Applications of Oxidation/Reduction Titrations. Lecture 6

Transcription

1 Applications of Oxidation/Reduction Titrations Lecture 6

2 Pretreatmentauxiliary oxidizing/reducing reagent Ex: when a sample containing iron is dissolved, the resulting solution usually contains a mixture of iron(ii) and iron(iii) ions We must first treat the sample solution with an auxiliary reducing reagent to convert all of the iron to iron(ii) To be a preoxidant or a prereductant: a reagent must react quantitatively with the analyte reagent excess must be easily removable

3 Auxiliary Reducing Reagents A number of metals are good prereducing agents: Ex: Zn, Al, Cd, Pb, Ni, Cu, Au(with Cl - ) and etc. Sticks or coils of the metal can be immersed directly in the analyte solution. After reduction is judged complete, the solid is removed The analyte solution must be filtered to remove granular/powdered forms of the metals

4 Auxiliary Oxidizing Reagents Sodium bismuthate is a powerful oxidizing agent capable, for example, of converting manganese(ii) quantitatively to permanganate ion Oxidations are performed by suspending the bismuthate in the analyte solution and boiling for a brief period The unused reagent is then removed by filtration

5 Other Oxidizing Agents Ammonium peroxydisulfate, (NH 4 ) 2 S 2 O 8 In acidic solution, it converts chromium(iii) to dichromate, excess reagent removal by boiling Peroxide Peroxide is a convenient oxidizing agent. Either a solid sodium salt or a dilute solution of the acid excess reagent removal by boiling

6 Applying Standard Reducing Agents Since standard solutions of most reductants tent to react with atmospheric oxygen: reductants are seldom used for the direct titration of oxidizing analytes indirect methods are used instead Two common reductants: Iron(II) ions Thiosulfate ions (S 2 O 3 2- )

7 Iron(II) Solutions Easily prepared from Iron(II) ammonium sulfate, Fe(NH 4 ) 2 (SO 4 ) 2 6H 2 O, or Related iron(ii) ethylenediamine sulfate, FeC 2 H 4 (NH 3) 2 (SO 4 ) 2 4H 2 O. Air oxidation prevention Air oxidation takes place rapidly in neutral solutions but is inhibited in the presence of acids. the most stable preparations is about 0.5 M in H 2 SO 4. Excess iron(ii) titration by potassium dichromate or cerium (IV) Application: Organic peroxide, hydroxylamine, chromium(vi), cerium (IV), molybdenum (VI), and nitrate, chlorate, perchlorate ions

8 Sodium Thiosulfate Thiosulfate ion (S 2 O 3 2- ) is a moderately strong reducing agent that has been widely used to determine oxidizing agents by an indirect procedure: Adding an unmeasured excess of potassium iodide (KI) to a slightly acidic solution (ph < 7)of the analyte. Reduction of the analyte produces a stoichiometrically equivalent amount of iodine. The liberated iodine is then titrated with sodium thiosulfate, Na 2 S 2 O 3 (one of the few reducing agents that is stable toward air oxidation). Ex:

9 Standardizing Thiosulfate Solutions Potassium iodate is an excellent primary standard for thiosulfate solutions: Weighted amount of primary-standard grade reagent are dissolved in water containing an excess of potassium iodide Acidified the mixture with a strong acid: The liberated iodine is titrated with thiosulfate solution:

10 EXAMPLE 20-1 A solution of sodium thiosulfate was standardized by dissolving g KIO 3 ( g/mol) in water, adding a large excess of KI, and acidifying with HCl. The liberated iodine required ml of the thiosulfate solution to decolorize the blue starch/iodine complex. Calculate the molarity of the Na 2 S 2 O 3.

