Chapter 20. Applications of Oxidation/Reduction Titrations

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1 Chapter 20 Applications of Oxidation/Reduction Titrations 1

2 Auxiliary Oxidizing and Reducing Reagents The analyte in an oxidation/reduction titration must be in a single oxidation state at the outset; however, the steps preceding titration, such as dissolving the sample and separating interferences, convert the analyte to a mixture of oxidized states. For example, Iron solution usually contains a mixture of Fe 2+ and Fe 3+. Convert all of the ion to Fe 2+ by treating the sample solution with an Auxiliary Reducing Agent is important. If we plan to titrate with a standard reductant, pretreatment with an auxiliary oxidizing reagent is needed. To be useful as a preoxidant or a prereductant, a reagent must react quantitatively with the analyte and must be easily removable to avoid interfering in the titration. 2

3 Auxiliary Reducing Reagents A number of metals are good reducing agents and have been used for the prereduction of analytes. zinc, aluminum, cadmium, lead, nickel, copper, silver Sticks or coils of the metal can be immersed directly in the analyte solution. After reduction is judged complete, the solid is removed manually and rinsed with water. The analyte solution must be filtered to remove granular or powdered forms of the metal. After reduction of the analyte, the reducing agent is removed by filtration of by use of a reductor. 3

4 A Jones Reductor A typical Jones reductor has a diameter of about 2 cm and holds a 40 to 50cm column of amalgamated zinc. Amalgamation is accomplished by allowing zinc granules to stand briefly in a solution of mercury(ii) chloride, where the following reaction occurs: 2Zn (s) + Hg +2 Zn +2 + Zn(Hg) (s) Zinc amalgam is an effective reducing agent and has the inhibiting the reduction of H + by Zn. Solution are quite acidic can be passed through a Jones reductor without significant hydrogen formation. 4

5 Two Reductors In a Walden reductor, granular metallic silver held in a narrow glass column is the reductant. Silver is not a good reducing agent unless chloride or some other ion that forms a silver salt of low solubility is present. Prereductions with a Walden reductor are generally carried out from hydrochloric acid solutions of the analyte. The coating of AgCl produced on the metal is removed periodically by dipping a zinc rod into the solution that covers the packing. The Walden reductor is somewhat more selective than Jones reductor. 5

6 Auxiliary Oxidizing Reagents Sodium Bismuthate (NaBiO 3 ) is a powerful oxidizing agent. Can convert Mn 2+ to MnO 4 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. NaBiO 3(s) + 4H + + 2e BiO + + Na + + 2H 2 O Ammonium Peroxydisulfate (NH 4 ) 2 S 2 O 8 is a powerful oxidizing agent. In acidic solution, it converts Cr 3+ to Cr 2 O 7 2, Ce 3+ to Ce 4+, Mn 2+ to MnO 4 The half reaction is: S 2 O e 2SO 4 2 The oxidants are catalyzed by traces of Ag +. The excess reagent is easily decomposed by a brief period of boiling: 2 S 2 O H 2 O > 4SO O 2(g) + 4H + 6

7 Auxiliary Oxidizing Reagents Sodium Peroxide and Hydrogen Peroxide are also oxidizing agents. Peroxide is a convenient oxidizing agent either as the solid Na salt or as a dilute solution of the acid. The half reaction of H 2 O 2 in acidic solution is: H 2 O 2 + 2H + + 2e 2H 2 O E 0 = 1.78 V After oxidation is complete, the solution is freed of excess reagent by boiling: H 2 O 2 > 2H 2 O + O 2(g) 7

8 Applying standard reducing agents Standard solution of most reductants tend to react with O 2 they are seldom used for the direct titration of oxidizing analytes Indirect methods are used instead The two most common reductants are: Fe 2+ solutions Thiosulfate ions (S 2 O 3 2 ) Iron(II) Solutions Iron(II) gets rapidly oxidized by air in neutral solutions but oxidation is inhibited in the presence of acids, with the most stable preparations being about 0.5 M in H 2 SO 4. Oxidizing agents are conveniently determined by treatment of the analyte solution with a measured excess of standard iron(ii) followed by immediate titration of the excess Fe 2+ with a standard solution of K 2 Cr 2 O 7 or Ce 4+. 8

9 Applying standard reducing agents Sodium Thiosulfate Thiosulfate ion (S 2 O 3 2 ) is a moderately strong reducing agent that is used to determine oxidizing agents by an indirect procedure in which iodine is an intermediate. With iodine, thiosulfate ion is oxidized quantitatively to tetrathionate ion according to the halfreaction: 2S 2 O 3 2 S 4 O e Other oxidants can oxidize the tetrathionate ion to sulfate ion. Determine oxidizing agents: 1. Adding an unmeasured excess of KI to a slightly acidic solution of analyte 2. Reduction of the analyte and produce the equivalent amount of I 2 3. I 2 will be titrated with Na 2 S 2 O 3 Example: determine sodium hypochlorite in bleaches OCl + 2I + 2H + > Cl + I 2 + H 2 O (unmeasured excess KI) I 2 + 2S 2 O 2 3 > S 4 O I 9

