1 cm 3 solution While observing conductance of solutions, electrodes have to be used and they may not have exact surface area of 1cm 2 and distance of 1 cm. Hence the reciprocal of resistance does not give specific conductivity but a value proportional to it. Such measurements in solutions are difficult. Hence indirect method is employed to calculate specific conductance from the observed conductance. The ratio of length(l) and area(a) of electrodes in a conductivity cell is defined as cell constant(x) Then x becomes the ratio of specific conductance and observed conductance. Specific conductance κ v = x X observed conductance The units of cell constant are cm -1. For the determination of cell constant, the specific conductance of N/50 KCl solution at 25 0 C is taken as ohm -1.cm -1. Such a solution is prepared by dissolving 0.372g of pure KCl in 250ml of conductivity water and its conductivity is observed. Then Cell constant x = / observed conductance Once cell constant is determined it can be used for all experiments provided care is taken to see that the distances are not altered. Problems on cell constant and conductance: 1. A conductance cell has two parallel electrodes of 1.25 cm2 area placed 10.5 cm apart; filled with an electrolyte solution the resistance was 1995 Ω. Calculate the cell constant and specific conductance. Solution: Engineering Chemistry Page 29

31 UNIT II ELECTROCHEMISTRY Equivalent conductance of 0.05 N NaOH is 240 mho. cm 2. The electrode of the cell are 1 cm apart with area 1 sq. cm. calculate the specific conductance. Solution: 3. A cell whose resistance when filled with 0.1 N KCl is ohm. It has 6306 ohm when filled with 0.01N NaCl at 25 C. Calculate the cell constant, specific conductance of NaCl solution, equivalent conductance of NaCl solution and degree of ionization of NaCl. Solution: Given Specific conductivity of 0.1 N KCl is at 25 C and equivalent conductance at infinite dilution of NaCl at 25 C is (i) (ii) ( ( ) ) ( ) Ohm -1 cm -1 ( ) ( ) ( ) Ohm -1 cm 2 equiv -1 (iii) Degree of ionization λ α(nacl) = Ohm -1 cm 2 equiv -1 α (NaCl) = 7 Degree of ionization of NaCl = 0.31 c. Equivalent conductance of an electrolytic solution is defined as the conductance of all the ions present in one equivalent of the electrolyte in the solution at a given dilution. If one equivalent of the electrolyte is contained in V ml, then: ʌ eq = V x specific conductance of 1 cm 3 solution = Vx κ V = (1/N) L or (1000/N) ml (dilution) ʌ eq = (1000 κ /N) The unit of equivalent conductance is ohm -1 cm 2 eq -1 d. Molar conductivity of a solution is defined as the conductance of all the ions present in one mole of electrolyte in the solution. If M is the molar concentration in mol/l then: ʌ m = (1000 κ /M) Engineering Chemistry Page 30

32 UNIT II ELECTROCHEMISTRY The unit of molar conductivity is ohm -1 cm 2 mol -1 e. Effect of dilution on conductance: As dilution increases more and more (i.e addition of water), the ionization of the electrolyte increases and number of ions in any cubic volume decrease. As specific conductance is conductance of the ions present in one cc of solution, its value decreases with dilution. Since equivalent conductance and molecular conductance are products specific conductance and volume of solution, they also tend to increase with dilution. Ionization increases with dilution at attains a limiting value and further addition of water will not produce any further ionization. Consequently this limiting value of equivalence conductance is at infinite dilution Λ. 2. KOHLRAUSCH S LAW OF INDEPENDENT MIGRATION OF IONS According to the law, at infinite dilution, when dissociation is altogether assumed to be complete and all interionic effects vanish, each ion moves independently irrespective of the nature and presence of the co-ion and contributes a definite value of its share to the total molar conductance of an electrolyte, which depends only on its nature and not at all on the ion with which it is linked. Thus, molar conductivity at infinite dilution of any electrolyte is equal to the sum of the molar conductances of the cation and anion present, since each ion contributes a definite amount to the total conductance of the electrolyte, i.e., ʌ m = V + λ + + V λ Where V + and V - are the number of cations and anions per formula of electrolyte; λ + and λ - are molar conductivities at infinite dilution of cation and anion respectively. So is the case with equivalent conductance. For example, for acetic acid (CH 3 COOH), V + = V = 1 so λ m (CH 3 COOH) = λ (H + ) + λ (CH 3 COO ) And for MgCl 2 V + =1, V = 2 λ m (MgCl 2 ) = λ + (Mg +2 ) + 2λ (Cl ) 2.1 APPLICATIONS OF KOHLRAUSCH S LAW: Determination of molar conductivity of a weak electrolyte: As mentioned earlier, it is not possible to determine the values of ʌ m of a weak electrolyte by extrapolation of molar conductivity values at infinite dilution, because (A) even at very high dilutions, these electrolytes are not completely ionized (B) the curve does not follow a straight line path. Kolrausch s law has provided a method for finding the ʌ m for weak electrolytes from ʌ m measurements of strong electrolytes. Suppose we want to compute the ʌ m value for acetic acid, this value can be obtained from the ʌ m values of HCl, NaCl and CH 3 COONa, by Kolrausch s law we have ʌ m (HCl) + ʌ m (CH 3 COONa) ʌ m (NaCl) = [λ (H + ) + λ (Cl ) ] + [λ (CH 3 COO - ) + λ (Na + )] [λ (Na + ) + λ (Cl )] Engineering Chemistry Page 31

33 UNIT II ELECTROCHEMISTRY = λ (H + ) + λ (CH 3 COO ) = λ (CH 3 COOH) Thus the values of ʌ m for strong electrolytes can be obtained by extrapolation of ʌ c m graphs. Thus ʌ m value for any weak electrolyte can be obtained from the ʌ m values of the selected strong electrolytes Determination of degree of dissociation: The term degree of dissociation is the fraction of the total number of molecules dissociated into ions i.e., α c = We also know that the conductivity of a solution is due to the presence of ions in solution, greater the number of ions, greater is the conductivity. ʌ c m, the molar conductivity at a particular dilution No. Of molecules dissociated into ions at this dilution But at infinite dilution all molecules are in ionic form ʌ m Total number of molecules taken Dividing (i) by (ii), we get (i) (ii) ʌ c m /ʌ m Thus, degree of dissociation at any dilution is the ratio of the molar conductivity at that dilution to the molar conductivity at infinite dilution. The value of ʌ c m can be obtained by direct measurement; while ʌ m can be obtained with the help of Kolrausch s law, i.e., ʌ m = V + λ + + V λ Hence, the values of α c can be calculated using the law Determination of solubility of sparingly soluble salts: For ordinary purpose, ionic salts like AgCl, PbS, BaSO 4, CaCO 3, Fe(OH) 3, etc., are regarded as insoluble, but they do have a very small, but definite solubilities in water. The solubility of such a sparingly soluble salt is obtained by determining the specific conductivity (k) of a saturated salt solution. Since only a very small amount of salt is present in solution, the equivalent conductivity at such dilution can practically be taken as ʌ eq. For every sparingly soluble salts: ʌ c eq = ʌ eq + kυ The value of ʌ can be calculated with the help of Kolrausch s law. c υ = ʌ eq = ʌ eq = λ + + λ k k k But υ cm 3 of saturated solution contains = 1 eq 1000 cm 3 or 1 L solution contains = (1000/υ) eq Engineering Chemistry Page 32

34 UNIT II ELECTROCHEMISTRY Calculation of ionic product of water (K w ) Pure water is a poor conductor of electricity. But the conductivity and other measurements support that water ionizes to a very small extent producing equal number of H + and OH - ions. The ionization equilibrium is represented as H 2 O H + + OH - Since H + is associated to water molecule, 2H 2 O H 3 O + + OH - The equilibrium constant K is given by [H3O ] [OH K 2 [H O] 2 - ] K. [H 2 O] 2 = [H 3 O + ] [OH - ] Since, the ionization of water is negligible, [H 2 O] can be taken as constant K[H 2 O] 2 = K w = constant K w = [H 3 O + ] [H 2 O] The constant K w is known as ionic product of water at given temperature. The ionic product of water, K w at a given temperature, is defined as the product of the concentrations of H + and OH - ions in water or in aqueous solution. At 25 o C (room temperature), K w is 1.0 x moles 2 /Lit 2. As the temperature increases, ionization of water increases and the value of K w increases. K w = [H + ][OH - ] = x moles 2 /Lit 2 at 22 o C. Each water molecule dissociates to give equal number of opposite ions, [H + ] = [OH - ] K w = [H + ][OH - ] = [H + ] 2 or [OH - ] 2 = 1.0 x M 2 /Lit 2 [H + ] = [OH - ] = x 10 Moles /Lit = 1.0 x 10-7 moles/lit Problems on Kohlrausch law: 1. Calculate the equivalent conductivity at infinite dilution for chloro acetic acid from the following data. The λ α of HCl, NaCl and ClCH 2 COONa are respectively 426,126 and 90mho cm-1 equiv-1. Sol: λ α of ClCH 2 COOH is to be calculated. ClCH 2 COOH ClCH 2 COO - + H + λ α λ a + λ c Given the values of λ α of the three electrolytes which contains ClCH2COO - and H + are Engineering Chemistry Page 33

