Hardness of Water. (c) Dr. Payal Joshi

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Ophelia Short
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3 Hardness of Water Water containing dissolved salts of Ca and Mg is called hard water Hard water prevents lathering of soap Ca 2+ & Mg 2+ ions react with soaps which are salts of fatty acids to give insoluble scums or precipitates This property or tendency hardness of water

4 2C 17 H 35 COO - Na + + Ca 2+ (C 17 H 35 COO) 2 Ca +2Na + (scum) 2C 17 H 35 COO - Na + + Mg 2+ (C 17 H 35 COO) 2 Mg (scum) +2Na + Types of Hardness Temporary/ Alkaline/Carbonate Permanent/ Nonalkaline/Non-Carbonate

5 Other metal ions like Fe 2+, Mn 2+, Al 3+ also react with soap in a similar manner, thus contributing to water hardness However, for practical purposes only Ca 2+ and Mg 2+ content is considered for deciding water hardness

6 Temporary Hardness Due to dissolved bicarbonates of Ca & Mg in water It is so called since it can be easily removed simply by boiling Heat decomposes bicarbonates of Ca & Mg as follows

7 Permanent Hardness Due to dissolved sulfates, chlorides, nitrates of Ca & Mg in water It can be eliminated by special chemical methods Temporary & Permanent hardness added together gives Total hardness Expression of Hardness Dissolved Ca & Mg salts calculated as Calcium carbonate equivalent (mg CaCO 3 equivalent)

8 Classify the following impurities into temporary, permanent & non-hardness causing impurities Impurities Ca(HCO3)2 MgSO4 CaCl2 CO2 HCl Mg(HCO3)2 CaSO4 NaCl Type of hardness Temporary Permanent Permanent Non-hardness Non-hardness Temporary Permanent Non-hardness

9 Hardness of water in terms of Degree Clark ( 0 Cl) Cl 1 ppm Hardness of water in terms of Degree French ( 0 Fr) Fr 1 ppm Hardness of water in terms of mg/lit 1 mg/lit 1 ppm

10 A water sample has a hardness of 807 ppm. Find hardness in terms of 0 Cl (Degree Clark) and 0 Fr (Degree French) a) Hardness in terms of 0 Cl Since, Cl = 1 ppm Thus, 807 ppm 807 x 0.07 = Cl 1 b) Hardness in terms of 0 Fr Since, Fr 1 ppm Thus, 807 ppm 807 x 0.1 = Fr 1

11 Conversion to mg CaCO 3 equivalent = Mass of hardness producing substance x Eq.wt of CaCO 3 Eq.wt of hardness producing substance

12 Disadvantages of using hard water Domestic use: Washing : no lather formation, wastage of soap Bathing: no lather formation. Also the resulting ppt sticks on body Cooking: due to dissolved salts, boiling point of water is elevated, causing wastage of time & fuel. Drinking: bad effect on metabolic system. Calcium oxalate stones may develop in urinary tracts, if used regularly.

13 Industrial use: Textiles: loss of soap during washing of yarn, & fabrics. Ppt of Ca & Mg sticks on fabric. Fe, Mn salts leave colored spots on fabrics Sugar: crystallization of sugar is affected due to presence of sulfate ions. Dyeing: Dissolved Ca, Mg, Fe salts react with dye forming undesirable ppts giving poor shades of color on the fabric. Pharmaceuticals: If used in drug, injections, ointments results in undesirable products in them causing ill health

14 Determination of hardness by EDTA Method Determining total amount of Ca 2+ & Mg 2+ ions in water Titrating sample with standard EDTA solution using an organic dye indicator EBT (Eriochrome Black T) EDTA is a weak acid with the structure given below (a) Structure of ethylene diamine tetraacetic acid, EDTA (b) Structure of tetracarboxylate ion, [EDTA] 4, formed by dissociation of EDTA

15 NH 4 Cl+NH 4 OH Mg 2+ + EBT (blue) > [Mg EBT] ph = 10 [Mg 2+ Titration ---EBT] (wine red unstable complex) ---> [Mg EDTA] EDTA (stable)+ EBT (blue) Structure of [Mg EDTA] 2 chelate (1:1 complex), known as chelates Complexation Reaction

16 Preparation of Reagents Standard Hard Water: 1 gm pure dry CaCO 3 dissolved in min.quantity of dil.hcl. Solution evaporated to dryness on water bath & residue left is dissolved in distilled water & solution is diluted to 1 lit=> Hardness of solution= 1 mg of CaCO 3 equivalent EDTA solution, EBT indicator => prepared Buffer solution : NH 4 Cl + conc. NH 3