11 Applications of Sodium Thiosulfate Solutions

12 Applying Standard Oxidizing Agents The choice among the most widely used oxidizing reagents: The strength of the analyte as a reducing agent The rate of reaction between oxidant and analyte The stability of the standard oxidant solutions The cost The availability of a satisfactory indicator

13 5 most widely used volumetric oxidizing reagents

14 Strong Oxidants - Potassium Permanganate and Cerium(IV) Half-reactions are For permanganate ion occurs only in solutions that are 0.1M or greater in strong acid in less acidic media, the product may be Mn(III), Mn(IV), or Mn(VI), depending on conditions For cerium (IV) In 1M sulfuric acid, formal potential is 1.44V In 1M perchloric acid and 1M nitric acid, the potentials are 1.7V and 1.61V, respectively. Solutions of cerium (IV) are found of limited applications here.

15 Comparing the Two Reagents In sulfuric acid Solutions of cerium(iv) are stable indefinitely permanganate solutions decompose slowly In hydrochloric acid cerium(iv) solutions in sulfuric acid do not oxidize chloride ion and can be used to titrate hydrochloric acid solutions of analytes permanganate ion cannot be used with hydrochloric acid solutions of analytes Cerium(IV) is that a primary-standard grade salt of the reagent is available However, the popularity of permanganate solutions is their modest cost

16 Detecting the End Points Potassium permanganate solution is intense purple color, which is sufficient to serve as an indicator for most titrations. The permanganate end point is not permanent because excess permanganate ions react slowly (slow that the end point fades only gradually over a period of perhaps 30 seconds) with the relatively large concentration of manganese(ii) ions Solutions of cerium(iv) are yellow-orange, but the color is not intense enough to act as an indicator in titration. The most widely used indicator with standard solutions of cerium (IV) is the iron(ii) complex of 1,10-phenanthroline

17 The Preparation and Stability of Standard Solutions Aqueous solutions of permanganate are not entirely stable: This reaction is catalyzed by light, heat, acids, bases, manganese(ii), and manganese dioxide. Removal of manganese dioxide by filtration(not paper filtering!!) before standardization markedly improves the stability of standard permanganate solutions Standardized permanganate solutions should be stored in the dark. Solutions containing excess standard permanganate should never be heated

18 EXAMPLE 20-2 Describe how you would prepare 2.0 L of an approximately M solution of KMnO 4 ( g/mol). Dissolve about 3.2 g of KMnO4 in a little water Add water to bring the volume to about 2.0 L Heat the solution to boiling for a brief period, and let stand until it is cool. Filter through a glass-filtering crucible and store in a clean dark bottle

19 Solutions of cerium (IV) preparation The reagent is dissolved in a solution that is at least 0.1M in sulfuric acid to prevent the precipitation of basic salts Sulfuric acid solutions of cerium(iv) are remarkably stable and can be stored for months or heated at 100 C for prolonged periods without a change in concentration

20 Standardizing Permanganate and Ce(IV) Solutions Sodium oxalate is widely used for standardization: Reaction with permanganate: The reaction is complex and proceeds slowly even at elevated temperature unless manganese(ii) is present as a catalyst (autocatalysis) Reaction with Cerium(IV): standardizations against sodium oxalate are usually performed at 50 C in a hydrochloric acid solution containing iodine monochloride as a catalyst

21 EXAMPLE 20-3 You wish to standardize the solution in Example 20-2 against primary standard Na 2 C 2 O 4 ( g/mol). If you want to use between 30 and 45 ml of the reagent for the standardization, what range of masses of the primary standard should you weigh out? For a 30-mL titration: For a 45-mL titration: should weigh to 0.15-g samples of the primary standard

22 EXAMPLE 20-4 A g sample of primary-standard Na 2 C 2 O 4 required exactly ml of the permanganate solution in Example 20-2 to reach the end point. What was the molarity of the KMnO 4 reagent?