35 UNIT II ELECTROCHEMISTRY H + Cl -, Na + Cl -, ClCH 2 COO - Na + = I + III - II = H + Cl - + ClCH 2 COO - Na + - Na + Cl - + I II III = ClCH 2 COO - H + λ a and λ c are present in electrolyte I and II which is to be added and the excess Na + and Cl - are to subtracted. Add the λ α values of HCl and ClCH 2 COONa and subtract the ClCH 2 COOH value of NaCl. The λ α of ClCH 2 COONa is obtained. = = 390 mho cm -1 equiv The specific conductance of saturated solution of silver chloride at 18 o C is 1.24x10-6 mho. The mobilities of Ag + and Cl - ions at this temperature are 53.8 and 65.3 respectively. Calculate the solubility of silver chloride in grams per liter. Sol. K v = 1.2x10-6 λ α = λ a + λ c λ α = = λ v = K v + V V = λ α /K v 10 6 ml 10 6 ml of contains 1 gm equivalent of the AgCl = (143.5 gms of AgCl) 1000ml of water should contain AgCl = 10-6 Solubility of AgCl = x 10-3 gms/ 1000cc. 3. Conductometric titrations Conductometric titration is instrumental method of volumetric analysis based on the change in conductance of the solution, at the equivalence point (or end point) during titration. This method is based on the fact that conductance of an aqueous solution, containing an electrolyte, depends upon: (i) the number of free ions in the solution and their nature; (ii) the charge on the free ions, (iii) mobility (or speed) of the ions. During the course of titration (i.e., addition of one electrolytic solution to that of another), the number of free ions in the solution changes. Not only that, even the identity of the ions also changes. As a result of this, conductance of the solution (contained in cell) also undergoes a change. Let us take a few specific cases for illustration. Acid- base titrations (a) Titration of a strong acid ( HCl) with a strong base ( NaOH) Before base is added, the conductivity of acid solution is high (mainly due to the presence of highly mobile H + ions). This is represented by point A on the curve. The acid is taken in the conductivity cell and the alkali in the burette. On gradual addition of NaOH from the burette, highly mobile H + Engineering Chemistry Page 34

36 UNIT II ELECTROCHEMISTRY ions of the acid are removed by the OH - ions added from the burette to form nearly nonconducting water molecules. H + Cl - + Na + OH - Na + Cl - + H 2 O Acid Base Salt Water (non- conducting) Hence, the conductivity of the solution decreases progressively, till the equivalence point B is reached. On further addition of NaOH, the conductivity of the solution will raise along the curve BC (due to the addition and presence of highly mobile OH - ions to the solution since they are no more neutralized). Thus, the descending branch of the curve (i.e., AB) gives the conductance of a mixture of acid and salt, and the ascending branch of the curve no excess of either acid or base and hence, the intersection corresponds to the equivalence point. A H + Cl + Na + Cl - Na + Cl - + Na + OH - C B Equivalence point Volume of strong base solution Fig. 1. Coductometric titration of a strong acid and strong base (b) Titration of a weak acid ( acetic acid) with a strong base (sodium hydroxide): The titration curve has the form of A, B, C. Acetic acid (a weak acid) has low conductivity, as represented by A. As NaOH is added, the poorly conducting acid is converted into highly ionized salt, CH 3 COONa. CH 3 COOH + NaOH CH 3 COO - Na + + H 2 O (unionized) Poorly conducting acid Highly ionized salt And consequently, the conductivity goes up along AB. When the acid is neutralized, further addition of the alkali causes a sharp rise in conductance along BC (due to addition of more conducting OH - ions). The intersection of AB and BC, therefore, represents the equivalence point. C Na + Ac - + Na + OH - Equivalence point B A HAc + Na + Ac - Volume of strong base solution Fig. 2. Coductometric titration of a weak acid and strong base Engineering Chemistry Page 35

37 UNIT II ELECTROCHEMISTRY (c) If strong acid ( HCl) is titrated against a weak base ( NH 4 OH): the conductance in the beginning starts falling (due to the removal of H + ions) to form practically unionized water plus slow moving NH 4 + ions H + Cl - + NH + 4 OH - Strong acid weak base NH 4 + Cl - + H 2 O Salt practically unionized However, when the entire acid is neutralized, further addition of poorly ionized ammonium hydroxide does not cause any appreciable change in the conductance. The shape of the curve thus obtained and the intersection corresponding to the equivalence point are shown in the figure. H + Cl + NH 4 + Cl - NH 4 OH + NH 4 Cl - Equivalence point Volume of NH 4 OH solution Fig. 3. Coductometric titration of a strong acid and weak base (d) If it is required to titrate a weak acid ( CH 3 COOH) against a weak base (NH 4 OH), the addition of ammonium hydroxide may even cause a decrease in conductance in the beginning, because the common ions formed depresses the dissociation of their respective electrolyte (i.e., CH 3 COOH, NH 4 OH). CH 3 COOH + NH 4 OH NH CH 3 COO - + H 2 O Weak acid weak base highly ionized salt practically unionized However, on further addition of ammonium hydroxide, an increase in the conductivity of the solution results, the conductance of the highly ionized salt (e.g., ammonium acetate) exceeds the conductance of the weak acid (acetic acid) it replaces. After the neutralization of the acid, further addition of poorly ionized ammonium hydroxide does not cause any appreciable change in the conductance. NH 4 + Ac - + NH 4 OH Equivalence point Volume of NH 4 OH solution Fig. 4. Coductometric titration of a weak acid against a weak base Engineering Chemistry Page 36

38 UNIT II ELECTROCHEMISTRY In a similar way redox titrations, precipitation titrations can also be performed and results are obtained in a better way. ADVANTAGES OF CONDUCTOMETRIC TITRATIONS 1. They give accurate end-points than the conventional visual titrations, where end point is detected only after adding excess of the titrant. One titration is enough. 2. There is no need to use indicator here, since end point is determined graphically by intersection of lines and many errors are also minimized due to graphical method. 3. They are useful in the titrations involving colored solutions, where the color change of indicator is not clear for detection. 4. They are very useful in the titrations of very dilute solutions 5. They are useful for titrating weak acids against weak alkalis, which otherwise do not give sharp endpoints in visual titrations 6. In this method, no keen observation may not be necessary near the end point, since it is obtained graphically and hence possible errors are minimized. 4. Electrochemical Cell (Galvanic cell) An electrochemical cell is a device in which a redox chemical reaction is utilized to get electrical energy. An electrochemical cell is generally referred to as voltaic cell or galvanic cell. The electrode where the oxidation occurs is called anode and the electrode where reduction occurs, is called cathode. Example: Daniels Cell, Leclanche cell The Daniel cell (Figure above) consists of zinc electrode dipped in ZnSO 4 solution and copper electrode, dipped in CuSO 4 solution. The two solutions are separated by salt bridge so as to avoid direct contact or mixing with each other. The electrode reactions in Daniel cell are At anode: Zn Zn e - (Oxidation) At Cathode: Cu e - Cu (Reduction) Cell Reaction: Zn + Cu 2+ Zn 2+ + Cu Zn has more tendency to form Zn 2+ and hence Zn metal acquires a negative charge; and Cu 2+ has more tendency to get deposited as Cu. Hence, copper electrode becomes positively charged. As a result, the Engineering Chemistry Page 37

39 UNIT II ELECTROCHEMISTRY electrons via the external circuit constitutes the electric current in the opposite direction. The emf of the cell is 1.1 volts. Direction of electron (e - ) flow (-) (+) Anode Zn ZnSO 4 (aq) CuSO 4 (aq) Cu Cathode Direction of electric current flow 4.1 Representation of a Galvanic Cell A galvanic cell can be represented as follows; a) Anode is written on the left hand side; while the cathode is written on the right hand side. b) The electrode on the left (anode) is represented by writing the metal or solid phase first and the electrolyte separated by a vertical line or semicolon Zn(s) Zn 2+ (aq) or Zn(s); Zn 2+ c) The cathode of the cell is written on the right hand side. In this case, the electrolyte is represented first and the metal or solid phase, thereafter separated by a vertical line or semicolon. Cu 2+ Cu(s) or Cu 2+ ; Cu(s) d) A salt bridge is indicated by two vertical lines, separating the two half cells. Thus, applying above considerations to Daniel Cell, we may represent it as Zn(s) Zn 2+ (1M) Cu 2+ (1M) Cu(s) 5. Electrochemical series: It is the series of the elements arranged in increasing order of their standard electrode reduction potentials. Metal ion Standard reduction potential in volts Li + e - Li K + + e - K Ca e - Ca Na + + e - Na Mg e - Mg Al e - Al Zn e - Zn Cr e - Cr Fe e - Fe Ni e - Ni Sn e - Sn Pb e - Pb Fe e - Fe Engineering Chemistry Page 38

40 UNIT II ELECTROCHEMISTRY H + + e - ½ H 0.00 Cu e - Cu Ag + + e - Ag Pt e - Pt Au + + e - Au ½ F 2 + e - F The electro chemical series provide valuable information regarding: Relative ease of oxidation or reduction A system with high reduction potential has a great tendency to undergo reduction. For example, the standard reduction potentials of F 2 /F - System and Li + /Li System is V and -3.05V respectively. The former one can easily gain electrons than the later one. So F 2 can easily be reduced to F - and Li is easily oxidized to Li Replacement Tendency Metal with greater oxidation potential can displace metals with lower oxidation potentials from their salt solution. the solution of CuSO 4. For Example, Cu 2+ has more tendency to replace Zn. Zinc will displace copper from Predicting spontaneity of redox reactions Positive value of E of a cell reaction indicates that the reaction is spontaneous. If the value of E is negative, the reaction is not feasible Calculation of Equilibrium Constant The standard electrode potential E 0 RT ln K nf Hence, log K eq eq 2.303RT log K nf Engineering Chemistry Page 39 eq 0 0 nf x E ne 2.303RT V 6. ELECTROMOTIVE FORCE (EMF) Electromotive force is the difference of potential produced by sources of electrical energy, which can be used to drive current through electrical circuit. It is not a force but a scalar quantity expressed in volts. 6.1 Electrode Potential A metal (M) consists of metal ions (M n+ ), with the valence electrons that bind the metal atoms together. If a metal is in contact with a solution of its own salt, the following two chemical reactions will take place.