17 Standardization of EDTA solution Pipette 50 ml of std hard water (SHW) in conical flask. Add ml buffer soln (ph=10), 4-5 drops of EBT indicator. Solution is titrated against EDTA till colour changes from wine red to deep blue (V1 ml) Water Sample Analysis (Estimation) Total hardness : 50 ml hard water sample titrated against EDTA (V2 ml) Permanent hardness : 50 ml hard water sample is boiled for min, filtered, diluted with D/W to make up to 50 ml & titrated against EDTA (V3 ml) Temporary Hardness = (Total Permanent) hardness

18 Softening of Water Process of removing hardness/ reducing concentration of hardness causing salts from water Clark s Process Calculated amount of lime [Ca(OH) 2 ] added to hard water Bicarbonates are converted into insoluble carbonates & removed by settling & filtration Lime Soda Process Lime treatment followed by addition of soda ash (Na 2 CO 3 ) Calcium ions in hard water are removed as CaCO 3 and Mg as Mg(OH) 2

19 Lime soda method

20 Reactions with Lime Lime removes free acids 2HCl + Ca(OH)2 CaCl2 + 2H2O H2SO4 + Ca(OH)2 CaSO4 + 2H2O Lime removes temporary hardness Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O Mg(HCO3)2 + 2Ca(OH)2 2CaCO3 + Mg(OH)2 + 2H2O Lime removes permanent hardness MgCl2/MgSO4 + Ca(OH)2 Mg(OH)2 + CaCl2/CaSO4 Lime removes dissolved iron and aluminium salts Al2(SO4)3 + 3Ca(OH)2 2Al(OH)3 + 3CaSO4 FeSO4 + Ca(OH)2 Fe(OH)2 + CaSO4

21 Reactions with soda Removes permanent hardness due to Ca salts CaCl2 + Na2CO3 CaCO3 + 2NaCl CaSO4 + Na2CO3 CaCO3 + Na2SO4 NaAlO 2 + 2H 2 O NaOH + Al(OH) 3 (hydrolysis) Al(OH)3 gelatinous ppt that traps fine ppt/particles Residual Hardness = ppm The method discussed is called Cold Lime Soda Process

22 Hot lime soda softener

23 Hot process is operated at temperature close to boiling point of the solution, ( C) Reaction proceeds faster Softening capacity of hot process increases manifold Precipitate & sludge formed settle down rapidly, hence no coagulants needed Much of the dissolved gases (like CO 2 & air) driven out of water Viscosity of softened water is lower, so filtration becomes easier Hot-Lime soda process produces water of comparatively lower residual hardness of ppm

24 Amount of Lime (in kg) = 74/100 [Temp Ca 2+ + (2 Temp Mg 2+ ) + perm Mg 2+ + Fe 2+, Al 3+ + H + (HCl/H 2 SO 4 ) + HCO NaAlO 2 ] Vol of water x 100/%pure Amount of Soda (in kg) 106/100 [Perm (Ca 2+ +Mg 2+ +Fe 2+ +Al 3+ + H + - HCO 3- ] Vwater x /%pure

25 Ion-Exchange Process Reversible exchange of ions of same charge between mobile liquid phase & an insoluble solid (stationary phase) Ion-exchanger is an insoluble material liberating counter ions by electrolytic dissociation Cation exchanger: High mol.wt, cross-linked polymer containing sulfonic (-SO 3 H), carboxylic (-COOH), or phenolic (-OH) as a part of resin and equivalent amount of cations H-R (resin) + Na + Na-R (resin) + H + 2NaR (resin) + Ca 2+ CaR 2 (resin) + 2Na +

26 Anion exchanger : Is a polymer containing quaternary ammonium groups (-N + R 2 ) containing equivalent amount of anions, Cl -, OH -, etc 2RCl + SO 4 2- R 2 (SO 4 ) + 2Cl - (resin) (solution) (resin) (solution) Water from which all the cations & anions are removed is called demineralized or deionized water. Water is first passed to cation exchanger in acid form. All cations will be exchanged for H + ions Next, water coming from cation exchanger is passed into anion exchanger in basic form and anions are exchanged for OH --

27 Major Impurity=> CaSO 4 2RCOOH + Ca 2+ (RCOO) 2 Ca + 2H + 2R OH + SO 4 2- R 2 SO 4 + 2OH - H + + OH - H 2 O

28 Regeneration With continued operations, exchange capacity of the column decreases Adding a strong acid regenerates cation resin and the anion resin is regenerated by adding strong base. (RCOO) 2 Ca + HCl 2RCOOH + Ca 2+ R 2 SO 4 + NaOH R OH + SO 4 2-

29 Advantages: (i) Process can be used to soften highly acidic or alkaline water (ii) Water of low hardness(2 ppm) can be obtained Disadvantages: (i) Equipment is costly (ii) Turbidity of water to be treated should be below 10 ppm

30 Zeolite or Permutit Process Hydrated sodium alumino silicate Na 2 O.Al 2 O 3.xSiO 2.yH 2 O Zeolite is capable of exchanging reversibly their sodium ions for hardness producing ions in water.