10 Applying standard reducing agents Sodium Thiosulfate Detecting End Points in Iodine/Thiosulfate Titrations Iodine titrations are often performed with starch as an indicator. The deep blue color develops in the presence of I 2 with bamylose. Red adduct forms when aamylose with I 2, but it is not reversible and is undesirable. The commercial soluble starch contains bamylose only. Aqueous starch decompose fast in the air due to the bacterial action and it can be inhibited by adding Hg 2+ or CHCl 3 as bacteriostat. Starch decomposes in the solution with high I 2 concentration. In titrations of excess I 2 with Na 2 S 2 O 3, addition of the indicator must be deferred until most of the I 2 has been reduced. 10

11 Applying standard reducing agents Sodium Thiosulfate Stability of Sodium Thiosulfate Solutions Although Na 2 S 2 O 3 solutions are resistant to air oxidation, it decompose to give sulfur and hydrogen sulfite ion: S 2 O H + HSO 3 + S (s) ph, the presence of microorganisms, the concentration of the solution, the presence of copper(ii) ions, and exposure to sunlight affect the reaction rate. The decomposition reaction increases when it becomes acidic. The most important cause for the instability of Na 2 S 2 O 3 is bacteria that metabolize thiosulfate ion to sulfite and sulfate or elemental sulfur. Sterile solution, keep ph between 9 and 10, or the presence of bactericide such as chloroform or Hg 2+ can increase stability. 11

12 Applying standard reducing agents Sodium Thiosulfate Standardizing Thiosulfate Solutions Potassium iodate (KIO 3 ) is an excellent primary standard for thiosulfate solutions. The KIO 3 and the excess of KI in the acidic solution: IO 3 + 5I + 6H + > 3 I H 2 O The liberated I 2 is then titrated with Na 2 S 2 O 3, I 2 + 2S 2 O 3 2 > S 4 O I So 1 mol IO 3 = 3 mol I 2 = 6 mol S 2 O 3 2 Other primary standards for sodium thiosulfate are potassium dichromate, potassium bromate, potassium hydrogen iodate, potassium hexacyanoferrate(iii), and metallic copper. All these compounds liberate I 2 when treated with excess KI. 12

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14 Applying standard reducing agents Sodium Thiosulfate Applications of Sodium Thiosulfate Solutions Several substances can be determined by the indirect method involving titration with Na 2 S 2 O 3. 14

Applying Standard Oxidizing Agents Table shows 5 of the most widely used volumetric oxidizing reagents. Standard potential ranges from 0.5 to 1.5V.

15 Applying Standard Oxidizing Agents Table shows 5 of the most widely used volumetric oxidizing reagents. Standard potential ranges from 0.5 to 1.5V. The choice of agent depends on 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, and the availability of a satisfactory indicator. 15

16 Applying Standard Oxidizing Agents The Strong Oxidants: Potassium Permanganate and Cerium(IV) Solutions of permanganate ion and cerium(iv) ion are strong oxidizing reagents whose applications closely parallel one another. The halfreactions are: The formal potential for Ce 4+ in 1M perchloric acid and 1M nitric acid are 1.70 V and 1.61 V; however, Ce 4+ is not stable in these two solution. In less acidic conditions, the product of MnO 4 may be Mn 3+, Mn 4+, or Mn 6+ depending on conditions. Solutions of cerium(iv) in sulfuric acid are stable indefinitely, but permanganate solutions decompose slowly and thus require occasional restandardization. Cerium(IV) solutions in sulfuric acid do not oxidize chloride ion and can be used to titrate hydrochloric acid solutions of analytes. 16

17 Applying Standard Oxidizing Agents The Strong Oxidants: Potassium Permanganate and Cerium(IV) Permanganate ion cannot be used with hydrochloric acid solutions unless special precautions are taken to prevent the slow oxidation of chloride ion that leads to overconsumption of the standard reagent. A further advantage of cerium(iv) is that a primarystandardgrade salt of the reagent is available, thus making possible the direct preparation of standard solutions. Despite the advantages, potassium permanganate is more widely used. 1. Color of MnO 4 solution (deep violet), intense enough to be an indicator in titration. (Mn 2+ : light pink, MnO 4 2 : dark green, MnO 4 3 : deep blue) 2. Lower cost of KMnO 4 solution. 3. Ce 4+ may form precipitates of basic salts in solutions that are less than 0.1M in strong acid. 17

18 Applying Standard Oxidizing Agents Detecting the End Points Potassium permanganate solution has an intense purple color, which is sufficient to serve as an indicator for most titrations. When low conc. of KMnO 4 solution is used, diphenylamine sulfonic acid or the 1,10phenanthroline complex of iron(ii) can provide a sharper end point. The end point is not permanent because excess permanganate ions react slowly with the relatively large concentration of manganese(ii) ions present at the end point. 2MnO 4 + 3Mn H 2 O <> 5MnO 2(s) + 4H + The equilibrium constant is about 10 47, indicating that the equilibrium conc. of MnO 4 is very small. But the reaction rate is slow to make the end point fades gradually over 30 sec. Solutions of cerium(iv) are yelloworange, but the color is not intense enough to act as an indicator in titrations. Several ox/red indicators can be used for titration with Ce 4+, such as iron(ii) complex of 1,10phenanthroline. 18