41 UNIT II ELECTROCHEMISTRY a) Positive metallic ions passing into solution M M n+ + ne - (Oxidation) b) Positive ions get deposited on the metal electrode M n+ + ne - M (Reduction) Zn 2+ ions moving from Zn electrode to solution leaving free electrons Zn Rod Zn 2+ Zn Cu 2+ Cu Rod Cu 2+ Cu 2+ ions entering the copper metal leaving behind free negativly charged ions in solution ZnSO 4 Soution Zn 2+ ions Cu 2+ ions CuSO 4 Solution Fig. 6. Electrode potential In the first case, n electrons are left behind on the metal and it acquires negative charge. In the second case, the metal acquires a positive charge. A dynamic equilibrium is ultimately established in the reaction. At equilibrium, the potential difference between the metal and solution attains a constant value, which is called the Electrode potential. Thus, Electrode potential of a metal is the measure of tendency of a metal to lose or gain electrons, when it is in contact with a solution of its own salt of unit molar concentration at 25 o C. As a convention, the tendency of an electrode to lose electrons is termed as oxidation potential (+X) and the tendency of an electrode to gain electrons as reduction potential (-X). 6.2 Redox Reaction Oxidation involves loss of electrons and reduction involves gain of electrons those electrons. In other words, oxidation and reduction must always go side by side and are not independent in an electro chemical cell. If we place zinc metal in a solution of copper sulphate, immediate deposition of Cu takes place and metal zinc goes into solution as Zn +2 Zn(s) + Cu 2+ (aq) Zn 2+ (aq) + Cu(s) In this change, the zinc atom (Zn) is oxidized to Zinc ion (Zn 2+ ), losing electrons; while the copper ion (Cu 2+ is reduced to copper atom, gaining those electrons. 2 Zn(S) Zn (aq) 2e (Oxidation ) 2 Cu (aq) 2e Cu(S) (Reduction) 2 2 Zn(S) Cu (aq) Zn (aq) Cu(S) (Redox) The overall reaction is called redox or oxidation reduction reaction. Each of these reactions is known as a half-reaction. The reaction, in which loss of electrons takes place, is called oxidation- Engineering Chemistry Page 40

42 UNIT II ELECTROCHEMISTRY half reaction; while the other reaction, in which gain of electrons takes place, is called reduction half reaction. Thus each electro chemical cell is made up of two electrodes which are dipped in their salt solutions and connected through a salt bridge and give emf on connection in an external circuit. Each of these electrodes will have its potential. An electrode dipped in its salt solution is a half cell and its potential called single electrode potential, which cannot be measured directly. 6.3 EMF of an Electrochemical Cell An electrochemical or galvanic cell is obtained by coupling two half cells. Mathematically, the emf of an electrochemical cell is the algebraic sum of the single electrode potentials; provided proper signs are given according to the actual reaction taking place on the electrodes. Standard EMF of the cell = [standard oxidation potential at oxidation half cell reaction + Standard reduction potential of reduction half cell reaction] = [standard reduction potential at reduction half cell reaction - Standard reduction potential of oxidation half cell reaction] [oxidation potential = - reduction potential] E cell = E cathod E anode E cell = E right E left ; Where E cell = e.m.f. of the cell E right = reduction potential of right hand side electrode E left = reduction potential of left hand side electrode The positive value of E cell indicates that the cell reaction is feasible. Problems on EMF 1. Calculate the e.m.f the following reaction at 25 o C Cu ++ + Zn Zn ++ + Cu E o Zn (Ox) = V E o Cu (Ox) = V E (Cell) = E (R) - E (L) = (-0.763) = = 1.1volt 2. The potential of the cell Zn Zn 2+ aq (1M) Cu 2+ aq (0.1M) Cu at 25 o C is 1. volt. calulate the potentilal of the cell Zn Zn 2+ aq (0.5M) Cu 2+ aq (0.01M) Cu at the same temperature Engineering Chemistry Page 41

43 UNIT II ELECTROCHEMISTRY E (cell) = E o (cell) E (cell) = 1.1 v 2 log 10 Cu 2+ Zn Measurement of Electrode Potential It is not possible to know the absolute value of a single electrode potential. We can only measure the difference in potential between two electrodes potentiometrically, if we fix arbitrarily the potential of any one of electrode. For this purpose, the potential of a standard hydrogen electrode (SHE) or normal hydrogen electrode (NHE) has been arbitrarily fixed as zero. = log = X log = X = = V 0.5 The SHE or NHE contains a platinum foil electrode coated with platinum black (for better adsorption of hydrogen gas) in contact with 1M HCl solution through which hydrogen gas is bubbled at a constant rate of one atmosphere pressure. This electrode is represented as.. Pt(s), H 2 (g) (1 atm); H + (1 M) The following reactions take place at this electrode H + (aq) + e - ½ H 2 (g) Reduction ½ H 2 (g) H + (aq) + e - Oxidation Thus when both of these reactions can take place at the electrode, it is capable of acting as anode or cathode depending on the other electrode coupled, to form the cell. The EMF of the cell is measured with a potentiometer and since the single electrode potential value of the hydrogen electrode is Engineering Chemistry Page 42

44 UNIT II ELECTROCHEMISTRY arbitrarily taken as zero, the single electrode potential of the other electrode can be calculated, since the EMF value determined is the algebraic sum of the values of single electrode potentials. In order to measure the electrode potential ( for eg; Zn dipped in ZnSO 4 ), this electrode is coupled with SHE through a salt bridge. Zinc electrode acts as anode, so the electrode potential is oxidation potential with a value V and consequently, its reduction potential is -0.76V. potential of copper electrode is +0.34V. 6.5 Expression for single electrode potential inter-related. Consider a general redox reaction M n+ (aq) + ne - M(S) The electrode For a reversible reaction, the free energy change ( G) and its equilibrium constant (K) are G [Product] G 0 RT ln [Reactant] Where G 0 is known as the standard free energy change. In a reversible reaction, the electrical energy is produced at the expense of the free energy i.e. Electrical work done - G = nfe and G 0 = - nfe 0 = Electrical energy produced = Quantity of electricity flow x EMF For every one mole electrons transferred in the cell reaction the quantity of electricity that flows through the cell is one faraday (1F= 96500) if n moles of electrons are transferred. Then, Electrical work done = nf. E cell Where E is the electrode potential; E 0 is the standard electrode potential; F = Faraday (or 96,500 coulombs). Consequently, 0 - nfe - nfe M RT ln n M 1 - nfe 0 RT ln n M - nfe - nfe 0 RT ln M n ( Concentration of M is unity) E E E 0 0 RT ln M nf n RT log M nf n Engineering Chemistry Page 43

45 UNIT II ELECTROCHEMISTRY This expression is known as Nernst s equation for electrode Potential. From the above equation, it is clear that i) If concentration of solution [M n+ ] is increased, the electrode potential increases and vice versa. ii) If temperature is increased, electrode potential is increased and vice-versa. When the elements are arranged in increasing order of reduction potential, a series called electro chemical series is obtained. 6.6 Nernst equation for a cell reaction Consider the Daniel Cell Zn ZnSO 4 (aq) CuSO 4 (aq) Cu Reaction is Zn(S) + Cu 2+ (aq) Zn 2+ (aq) + Cu(S) RT E cell Zn = E 0 cell log 2 nf Cu At 298K, the Nernst equation can be written as E cell (ECathode E Anode) log 2 n Zn Cu E ) log Cu Cu Zn Zn 2 (E 2 7. Potentiometric titrations n Potentiometric titration is an important application of emf measurement. In this method, a cell is constructed, in which at least one of the electrodes is reversible with respect to one of the ions taking part in the titration reaction. Theory: The potential of an electrode dipping in a solution of an electrolyte depends on upon the concentration of active ions (i.e., which changes the electrode potential) E = E 0 + (RT/nF) log C A small change in the active ion concentration in the solution changes the electrode potential to the corresponding level. During the course of titration, the concentration of the active ion Cu Zn Engineering Chemistry Page 44

46 UNIT II ELECTROCHEMISTRY decreases, thereby electrode potential of the indicator electrode decreases. Thus, measurement of indicator electrode potential can serve as a means of detection of equivalence point of the titration reaction. The potential of the indicator electrode is, usually measured potentiometrically by connecting it to a reference electrode like saturated calomel electrode. Detection of end point: The emf of a cell changes by the addition of small volumes of titrant, so the concentration of reversible ion in contact with indicator electrode changes. The change in emf with every small addition is recorded. The change of potential will be slow at first, but at equivalence the point change will be sharp or quite sudden with a jump or rise i n potential. The values of potentials are plotted against corresponding volume of titrant added. A curve (a) like the one shown in figure is obtained. The end point corresponds to the point of inflexion, i.e., point where the slope of the curve is maximum as shown. If the inflexion is not sharp alternatively, the change in emf with every small addition of titrant is plotted against volume V to obtain a curve shown in (b). The maximum of the curve b gives the end point. Maxima Emf of cell (E) End point E/ V End point V (Volume of titrant) V (Volume of titrant) (a) (b) Fig. 11. (a) In potentiometeric titration, the point of inflation is the end-point, (b) plot of E/ V against volume (V) Maxima gives the more acurare end-point. There are three important types of potentiometric titrations, which are described below. a. Acid base reactions: In acid base titrations, quinhydrone electrode is employed as the indicator electrode. The reference electrode is, generally the saturated calomel electrode. A definite volume of the given acid solution is taken in a 100ml beaker. To it a pinch of quinhydrone is added and a stirrer and platinum electrode are placed in it. This electrode is then connected to saturated calomel electrode through a potentiometer. On adding standard alkali solution from the burette, the emf of the cell increases at first slowly, but at the end point the rate of change of potential will be suddenly quite large with a possible jump or drop in potential. The end point of the titration is then located by plotting ΔE/ΔV versus V as shown in figure (11b) and the volume of titrant corresponding to the peak in the curve gives end point. The advantage of such a titration lies in the fact that this method can be used for titration of coloured solutions. Engineering Chemistry Page 45

47 V UNIT II ELECTROCHEMISTRY b. Oxidation-reduction reactions: Titrations involving oxidizing agents such as potassium dichromate, potassium permanganate etc and reducing agents like ferrous ammonium sulphate can be followed potentiometrically by using platinum indicator electrode.. On adding potassium dichromate from the burette, emf of the cell will increase first slowly, but at the equivalence point, there will be sudden jump or drop in the potential since the change in the ratio of Fe +2 /Fe +3 ions concentration, is quite rapid at the equivalence point. Advantages of potentiometric titrations: E = E 0 + (RT/nF) log (Fe +2 )/ (Fe +3 ) 1. Coloured solutions, where the use of indicator is not possible can be estimated by potentiometric titrations. 2. Since no indicator is necessary, there is no problem with regard to the choice of indicators based on ph value of the solutions. 3. Since end point is determined graphically, many errors in titration are minimized and single titration is enough. 4. Polybasic acids can be titrated in steps corresponding to different steps of neutralization. 5. Dependence on colour and external indicators are avoided for redox titration by Potentiometric titrations. 6. Solutions containing more than one halide can be analyzed in a single titration against silver nitrate. 8. Concentration Cell In a galvanic cell, electrical energy arises from the decrease in free energy (- G) of the chemical reactions taking place in the cell. In a concentration cell, there is no net chemical reaction. The electrical energy in a concentration cell arises from the transfer of a substance from the solution of a higher concentration (around one electrode) to solution of lower concentration (around the other electrode). A concentration cell is made up of two half cells having identical electrodes, identical electrolyte, except that the concentrations of the reactive ions at the two electrodes are different. The two half cells may be joined by a salt bridge. For example Ag AgNO AgNO Ag + 3 (C 1 ) Salt bridge 3 (C 2 ) of stand (Con) NH 4 NO 3 Ag - or Ag + AgNO 3 (C 1 M) AgNO 3 (C 2 M) E Cell n C log C 2 1. at 25 0 C. (C 2 C 1 ) Engineering Chemistry Page 46