31 Hard water enters from top at a specified rate & passes over a bed of sodium zeolite kept in a cylinder. Softened water is collected at the bottom of cylinder & is taken out from time to time.

32 Theory: When hard water is passed over a bed of sodium zeolite, Ca 2+, Mg 2+ ions are taken up by the zeolite simultaneously releasing equivalent Na + ions in exchange for them. CaCl 2 + Na 2 Ze CaZe + 2NaCl MgSO 4 + Na 2 Ze MgZe + Na 2 SO 4

33 Regeneration: When Zeolite is completely converted into calcium & magnesium Zeolites, it ceases to soften water i.e. it gets exhausted. It is generated by treating with 10% brine solution. CaZe + 2NaCl Na 2 Ze + CaCl 2 MgZe + 2NaCl Na 2 Ze + MgCl 2

34 Advantages: Water of about 10 ppm hardness is produced. Process automatically adjusts itself for different hardness of incoming water. Requires less skill in maintenance as well as operation.

35 Disadvantages: Treated water contains more sodium salts Method only replaces Ca 2+ & Mg 2+ ions by Na + ions, but leaves all ions (HCO 3 - & CO 3 2- ) in soft water Such soft water containing NaHCO 3, Na 2 CO 3 etc decomposes to give dissolved CO 2 which cause corrosion in steel container walls

36 Drinking Water Purification

37 Removal of microorganisms Process of destroying or killing disease-producing microorganisms from water and making it safer for use is called disinfection Chemicals or substances added to water for killing bacteria are known as disinfectants Chlorination Bleaching powder treatment Ozonization

38 Chlorination Chlorine (gas or in concentrated solution form) produces hypochlorous acid Cl 2 + H 2 O HOCl + HCl Hypochlorous acid, so produced kills the bacteria. Bacteria + HOCl Bacteria killed HOCl H + + OCl - Hypochlorite ion inactivates the enzymes present in microorganisms leading to death

39 Action of bleaching powder Ca(OCl) 2 + H 2 O Ca(OH) 2 + Cl 2 Cl 2 + H 2 O HCl + HOCl (hypochlorous acid) Germs + HOCl Germs killed Only calculated quantity of bleaching powder (calcium hypochlorite) should be used, since an excess of it gives bad taste and smell to treated water

40 Action of Ozone Ozone is an excellent disinfectant produced by passing electric discharge through oxygen 3O 2 2O 3 Ozone is highly unstable and breaks down liberating nascent oxygen O 3 O 2 + [O] Nascent oxygen kills all bacteria and oxidizes organic matter present in water

41 Reverse Osmosis Semipermeable membrane Pressure Initial condition Equilibrium Reverse Osmosis

42 Ultrafiltration Ultrafiltration is a cross flow separation process- similar to reverse osmosis Pore size of the membrane used in ultrafiltration is in the range of microns Low MW compounds pass through membrane & high MW compounds are retained in the membrane. It is suitable for retaining biomolecules, bacteria, viruses, polymers, colloidal particles and sugar molecules.

43 The stream of liquid that comes out through membrane is permeate and the other side of the liquid stream (which contains the macromolecules) is called the concentrate (retained by the membrane)

44 Applications Pretreatment in sea water desalination plants in combination with reverse osmosis Sterile filtration of drinking and beverage water Removal of metal hydroxides in wastewater treatment

Water Technology. 2C 17 H 35 COONa + CaCl 2 (C 17 H 35 COO) 2 Ca (scum) + 2NaCl 2C 17 H 35 COONa + MgSO 4 (C 17 H 35 COO) 2 Mg(scum) + Na 2 SO 4

Water Technology. 2C 17 H 35 COONa + CaCl 2 (C 17 H 35 COO) 2 Ca (scum) + 2NaCl 2C 17 H 35 COONa + MgSO 4 (C 17 H 35 COO) 2 Mg(scum) + Na 2 SO 4 Water Technology Contents: Hardness of water. Determination of hardness of water by EDTA titration. Numericals based on hardness of water and EDTA method. Softening Methods: Hot and Cold Lime-Soda Method,