23 Using Potassium Permanganate and Cerium(IV) Solutions

24 EXAMPLE 20-5 Aqueous solutions containing approximately 3% (w/w) H 2 O 2 are sold in drug stores as disinfectants. Propose a method for determining the peroxide content of such preparation using the standard solution described in Examples 20-3 and Assume that you wish to use between 30 and 45 ml of the reagent for a titration. The reaction is The amounts of KMnO 4 in 35 to 45 ml of the reagent are: The amounts of H 2 O 2 consumed or Samples should weigh between 1.1 and 1.5 g. These should be diluted to perhaps 75 to 100 ml with water and made slightly acidic with dilute H 2 SO 4 before titration

25 Potassium Dichromate Dichromate ion has analytical applications: generally carried out in solutions about 1M in HCl or H 2 SO 4 (half reaction about 1.0 to 1.1V) indefinitely stable can be boiled without decomposition do not react with hydrochloric acid modest cost Disadvantage: Lower electrode potential when comparing to cerium(iv) and permanganate ion

26 Preparing Dichromate Solutions For most purposes: reagent-grade potassium dichromate solid is dried at 150 C to 200 C before being weighed. Indicator: Diphenylamine sulfonic acid is an excellent indicator for titration. The oxidized form of diphenylamine sulfonic acid is violet, and its reduced form is essentially colorless

27 EXAMPLE 20-6 A 5.00-mL sample of brandy was diluted to L in a volumetric flask. The ethanol (C 2 H 5 OH) in a mL aliquot of the diluted solution was distilled into ml of M K 2 Cr 2 O 7 and oxidized to acetic acid with heating. The reaction is After cooling, ml of M Fe 2+ was pipetted into the flask. The excess Fe 2+ was then titrated with 7.46 ml of the standard K 2 Cr 2 O 7 to a diphenylamine sulfonic acid end point. Calculate the percent (w/v) C 2 H 5 OH (46.07 g/mol) in the brandy.

28 Total amount K 2 Cr 2 O 7 : Amount K 2 Cr 2 O 7 consumed by Fe 2+ : Amount K 2 Cr 2 O 7 consumed by C 2 H 5 OH: =( ) mmol K 2 Cr 2 O 7 = mmol K 2 Cr 2 O 7 Mass of C 2 H 5 OH:

29 Iodine Iodine is a weak oxidizing agent used primarily for the determination of strong reductants: where I 3 - is the triiodide ion. Low stability Because of the smaller electrode potential, iodine solutions have relatively limited application. The low potential, however, has important advantage of A degree of selectivity that makes possible the determination of strong reducing agents in the presence of weak ones. iodine is the availability of a sensitive and reversible indicator for the titrations.

30 Properties of Iodine Solutions Not very soluble in water (0.001 M). Hence, iodine is ordinarily dissolved in moderately concentrated solutions of potassium iodide. Lack stability because The volatility of the solute Losses of iodine from an open vessel occur in a short time Iodine slowly attacks most organic materials Air oxidation of iodide:

31 Use Iodine as an Oxidizing Agent

32 Potassium Bromate as a Source of Bromine Primary-standard potassium bromate is available from commercial sources and can be used directly to prepare standard solutions that are stable indefinitely. Not frequently used for direct titration Convenient and widely used stable source of bromine: The primary use of standard potassium bromate is the determination of organic compounds that react with bromine.

33 Bromine for Substitution Reactions Addition reactions

34 EXAMPLE 20-7

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36 Determining Water with the Karl Fischer Reagent Karl Fischer titration: a procedure for the determination of water in various types of solids and organic liquids: The stoichiometry can vary from 2:1 to 1:1 depending on the presence of acids and bases in the solution

37 Determining Water with the Karl Fischer Reagent I 2 and SO 2 react in the presence of pyridine and water to form pyridinium sulfite and pyridinium iodide: where I 2, SO 2, and SO 3 are shown as complexed by the pyridine. The pyridinium sulfite can also consume water (1) undesirable because it is not as specific for water (2) can be prevent by having a large excess of methanol The stoichiometry is one mole of I 2 per mole of H 2 O present

38 Pyridine-free reagents Pyridine has objectionable odor. Pyridine-free reagents are commercially available: The stoichiometry is again one mole of I 2 per mole of H 2 O present