48 UNIT II ELECTROCHEMISTRY The general equation for emf of such cell is given by In this cell, the following reactions occur 2.303RT C ECell log nf C 2 1 At anode : M M n+ (C 1 ) + n e- At cathode : M n+ (C 2 ) + n e- M Cell reaction : M n+ (C 2 ) M n+ (C 1 ) The emf so developed is due to the mere transference due to concentration gradient of metal ions from the solution of higher concentration (C 2 ) to the solution of lower concentration (C 1 ). 9. Ion Selective Electrode: An electrode that responds to the activity of a particular ion is called ion-selective (or) ion-sensitive electrode (ISE). ISE are prepared basing on the principle that membrane potentials are developed due to concentration gradient. Principle: Whenever two solutions of different concentrations are separated by a membrane, a potential difference arises across the membrane due to the unequal distribution of ions in the solutions. This potential difference is known as membrane potential. This membrane potential difference will be measured by ion-selectometers with the help of ion selective electrodes. There are six types of ISE based on the membranes used in the electrode: 1. Glass membrane electrode 2. Liquid membrane electrode 3. Solid membrane electrode 4. Gas sensing electrode 5. Enzyme based electrode 6. Bio catalytic membrane electrode 9.1 Glass membrane electrode: The electrode has a thin specially made glass membrane, which is selective to various univalent cations. The internal reference electrode enables electrical contact between the inner surface of the membrane via the reference solution and ion electrometer. Engineering Chemistry Page 47

49 , UNIT II ELECTROCHEMISTRY Glass Electrode: It contains a bulb containing 0.1M HCl and a silver wire coated with silver chloride which acts as internal reference electrode. It is immersed in a solution whose hydrogen ion concentration is to be determined. The upper end of the glass electrode is sealed. A potential develops between two surfaces of the membrane and PD developed is proportional to the difference in ph value. Hence, the potential difference between the outer surface of the glass bulb the solution into which the glass bulb immersed is varied and so, the overall potential is governed by the hydrogen ion concentration of the test solution a glass electrode is combined with SCE and placed in a solution and the ph is calculated. Ag/AgCl 0.1M KCl 0.1M HCl Glass memberane Fig. 12. Glass memberane electrode The glass used for construction of the glass electrode is a special glass of approximate composition of SiO 2 (72%), Na 2 O (22%) and CaO (6%). Such electrodes satisfactorily work over the ph range Glass electrode are easy for operation, not easily poisoned. The equilibrium is rapidly reached with accurate results. However glass electrodes can be used in ph range 0-10 and at higher ph the glass attacked by alkalis. 9.2 Solid Membrane Electrode The active membrane is a single crystal of LaF 3 doped with europium (II) to lower its electrical resistance and facilitate ionic charge transport. The LaF 3 crystal, seated into the end of a rigid plastic tube, is in contact with the internal and external solutions. Typically the internal solution is 0.1M each NaF and NaCl; the fluoride ion activity controls the potential of the inner surface of the LaF 3 membrane, and the chloride ion activity fixes the potential of the internal Ag/AgCl reference electrode the electro chemical cell for this electrode. Ag/AgCl (s), Cl - (0.1M), F - (0.1M)/LaF 3 crystal/test solubility //Reference electrode It obeys Nernst relation of the form E = constant + RT ln [F - ] int /[F - ] ext F Since [F] int is constant, the equation simplifies to E = constant ph. At 25 o C. In solid membrane electrodes, the glass membrane is replaced by an ion conducting membrane. Fluoride electrode is example which is used to measure fluoride ions accurately quickly and economically. It consists of single crystal of LaF 3 as membrane. It contains an internal reference electrode and internal fluoridestandard electrode and LaF 3 ion exchange. Fig. 13. Solid Membrane Electrode Initial filling solution Reference electrode Solid state ionic conductor Engineering Chemistry Page 48

50 , UNIT II ELECTROCHEMISTRY The fluoride electrode also responds to hydroxide ion concentration. Hence, the hydroxide ion concentration is kept constant with buffer solutions. It contains of 0.25M acetic acid, 0.75M sodium acetate, 1M sodium chloride and 1mg sodium citrate. Sodium citrate masks Al 3+ and Fe 3+ which interfere by complexing Fluoride. The buffer controls the overall ionic strength as well as the ph. This electrode is highly useful in environmental studies, fluoride and other ions. 9.3 Liquid Membrane Electrode Liquid membrane or ion exchange electrodes are prepared using an organic liquid ionexchanger which is immiscible with water or with ion sensing material is dissolved in an organic solvent which is immiscible with water. The solvent is placed in a tube sealed at the lower end by a thin hydrophobic membrane such as cellulose acetate paper; aqueous solutions will not penetrate this film. Liquid membrane electrode consists of a double concentric tube arrangement in which the inner tube contains the aqueous reference solution and internal reference electrode. The outer compartment contains organic liquid ion exchanger reservoir which occupies the pores of a hydrophobic membrane. Calcium responsive electrode is an example for the liquid membrane electrode. It contains calcium salts, bis (2-ethyl hexy1) phosphoric acid (d 2 EHP) and dissolved in straight chain alcohols (or) di decyl hydrogen phosphate dissolved in din-octyle phenyl/phosphate. 9.4 Enzyme based electrode Ag/AgCl ref. electrode Internal aq. solution Ion exchange reservoir Liquid internal exchange layer with in porous memberane Porous memberane Fig. 14. Liquid Membrane Electrode These electrodes make use of an enzyme to convert the substance to be determined into an ionic product, which can itself be detected by a known ion selective electrode. A typical example is the urea electrode in which the enzyme urease is employed to hydrolyse urea and the progress of the reaction can be followed by means of a glass electrode, which is sensitive to ammonium ions. The final concentration of ammonium ions determined can be related to the urea present. NH 2 CONH 2 + H 2 O + H + urease + 2NH 4 + CO 2 The enzyme is incorporated in a poly acrylamide gel, which is allowed to set on the bulb of the glass electrode and may be held in position by a nylon gauze. Then the electrode is inserted in to a solution containing urea. Ammonium ions produced, diffuse through the gel and cause a response by the ammonium ion probe. Engineering Chemistry Page 49

51 UNIT II ELECTROCHEMISTRY Gas Sensing Electrode The construction of ammonia sensing glass electrode is shown. Dissolved ammonia from the sample diffuses through a gas permeable fluoro carbon membrane until a reversible equilibrium is established between the ammonia level of the sample and internal filling solution. Hydroxide ions are formed in the internal filling solution by the reaction of ammonia with water. Ref.electrode Internal filling solution Sensing element Memberane Fig. 15. Gas Sensing Electrode NH 3 + H 2 O NH + 4 +OH - The hydroxide ion concentration level of the internal filling solution is measured by the internal sensing element, which is directly proportional to the level of ammonia in the sample Bio-catalytic Membrane Electrode In bio- catalytic membrane electrodes, a bio-catalyst is immobilized at the surface of an electro chemical sensor (membrane electrode). The membrane electrode may be ion-selective (glass, solid (or) polymer) or gas sensing (NH 3, CO 2 (or) H 2 S) electrode. The biocatalysts may be an enzyme, tissue, or bacteria. Bio-catalytic electrode life time is dependent Memberan on the stability of the bio-catalyst, which in turn is dictated by (1) methods of the bio-catalyst immobilization (2) ph of the solution, (3) storage Bulk solution conditions and (4) presence of activators (or) inhibitors. The average reported life time for gas sensing based bio-catalytic membrane electrodes is 28 days. The range of life time is 0.4 to 240 days. Biocatalytic layer The electrodes have excellent selectivity characteristics owing to bio-catalytic specificity. Fig. 16. Biocatalytic Membrane Electrode 10. Batteries A device that stores chemical energy for a release at a desired later stage as electricity is known as cell and a series of such cells in a group is called a battery. They are commercial portable source of electro-chemical cells which supply DC at a constant voltage. They convert chemical energy of a selected reaction into electrical energy. Batteries can be any of the three types Classification Engineering Chemistry Page 50

52 UNIT II ELECTROCHEMISTRY Primary Battery (or Primary cells): These are the cells in which the cell reaction is not reversible. When the reactants have for the most part been converted into products, no more electric current is produced and the battery becomes dead. They can be used as long as the reactants are active and can not be recharged. Eg: Leclanche cell Secondary Battery (or Secondary Cells): Here the cell reaction can be reversed by passing the direct electric current in opposite direction. A secondary battery may be used through a large number of cycles of discharging and charging. Eg; Lead storage cell Flow battery/ Fuel cells: These are the cells in which materials (reactants, products, electrolytes) pass through the battery, which is simply an electrochemical cell that converts chemical to electrical energy. Energy can be withdrawn indefinitely as long as fuel supply is maintained. They do not store energy and cannot be reused. Eg; Hydrogen oxygen fuel cell 10.2 Dry or Leclanche Cell This is a cell is without fluid component and hence is conveniently portable. The anode of the cell is a zinc can, containing an electrolyte consisting of solid NH 4 Cl, ZnCl 2 and MnO 2, to which starch is added to make it thick paste to prevent leakage on transport. A graphite rod acts as a cathode and is immersed in the electrolyte in the centre of the cell as displayed in the figure below: Metal cap (+ ve) Insulating washer collar to keep rod in postion Zinc Cup (negative) Graphite rod Mixture of mangnese (iv) oxide, graphite, ammonium chloride and zinc chloride The cathode reaction is quite complex. At Cathode: Metal cover (negative) Fig. 17. Oxidation- reduction potentiometric titration 2MnO 2 (s) + H 2 + 2e - Mn 2 O 3 (s) + 2OH (aq) NH 4 (aq) + OH (aq) NH 3 (g) + H 2 O (l) ZnCl 2 (s) + 2NH 3 (g) [Zn (NH 3 ) 2 ]Cl 2 (s) 2MnO 2 + 2NH 4 Cl (aq) + 2e - [Zn(NH 3 ) 2 ]Cl 2 (s) + Mn 2 O 3 + H 2 O At Anode: Zn(s) Zn 2+ (aq) + 2e - Net reaction: Zn(s) + 2NH 4 Cl (aq) + 2MnO 2 (s) Mn 2 O 3 (s) + [Zn(NH 3 ) 2 ]Cl 2 (s) + H 2 O Engineering Chemistry Page 51

53 UNIT II ELECTROCHEMISTRY Since various reactions involved cannot be reversed by passing electricity back through the cell, this cell is irreversible and for one time use only. It gives a voltage of about 1.5V, which can be increased further to desired level by connecting such cells in series. Dry cell finds wide variety of applications in flash-lights, transistor radios, calculators. Disadvantages 1. When current is drawn rapidly from it, products build up on the electrodes, thereby causing a drop in the voltage. 2. Since the electrolytic medium is acidic, zinc metal dissolves slowly, thereby cell rundown is slow. 3. Since the reactants are in acidic medium, zinc undergoes corrosion at a faster rate and decreases the life of the cell Alkaline Battery: This is an improved form over the dry cell, in which NH 4 Cl is replaced by KOH as an electrolyte. Zinc in powdered form is mixed with KOH to get a Gel. Graphite anode rod is surrounded by a paste containing MnO 2. The outside body is made with zinc. The cell reactions are Anode: Zn(s) + 2OH - (aq) Zn(OH) 2 (S) + 2e - Cathode: 2MnO 2 (s) + H 2 O(l) + 2e - Mn 2 O 3 (s) + 2OH - (aq) Net reaction: Zn (S) +2MnO 2(S) H 2 O (l) Zn(OH) 2(S) + Mn 2 O 3(S) The main advantages of alkaline cell over dry cell are i) Zinc does not dissolve as readily in a basic medium and hence is not corroded. ii) Alkaline battery maintains a better voltage as the current is drawn from it iii) The life of alkaline battery is longer than dry cell, since there is no corrosion of Zn. Uses: These are used in camera exposure controls, calculators, watches etc Nickel Cadmium Battery: This consists of a cadmium anode and cathode composed of a paste of NiO (OH) (s). The cell reaction is Anode: Cd(s) + 2OH - (aq) Cd(OH) 2 + 2e - Cathode: 2NiO(OH)(s) + 2H 2 O + 2e - 2Ni(OH) 2 (s) + 2OH - (aq) Net reaction: 2NiO(OH)(s) + Cd(s) + 2H 2 O(l) Cd(OH) 2 (s) +2Ni(OH) 2 (s) The reaction can be readily reversed, because the reaction products, Ni(OH) 2(S) and Cd(OH) 2, adhere to the electrode surface. It is portable, rechargeable cell with a fairly long life >20 years with a high amperage and maintains a constant voltage of 1.4 to 1.45 volts. It can be left for long periods of time without any appreciable deterioration, since no gases are produced during discharging or charging. Engineering Chemistry Page 52

54 UNIT II ELECTROCHEMISTRY Uses: They are used in electronic calculators, electronic flash units, cordless electronic shavers, transistors and other battery powered small tools, emergency power supply and military tools Mercury Battery This is a tiny cell used for special medical applications and in advanced electronics. The anode is zinc amalgam and strong alkaline paste containing KOH, Zn(OH) 2 and HgO serves as a cathode. These are enclosed in a steel case, where cathode and anode are separated by a paper driver, which allows the migration of ions. The cell reaction is Anode : Zn amalgam (s) + 2OH - (aq) ZnO (s) + H 2 O (l) + 2e - Cathode: HgO (s) + H 2 O (l) + 2e - Hg(l) + 2OH - (aq) Net reaction: Zn amalgam (s) + HgO (s) ZnO (s) + Hg (l) Since there is no change in the electrolyte composition during its operation and overall reaction involves only solid substances, it provides nearly constant voltage of 1.34V for 95% of its life time. It is more expensive than ordinary dry cell but has specific applications. Uses: It gives excellent performance in heart pacemakers, hearing aids, light meters, digital watches etc Fuel Cells: A fuel cell is an electro-chemical cell which converts the chemical energy of an easily available fuel -oxidant system directly into electricity. The chemical energy is provided by the fuel and the oxidant is stored outside the cell and maintains a continuous supply. The essential process in a fuel cell is Fuel + oxygen Oxidation products + electrical energy Characteristics of a fuel cell They do not store chemical energy. Reactants are to be supplied constantly, while products are removed constantly. A fuel cell resembles an engine than a battery. The efficiency of the fuel cell is about twice that of a conventional power plant for generating electricity. Fuel cell generators are free from noise, vibration, heat transfer thermal pollution and other problems normally associated with conventional power plants. Despite these characteristics, the fuel cells are not yet commercialized in operation. A major problem lies in the choice and availability of suitable catalysts (for electrodes) able to function efficiently for long periods to time without deterioration and contamination H 2 -O 2 Fuel cell: One of the simplest and most successful fuel cell is hydrogen oxygen fuel cell, which consists essentially of an electrolytic solution such as 25% KOH solution and two inert porous electrodes. Hydrogen and oxygen gases are bubbled, through the anode and the cathode compartment. Engineering Chemistry Page 53

55 UNIT II ELECTROCHEMISTRY The following reactions take place. Anode: 2H 2 (g) + 4OH - (aq) 4H 2 O(l) + 4e - Cathode: O 2 (g) + 2H 2 O (l) + 4e - 4OH - (aq) Net reaction: 2H 2 (g) + O 2 (g) 2H 2 O (l) The standard emf of the cell is E 0 = E 0 ox+e 0 Red = = 1.23V. In actual practice, the emf of the cell is 0.8 to 1.0V. The product discharged by the cell is water. Usually, a large number of these cells are stacked together in series to make a battery, called the fuel cell battery or fuel battery. The electrodes must meet the stringent requirements. They must i) be good conductors ii) iii) iv) be good electron sources or sinks not to be consumed or deteriorated by the electrolyte heat or electrode reactions. be excellent catalysts for the reactions that take place on their surface. When hydrogen is used as the fuel, the electrodes are made of either graphite impregnated with finely divided platinum, or a 75:25 alloy of Palladium and Silver or nickel. Electrolytes used are aqueous KOH or H 2 SO 4 or ion-exchange resin saturated with water. For low temperature operating fuel battery (-54 o C to 72 o C), potassium thiocyanate dissolved in liquid ammonia is employed. Applications: Hydrogen Oxygen fuel cells are used as auxiliary energy source in space vehicles, submarines and other military vehicles. It has light weight and the product out come is useful water. The initial cost is high but the maintenance cost is low Other fuel cells In addition to H 2 /O 2 system, a number of other fuel cells are also developed. i) Propane oxygen fuel Cell ii) The half reactions are Anode: C 3 H 8 (g) + 6H 2 O (l) 3CO 2 (g) + 2OH + + 2Oe - Cathode: 5O 2 (g) + 2OH + (aq) + 2Oe - 10H 2 O (l) Net reaction: C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O (l) Methyl alcohol oxygen fuel Cell The half reactions are Anode: CH 3 OH (l) + H 2 O (l) CO 2 (g) + 6H + (aq) + 6e - Cathode: 3 2 O 2 (g) + 6H + + 6e - 3H 2 O (l) Net reaction: CH 3 OH (l) O 2 (g) CO 2 (g) + 2H 2 O (l) Engineering Chemistry Page 54

56 UNIT II ELECTROCHEMISTRY Assignment Questions 1. (a) Define transport number and ionic mobilities (b) State and explain Kohlrausch law (c) Discuss the applications of Kohlaursch law. 2. (a) Define and explain Specific conductance, equivalent conductance and molar conductance. (b) Show that the degree of dissociation of a weak acid is reciprocal of the square root its concentration. 3. Discuss in detail about various Conductometric titrations. 4. What is a galvanic cell? Explain its construction, working and applications 5. Define electrode potential. Derive an expression for the determination of electrode potential. 6. What is electrochemical series? Derive an expression for Nernst Equation. 7. Discuss in detail about Potentiometric titrations. 8. What are ion selective electrodes? Explain the construction working and applications of (a) NHE (b) calomel electrode (c) glass membrane electrode (d) Ag-AgCl electrode. 9. (a) What is a battery? Explain the classification of batteries (b) Explain the construction and working of Leclanche cell and Ni-Cd battery (c) Explain in detail about concentration cells 10. What is a fuel cell? Explain the construction and working of Hydrogen-Oxygen fuel cell and Methanol Oxygen fuel cell. Engineering Chemistry Page 55

57 UNIT-III MATERIAL CORROSION NOTE CORROSION Syllabus: Causes and effects of corrosion Theories of corrosion (dry, chemical and electrochemical corrosion) Factors effecting corrosion Corrosion control methods Cathode protection Sacrificial anodic, impressed current methods Surface coatings Methods of application on metals (Hot dipping, galvanizing, tinning, cladding, electroplating, electro less plating) Organic surface coatings Paints Their constituents and their functions. Objectives: The problems associated with corrosion are well known and the engineers must be aware of these problems and also how to counter them OUTLINES Introduction Theories of corrosion Galvanic series Types of corrosion Factors influencing corrosion Corrosion control methods Protective coatings Constituents of paints and their functions Engineering Chemistry Page 54

58 UNIT-III MATERIAL CORROSION NOTE Introduction: The phenomenon of deterioration and destruction of matter by unwanted, unintentional attack of the environment leading to loss of matter starting at its surface is called corrosion. Examples are rusting of iron, formation of mill scales, tarnishing of silver, formation of a green film of basic carbonate (CuCO 3.Cu (OH) 2 ) on the surface of copper etc. The basic reason for corrosion is that metals are more stable as their minerals/compounds than in pure state with few exceptions like gold etc. Corrosion is a challenge for engineering materials due to enormous loss of material in corrosion. 2. Theories of corrosion- types Corrosion is broadly classified into two types. 1. Dry or chemical corrosion 2. Wet or electrochemical corrosion 2.1 Dry or chemical corrosion This type of corrosion takes place by the direct attack of gases present in atmosphere such as O 2, CO 2, H 2 S, SO 2, halogens, etc., with metal surfaces in the immediate vicinity. Dry corrosion is classified into three types. i) Oxidation corrosion ii) iii) Corrosion by other gases Liquid metal corrosion Oxidation corrosion: This is brought about by the direct action of oxygen on the metal surface in the absence of moisture. The oxygen atoms of the air are held close to the surface by means of weak Vander waal forces. Over a period of time, these forces results in the formation of weak bonds converting the metal into its corresponding metal oxide. The phenomenon is known as chemisorption. The following reactions are involved in oxidation corrosion. 2 M Mn ne - (Loss of electrons) (Oxidation) n O 2 + ne- 2 no - 2 (Gain of electrons) (Reduction) 2 M + n 2 O 2 2 Mn no 2 Engineering Chemistry Page 55

59 UNIT-III MATERIAL CORROSION NOTE Mechanism: Oxidation occurs at the surface of the metal first and forms a layer of deposit (oxide) that tends to restrict further oxidation. The nature of the oxide film formed plays an important role on the surface of the metal as it may be stable, unstable, volatile and porous. If a stable layer is formed on the surface, such a product prevents the exposure of the metal for further corrosion. If unstable oxidation product is formed, the product decomposes readily and may allow further corrosion. If the product formed is volatile in nature, it readily volatilizes, leaving behind fresh metal surface. This leads to rapid and excessive corrosion. Ex: Molybdenum oxide MoO 3 It a porous product is formed, an unobstructed and uninterrupted oxidation corrosion reaction takes place Pilling Bedworth Rule: According to this, an oxide product is protective or non-porous, if the volume of oxide is at least as great as the volume of metal from which it is formed. On the other hand, if the volume of oxide formed is less than the volume of the metal, the oxide layer is porous and nonprotective. Thus smaller is the specific volume ratio (Volume of metal oxide/volume of the metal), greater is the oxidation corrosion. Ex: Alkali& alkaline earth metals (Li, K, Na, and Mg) form oxides having volume less than the volume of metals. While Al forms oxides which is non-porous and protective. The specific volume ratios of Ni, Cr and W are 1.6, 2.0 and 3.6 respectively. Hence, the rate of oxidation of tungsten (W) is least, even at elevated temperatures. Engineering Chemistry Page 56

60 UNIT-III MATERIAL CORROSION NOTE Corrosion by other gases: This type of corrosion takes place by the chemical affinity of gases such as SO 2, CO 2, Cl 2, H 2 S, and F2 etc. The degree of attack depends upon the formation of protective or nonprotective films on the metal surface. Example, AgCl forms the protective films. SnCl 4 forms a volatile product, while attack of Fe by H 2 S gas forms a porous FeS film Liquid metal corrosion: This type of corrosion takes place due to chemical action of a flowing liquid metal on another solid metal surface or an alloy. Such corrosion occurs in devices used for nuclear power. The corrosion involves either dissolution of solid metal by a liquid metal or internal penetration of liquid metal into solid metal, which weaken the solid metal Wet corrosion This type of corrosion occurs when a conducting liquid is in contact with metal or when two dissimilar metals or alloys are either immersed or dipped partially in a solution. It involves the formation of two areas of different potentials in contact with a conducting liquid. One is named as anodic area where oxidation reaction takes place, the other is referred to as a cathodic area involving reduction. The metal at anodic area is destroyed either by dissolving or by forming a combined state, such as oxides. Hence corrosion always occurs at anodic areas. At cathode, the dissolved constituents gain the electrons forming non-metallic ions. The metallic ions and non-metallic ions diffuse towards each other forming product somewhere between anode and cathode Mechanism of wet or electro chemical corrosion: Electro chemical corrosion involves flow of electric current between anodic and cathodic areas. At anode, dissolution of metal takes place forming corresponding metallic ions. M M n+ + ne - On the other hand, at cathode, consumption of electrons takes place either by i) Evolution of hydrogen type ii) Absorption of oxygen type i) Evolution of hydrogen type: This type of corrosion occurs if the conducting medium is acidic in nature. For example, Iron dissolves and forms ferrous ions with the liberation of electrons. These electrons flow from anode to cathode, where H + ions are eliminated as hydrogen gas. Fe Fe e (Oxidation) 2 H e - H 2 (Reduction) Engineering Chemistry Page 57

61 UNIT-III MATERIAL CORROSION NOTE Fe + 2 H + Fe 2+ + H 2 ii) Absorption of oxygen type: A cathodic reaction can be absorption of oxygen, if the conducting liquid is neutral or aqueous and sufficiently aerated. Some cracks developed in iron oxide film cause this type of corrosion. The surface of iron is always coated with a thin oxide film. The crack developed will create an anodic area on the surface while the well coated metal parts act as cathode. The anodic areas are small and the cathodic areas are large. Corrosion occurs at the anode and rust occurs in between anode and cathodic areas. When the amount of oxygen increases corrosion is accelerated. ½ O 2 + H 2 O + 2 e - 2OH - (Reduction) The Fe 2+ ions formed at anode, and OH - ions formed at cathode, diffuse towards each other forming Fe (OH) 2 i.e., Fe OH - Fe(OH) 2 If enough oxygen is present, the Fe (OH) 2 is oxidized further to Fe(OH) 3. This eventually is converted in to rust [Fe 2 O 3 x.h 2 O] Difference between chemical Corrosion and electrochemical corrosion Chemical Corrosion Electrochemical Corrosion Engineering Chemistry Page 58

62 UNIT-III MATERIAL CORROSION NOTE It takes place in dry condition 2. It involves the direct chemical attack of environment of the metal. 3. It takes place on homogeneous and heterogeneous surfaces. 4. Corrosion product accumulates at the same place where corrosion is taking place. 5. Uniform corrosion takes place. 1. It takes place in wet condition such as in the presence of electrolytes. 2. It involves the formation of large number of galvanic cells. 3. It takes place on heterogeneous surfaces only. 4. Corrosion product accumulates at cathode, but corrosion takes place at anode. 5. Non Uniform corrosion takes place. 3. Galvanic series In the electrochemical series the elements are arranged in the increasing order of their reduction potential values. Galvanic series or electrochemical series is an arrangement of metals in the increasing order of their reduction potentials. The metals with more anodic character occupy the top positions in the series whereas the bottom positions are occupied by more cathodic metals. A metal top in the series is more anodic and undergoes corrosion faster than the metal below in the series. Examples: Mg, Zn, Al, Cd, Duralumin, steel, lead tin (solder), Pb, Sn, Cu and its alloys, Cupro Nickel, bronze, passive stainless steel, Ag, Ti, Graphite, Au, Pt. The noble character increases down this series. Active (or anodic) Noble (or cathodic) 1. Mg 2. Mg alloys 3. Zn 4. Al 5. Cd 6. Al alloys 7. Mild steel 8. Cast iron 9. High Ni Cast iron 10. Pb-Sn Solder 11. Pb 12. Sn 13. Lconel 14. Ni-Mo-Fe alloys 15. Brasses 16. Monel 17. Silver solder 18. Cu 19. Ni 20. Cr stainless steel stainless steel Mo stainless steel 23. Ag 24. Ti 25. Graphite 26. Au 27. Pt Although electrochemical series gives useful information regarding the chemical reactivity of metal it does not predict the corrosion behaviour of the metal several side reactions may take place which influence the corrosion reaction hence oxidation potentials of various metals and alloys are determined with SCE, immersing the metal and alloys in sea water when these oxidation potentials are arranged in the decreasing order of their activity the galvanic series arises. Engineering Chemistry Page 59

63 UNIT-III MATERIAL CORROSION NOTE Differences between electrochemical series and galvanic series: Galvanic series Electrochemical series 1. This series was developed by the study 1. This was developed by dipping of corrosion of metals and alloys in sea water without their oxide film. pure metals in their 1M salt solution 2. The position of the given metal may shift 2. The position of the metal is fixed 3. The corrosion of alloys can be studied 3. No information regarding alloys. from the series. 4. The position of a metal is different from that 4. The position of the metal is fixed. of the position of the alloy which contains the same metal in it. 5. The series predicts relative corrosion nature 5. The series predicts relative displacement nature. 6. The series comprises metals & alloys 6. This comprises metals & non-metals. 4. Types of corrosion 1. Galvanic corrosion 2. Concentration cell corrosion 3. Pitting corrosion 4. Waterline corrosion 5. Stress corrosion 6. Microbial corrosion 7. Intergranular corrosion 4.1. Galvanic corrosion When two dissimilar metals are electrically connected and exposed to an electrolyte, the metals higher in electrochemical series have a tendency of forming anode and undergo corrosion. For example, when zinc and copper are electrically connected either in acidic solutions or in their respective salt solution, zinc being more anodic by virtue of its position in electro chemical series, forms anode and copper automatically becomes cathode. Ex: Steel screws in a brass marine hardware, steel pipe connected to copper etc. Engineering Chemistry Page 60

64 UNIT-III MATERIAL CORROSION NOTE ,,,, 4.2. Concentration cell corrosion: This type of corrosion takes place, when a metal surface is exposed to an electrolyte of varying concentrations or varying aerations. The poorly oxygenated parts are more prone to become anodic areas. For example, when a zinc rod is partially immersed in neutral salt solution, the metal above the water line is more oxygenated, while the portion that is immersed has smaller oxygen concentration and thus become anodic. Hence a potential difference is created, which causes the flow of current between two differentially aerated areas of same metal. Zn Zn e - ½ O 2 + H 2 O + 2e - 2 OH - (Oxidation) (Reduction) The circuit is completed by migration of ions through the electrolyte and flow of electrons through the metal from anode to cathode Pitting corrosion It is defined as intense, localized, accelerated attack resulting in the formation of a pinholes, pits and cavities on the metal surface. Such a type of corrosion takes place when there is a breakdown, peeling or cracking of a protective film due to scratches, abrading action, sliding under load etc. Engineering Chemistry Page 61

65 UNIT-III MATERIAL CORROSION NOTE Waterline corrosion: When water is stored in a container or a steel tank, it is generally found that most of the corrosion takes place just beneath the line of water level. The area above waterline is highly oxygenated and acts as cathode, while the area just beneath the waterline is poorly oxygenated and becomes anodic site. This type of corrosion is also a consequence of differential aeration Stress corrosion: It is a combined effect of static tensile stress and the corrosive environment on a metal. An important example of this type of stress corrosion is caustic embrittlement. Corrosion due to caustic embrittlement A high pressure boiler is used for generation of steam. The water used for steam generation, usually contains small quantities of Na 2 CO 3, which decomposes to give caustic NaOH and liberate CO 2. Na 2 CO 3 + H 2 O 2 NaOH + CO 2 This makes the water alkaline and NaOH thus formed flows into minute air cracks and crevices present on the boiler surface and get deposited as caustic soda. NaOH thus deposited dissolves iron as sodium ferroate (Na 2 FeO 2 ) in cracks and crevices, where the metal is stressed. The sodium ferroate further decomposes giving Fe 3 O 4 (magnetite) with regeneration of NaOH, thereby enhancing further dissociation of Iron. 3 Na 2 FeO H 2 O 6 NaOH + Fe 3 O 4 + H 2 6 Na 2 FeO H 2 O + O 2 12 NaOH + 2 Fe 2 O 4 Caustic embrittlement can also be represented by means of an electro chemical equation. Engineering Chemistry Page 62

66 UNIT-III MATERIAL CORROSION NOTE Iron Conc. NaOH dil NaOH Iron - The caustic embrittlement can be prevented by adding tannin or lignin to the boiler water or by using Na 2 SO 4 in place of Na 2 CO 3 for water treatment Microbial corrosion: Metals undergo corrosion due to microbial action both in aerobic and anaerobic conditions. There are mainly 4 types of microbes which cause corrosion in nature. a) Sulphate reducing bacteria (Sporovobrio desulphuricous) b) Sulphur bacteria (Thioracillus) c) Iron and manganese bacteria d) Film forming bacteria. a) Sulphate reducing bacteria: This bacteria as a part of its metabolic activity, takes sulphates present in the soil along with water and air. The typical equations involving corrosion by sulphate reducing bacteria are given below. Anodic solution of iron 8 H 2 O 8 H OH - 4 Fe + 8 H + 4 Fe H Depolarization, due to activity of bacteria H 2 SO H H 2 S + 4 H 2 O Corrosion products Fe 2+ + H 2 S FeS + 2 H + 3 Fe OH - 3 Fe(OH) 2 b) Sulphur bacteria (Thioracillus): This is a kind of bacteria which has sulphur present in its cell, which as a part of metabolic activity, picks up the oxygen and moisture present in the soil and excrete sulphates making the soil acidic. This eventually leads to corrosion of buried metals. c) Iron and manganese bacteria: These bacteria consume Iron and Manganese deposits directly and digest them converting them into sulphides and hydroxides at optimum conditions of o C and ph 5-9. e) Film forming bacteria: These are usually algae and fungi which form a thin film on the surface of the metal accommodating the accumulation of dust, moisture leading to formation of differential concentration or differential aerations cell Intergranular corrosion Engineering Chemistry Page 63

67 UNIT-III MATERIAL CORROSION NOTE All solids have grain structures which when exposed to corrosive environment undergo corrosion because of formation of potential zones of areas within the crystal lattice. During crystallization of the metal, the impurities present in the materials get accumulated near the boundaries of grains, while the pure form of metal occupies the grain proper. This leads to the formation of two areas of different potentials, which makes the corrosion current to flow from the active grain boundary (anode) towards grain proper (cathode). Such a material when it is exposed to corrosive environment, grain boundaries are attacked readily causing corrosion Passivation: The phenomenon in which a metal exhibits extra corrosion resistance than that is expected from its position in the electro chemical series or galvanic series is known as passivation. This extra resistance towards corrosion is obtained due to formation of a very thin film of oxide layer ( mm of thickness). This thin film is non-porous, highly protective and of self-healing nature Soil corrosion: This type of corrosion depends on the presence of Salts, moisture, ph, bacteria, aeration and texture of the soil. Based on the texture of the soil, soils are of three types Graveled or sandy soil: These are loose soils having sufficient aeration. When an iron rod is buried in such a soil, it gets corroded because of undergoing differential aeration corrosion. The severity of corrosion also depends upon the type of product and the salts present Water logged soils: Corrosion of metals in waterlogged soils takes place due to microbial action following a wet mechanism Intermediate soils: These types of soils have gravel or sandy clay like matter along with moisture. Corrosion of metals occurs as a consequence of both differential aeration and microbial attack Erosion corrosion: It is caused by the combined effect of the abrading action of turbulent flow of gases, vapours and liquids and mechanical action of solids over a metal surface. 5. Factors influencing corrosion The rate and extent of corrosion, depends on the following characteristics i) Metal based factors ii) Environment based factors Engineering Chemistry Page 64

68 UNIT-III CORROSION Metal based factors a) Position in the galvanic series: When two metals or alloys are in electrical contact, in presence of an electrolyte, the more active metal (or higher up in the series) suffers corrosion. The rate and severity of corrosion depends upon the difference in their positions and greater is the difference, the faster is the corrosion of anodic metal/alloy. b) Over voltage: When a Zn rod (high in position in galvanic series) is placed in 1N H 2 SO 4, it undergoes corrosion forming a film and evolving hydrogen gas. The initial rate of corrosion is slow, because of over voltage (0.7V). However, if few drops of CuSO 4 are added, the corrosion rate of Zn is accelerated, as Cu gets deposited on Zn metal, there by the over voltage is reduced to 0.33V. The reduction is over voltage of the corroding metal/alloy accelerates the corrosion rate. c) Relative areas of cathodic and anodic parts: When two dissimilar metals or alloys are in contact, the corrosion of the anodic part is directly proportional to the ratio of areas of the cathodic part and the anodic part. Corrosion is more rapid, severe and highly localized, if the anodic area is small, because the current density at a smaller anodic area is much greater, and the demand for electrons (large cathodic area) can be met by smaller anodic areas only by undergoing corrosion more briskly. d) Purity of the metal: Impurities in a metal, cause heterogeneity, and forming electrochemical cells (at exposed parts) and the anodic part gets corroded. Example, Zinc metal containing Pb or Fe as impurity gets corroded. The rate and extent of corrosion increases with the increase in exposure and the extent of the impurities present. Corrosion resistance of a metal is increased by increasing its purity. f) Physical state of the metal The rate of corrosion is influenced by physical state of metal. The smaller the grain size of the metal or alloy, the greater will be its solubility and hence, greater will be its corrosion Environment based factors a) Temperature: With increase of temperature of environment, the reaction as well as diffusion rate increases, thereby corrosion rate is generally enhanced. b) Humidity of air: It is the deciding factor in atmospheric corrosion. Critical humidity is defined as the relative humidity above which the atmospheric corrosion rate of metal increases sharply. The corrosion of metal becomes faster in humid atmosphere, since the gases (CO 2, O 2, etc) and water vapour present in atmosphere furnish water to the electrolyte leading to the setting up of an electrochemical cell. Engineering Chemistry Page 65

69 UNIT-III CORROSION c) Presence of impurities in atmosphere: Atmosphere in the industrial areas contains corrosive gases like CO 2, H 2 S, SO 2 and fumes of HCl, H 2 SO 4 etc. In the presence of these gases and water vapour present, the acidity of the liquid, adjacent to the metal surface increases and electrical conductivity also increases. Consequently, the corrosion increases. d) Influence of ph: Generally, acidic media are more corrosive than alkaline and neutral media. Amphoteric metals (Al, Pb) dissolve in alkaline solutions as complex ions. For example, corrosion of Fe is slow in oxygen free water, but is increased due to the presence of oxygen. Corrosion of metals, readily attacked by acid, can be reduced by increasing the ph of the attacking environment. 6. Corrosion control (Protection against corrosion) Some of the corrosion control methods are described as follows Proper designing: The design of the material should be such that corrosion, even if it occurs, is uniform and does not result in intense and localized corrosion. Important design principles are: Avoid the contact of dissimilar metals in the presence of a corroding solution, otherwise the corrosion is localized on the more active metal and less active metal remains protected. a. When two dissimilar metals are to be in contact, the anodic material should have as large area as possible; whereas the cathodic metal should have as much smaller area as possible. b. If two dissimilar metals in contact have to be used, they should be as close as possible to each other in the electro chemical series. c. Whenever the direct joining of dissimilar metals is unavoidable, an insulating fitting may be applied in between them to avoid the direct metal to metal contact. d. The anodic metal should not be painted or coated, when in contact with a dissimilar cathodic metal. e. A proper design should avoid the presence of crevices between adjacent parts of structure, even in case of the same metal, since crevices permit concentration differences. f. Sharp corners and recesses should be avoided, as they are favorable for the formation of stagnant areas and accumulation of solids. Engineering Chemistry Page 66

70 UNIT-III CORROSION g. The equipment should be supported on legs to allow free circulation of air and prevent the formation of stagnant pools or damp areas Use of pure metal: Impurities in a metal cause heterogeneity, which decrease corrosion resistance of the metal. Hence corrosion resistance of any metal is improved by increasing its purity. Ex: Al, Mg. Ex: the corrosion resistance of Al depends on its oxide film formation, which is highly protective only on the high purity metal Using metal alloys: Corrosion resistance of most metals is best increased by alloying them with suitable elements. For maximum corrosion resistance, the alloy should be completely homogeneous Cathodic protection: The principle involved here is to force the metal to be protected as to behave like a cathode. There are two types of cathodic protections. i) Sacrificial anodic protection method: The metallic structure to be protected is connected by a wire to the more anodic metal, so that active metal itself get corroded slowly, while the parent structure is protected. The more active metal is called sacrificial anode, which must be replaced, when consumed completely. Metals commonly used as sacrificial anodes are Mg & Zn. Engineering Chemistry Page 67

71 UNIT-III CORROSION ii) Impressed current cathodic protection: An impressed current is applied in opposite direction to nullify the corrosion current, and convert the corroding metal from anode to cathode. Usually a sufficient D.C. is applied to an insoluble anode, buried in the soil and connected to the metallic structure to be protected (Fig. 16.). The anode is usually in a backfill (composed of cock breeze or gypsum), so as increase the electrical contact with the surrounding soil. This kind of protection technique is useful for large structures for long term operations Use of inhibitors: A corrosion inhibitor is a substance when added in small quantities to the aqueous corrosive environment, effectively decreases the corrosion of the metal. i) Anodic inhibitors: Anodic inhibitors stop the corrosion reaction, occurring at anode, by forming a precipitate with a newly produced metal ion. These are adsorbed on the metal surface in the form of a protective film or barrier. Examples are chromates, phosphates, tungstates and other transition metals with high oxygen content. ii) Cathodic inhibitors: In acidic solutions, the main cathodic reaction is evolution of hydrogen. a) 2H + (aq) + 2e - H 2 (g) Corrosion may be reduced either by slowing down the diffusion of hydrated H + ions to the cathode and/or by increasing the over voltage of hydrogen evolution. Engineering Chemistry Page 68

72 UNIT-III CORROSION The diffusion of H + ions is considerably decreased by organic inhibitors like amines, mercaptans, heterocyclic nitrogen compounds, substituted urea and thiourea, heavy metal soaps, which are capable of being adsorbed at metal surfaces. b) In neutral solutions, the cathodic reaction is H 2 O O 2 + 2e - 2 OH - (aq) Corrosion is controlled either by eliminating oxygen from the corroding medium or by retarding its diffusion to the cathodic areas. The oxygen is eliminated either by reducing agents (like Na 2 SO 3 ) or by de-aeration. The inhibitors like Mg, Zn or Ni salts tend to retard the diffusion of OH - ions to cathodic areas. 7. Protective coatings It is the oldest of the common procedures for corrosion prevention. A coated surface isolates the underlying metal from the corroding environment. i) The coating applied must be chemically inert to the environment under particular conditions of temperature and pressure. ii) The coatings must prevent the penetration of the environment to the material, which they protect. There are mainly three types of protective coatings a) Metallic coatings: b) Inorganic coatings (chemical conversion) ; c) Organic coatings (paints etc.,) 7.1. Metallic coatings: A metal is coated on the other metal, in order to prevent corrosion. These are of two types a) Anodic coatings: These are produced from coating-metals, which are anodic to the base metal. This provides the complete protection to the underlying base metal as long as the coating intact. However, the formation of the pores or cracks on the protective layer can set up severe galvanic corrosion leading to complete destruction of the base metal. E.g.: In case of galvanized steel, zinc, the coating-metal being anodic is attacked; leaving the underlying cathodic metal (iron) unattacked (Figure 17 ) Engineering Chemistry Page 69

73 UNIT-III CORROSION b) Cathodic coatings: These are obtained by coating a more noble metal having higher electrode potential than the base metal. The cathodic coating provides effective protection to the base metal only when they are completely continuous and free from pores, breaks or discontinuities. An example of cathodic coating is Tinning, coating of tin on iron (Figure 18 ) Methods of application of metallic coatings: a) Hot dipping: It is used for producing a coating of low-melting metal such as Zn, Sn, Pb, Al etc. on iron, steel and copper, which have relatively higher melting points. The process consists of immersing the base metal in a bath of the molten coating metal, covered by a molten flux layer (usually ZnCl 2 ). The flux cleans the base metal surface and prevents the oxidation of the molten coating metal. For good adhesion, the base metal surface must be very clean; otherwise it cannot be properly wetted by the molten metal. The two most widely applied hot dipping methods are: i) Galvanizing and ii) Tinning i) Galvanizing: It the process of coating iron or steel sheets with a thin coat of metallic zinc to prevent the sheets from rusting. (Figure. 19 ) Engineering Chemistry Page 70

74 UNIT-III CORROSION The base metal sheet of iron or steel is cleaned by acid pickling method with dilute sulphuric acid at C, washed and dried. It is then dipped in a bath of molten zinc and after taking out of bath it is passed between hot rollers to remove excess zinc and annealed (slow cooling). Galvanized utensils cannot be used for storing foods as zinc dissolves and forms toxic substances. ii) Tinning: The process of coating metallic tin over the iron or steel articles (Figure. 20) is called tinning. The surface the base metal i.e., iron sheet is cleaned by acid pickling with dilute sulphuric acid and passed through a bath of zinc chloride flux. The flue helps the molten metal to adhere to the iron metal sheet surface. Then the sheet is passed through the molten tin bath and pressed between two rollers with a layer of palm oil. The oil will help to protect the tin coated layer from any oxidation. The rollers also remove excess tin and produce a thin film of coating with uniform concentration. The tinned metal possesses good resistance against atmospheric corrosion and tin is nontoxic. Hence such containers can be safely used for storing food material. Comparison between galvanization and tinning Engineering Chemistry Page 71

75 UNIT-III CORROSION Galvanization Tinning 1. Coating of iron with zinc to prevent 1. Coating is done with tin corrosion 2. It protects the metal sacrificially 2. Protection is due to noble character of tin 1. Protection continues even if the coating is broken 4. Food materials cannot be stored in zinc coated containers as zinc easily dissolves in acid food stuffs and converts into toxic compounds. 5. The galvanized sheet is subjected to the process of annealing 6. Galvanized articles are good engineering meterials 3. Protection is provided only when coating is continuous 2. Tin Coating is non-toxic. So food items can be stored. 3. No annealing is necessary. 6. Tinned articles are used only for storing food c) Electro plating: The process of depositing or coating a metal on the surface of base metal/ non metal by electrolysis is called electro plating. It is widely adopted to coat base metals with protective metallic coatings of Cu, Ni, Zn, Pb, Sn, Au and Ag. Process: The metal surface is cleaned thoroughly. The article to be electroplated is made as cathode. The anode is made of pure metal, which is to be coated on the article. The electrolyte is the salt of the metal to be coated on the article. A direct current is passed through the electrolyte. The anode dissolves, depositing the metal ions from the solution on the article at cathode in the form of a fine thin metallic coating. Ex: Electroplating of gold: Cathode: Article to be electroplated Anode: A block of gold metal Electrolyte: Aqueous solution of AuCl 3 or potassium auro-cyanide K[Au(CN) 2 ] Factors affecting electroplating: Cleaning of the article is essential for strong adherent electroplating. Concentration of the electrolyte is a major factor in electroplating. Low concentration of metal in ions produces uniform, coherent metal deposition. Thickness of the deposit should be minimized order to get a strong adherent coating. Additives such as glue, boric acid etc. should be added to the electrolyte bath to get a strong adherent and smooth coating. Engineering Chemistry Page 72

76 UNIT-III CORROSION Battery Gold (Anode) Cathode AuCl 3 or K[Au(CN) 3 ] The electrolyte selected should be highly soluble and should not undergo any chemical reaction. ph of the electrolytic bath must be properly maintained to get the deposition effectively. Applications: It is widely used technique in industries and consumer goods. It can be used for both metals and non metals. In metals it prevents corrosion and in non metals it increases the strength d) Electro less plating: The deposition of a metal form its salt solution on catalytically active surface by a suitable reducing agent without use of electrical energy is called electro less plating or chemical plating. The metal ions are reduced to the metal which gets plated over the catalytic surface the metal surface is treated with acid (etching) and treated with reducing agent like formaldehyde. Heat treatment may be adopted. Electro less plating can be done for on conducting surfaces like plastic or printed circuit boards. Some times complexing agents stabilizers and buffer solutions may also be necessary this technique is widely used in electronic decorative equipment, automobile industry etc., e) Metal Cladding: It is the process by which a dense, homogeneous layer of coating metal is bonded (cladded) firmly and permanently to the base metal on one or both sides. The choice of the cladding metal depends on the corrosion resistance required for any particular environment. Engineering Chemistry Page 73

77 UNIT-III CORROSION Here, the metal to be protected is sandwiched between the two layers of the protecting metal. The whole combination is pressed by rollers under the action of heat and pressure. The cladding materials generally used are corrosion resistant. Examples: Al (Aluminium) Figure. 21) Ni, Cu, Pb, Ag etc. This method is widely adopted in air craft and automobile industry. f) Metal spraying: In this method, the molten metal is sprayed on the cleaned base metal with the help of a spraying gun. The metal surface must be rough. The metal to be sprayed in molten state is fed through a central barrel. A gaseous mixture (oxy acetylene) passing through a tube around the barrel burns at the orifice to melt the wire. The molten metal is then projected against the surface to be coated. This method is limited to low melting metals like Zn, Pb, Sn etc. Non metallic articles like glass, plastic and wood are also coated. g) Powder metal method: Here, finely divided powdered metal is sucked from the powder chamber and then heated, as it passes through the flame of the blow pipe. The blow-pipe disintegrates the metal into a cloud of molten globules, which are then adsorbed on the base metal surface. This method is limited to low-melting metals like Zn, Pb, Sn etc. This can be applied to fabricated structure and there is no possibility of damage Chemical Conversion Coating: These are inorganic surface barriers, produced by chemical or electro chemical reactions, brought at the surface of the base metal. Such coatings are particularly used as an excellent base for paints, lacquers, oils and enamels Phosphate coating: It is a conversion coating consisting of an insoluble crystalline metalphosphate salt formed in a chemical reaction between the substrate metal iron and phosphoric acid solution containing ions of metals (Zn, Fe or Mn) The reaction for the formation of zinc phosphate coating on the surface of base metal iron may be represented as Zn (H 2 PO 4 ) 2 + Fe + 4H 2 O Zn 3 (PO 4 ) 2. 4 H 2 O + Fe HPO 4 + H 3 PO 4 + H 2 Base metal Coating Engineering Chemistry Page 74