19 Applying Standard Oxidizing Agents The Preparation and Stability of Standard Solutions Aqueous solutions of MnO 4 are not stable due to water oxidation: 4MnO 4 + 2H 2 O <> 4MnO 2(s) + 3O 2(g) + 4OH Permanganate solutions are moderately stable because the decomposition reaction is slow. The decomposition is catalyzed by light, heat, acids, bases, Mn 2+ and particularly MnO 2. MnO 2 is a contaminant in the solid KMnO 4. Removal of MnO 2 by filtration and store in dark improves the KMnO 4 stability. Filter paper cannot be used for filtering because KMnO 4 reacts with it to form MnO 2. 19

20 The most widely used compounds for preparation of Ce 4+ solution are listed. 20

21 Standardizing Permanganate and Ce(IV) Solutions Sodium oxalate is a widely used primary standard. In acidic solutions, oxlate ions are converted to the undissociated acid. 2MnO 4 + 5H 2 C 2 O 4 + 6H + > 2Mn CO 2(g) + 8H 2 O The reaction between permanganate ion and oxalic acid is complex and proceeds slowly even at elevated temperature unless manganese(ii) is present as a catalyst. As the conc. of Mn 2+ increases, the reaction proceeds more and more rapidly as a result of autocatalysis. Sodium oxalate is also widely used to standardize Ce(IV) solutions. 2Ce 4+ + H 2 C 2 O 4 > 2Ce CO 2(g) + 2H + Cerium(IV) standardizations against sodium oxalate are usually performed at 50 C in a hydrochloric acid solution containing iodine monochloride as a catalyst. 21

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23 2 MnO C 2 O H + > 2 Mn CO 2(g) + 8 H 2 O 23

24 Using Permanganate and Ce(IV) Solutions The table lists some of the many applications of MnO 4 and Ce 4+ solutions to the volumetric determination of inorganic species. 24

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27 Potassium Dichromate Dichromate ion is reduced to Cr 3+ : green color Cr 2 O H + + 6e <> 2Cr H 2 O E o = 1.33V Dichromate titrations are generally carried out in solutions that are about 1 M in hydrochloric or sulfuric acid and the formal potential will be 1.0 ~ 1.1V. Potassium dichromate solutions are indefinitely stable, can be boiled without decomposition, and do not react with hydrochloric acid. The disadvantages are its lower electrode potential and the slowness of its reaction with certain reducing agents. 27

28 Potassium Dichromate Preparing Dichromate Solutions Potassium dichromate can be dried at 150 ~ 200 o C before being weighted. The orange color of a dichromate solution is not intense enough for use in endpoint detection. Diphenylamine sulfonic acid is an excellent indicator for titrations with this reagent. The oxidized form is violet and the reduced form is colorless. So, the color change observed is from green (Cr 3+ ) to violet (oxidized form). 28

29 Potassium Dichromate Applying Potassium Dichromate Solutions The principal use of dichromate is for the volumetric titration of iron(ii) in the presence of moderate concentrations of hydrochloric acid. Cr 2 O H + + 6Fe 2+ <> 2Cr H 2 O + 6Fe 3+ Dichromate with Fe 2+ has been widely used for INDIRECT determination of oxidizing agents. A measured excess of Fe 2+ solution is added to an acidic solution of analyte. The excess Fe 2+ is backtitrated with standard K 2 Cr 2 O 7 This method has been applied to the determination of nitrate, chlorate, permanganate, and dichromate ions as well as organic peroxides and other oxidizing agents. 29

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Iodine Iodine is a weak oxidizing agent used primarily for the determination of strong reductants. I 3 + 2e <> 3I E o = 0.

31 Iodine Iodine is a weak oxidizing agent used primarily for the determination of strong reductants. I 3 + 2e <> 3I E o = V Solutions are prepared by dissolving iodine in a concentrated solution of potassium iodide. With smaller electrode potential, iodine solution has Relatively limited application Advantageous: a degree of selectivity makes possible the determination of strong reducing agents in the presence of weak ones Advantageous: a sensitive and reversible indicator for titrations. 31

32 Iodine Iodine is not very soluble in water (0.001 M). Iodine is usually dissolved in moderately concentrated solutions of KI. I 2(s) + I <> I 3 K = 7.1 x 10 2 Iodine solution lack stability because Volatility of the solute: losses of iodine from an open vessel occur in a relatively short time even in the presence of an excess of I Iodine slowly attacks most organic materials => do not use cork or rubber stoppers to close containers of the iodine solution Air oxidation of iodide ion: 4I + O 2(g) + 4H + <> 2I 2 + 2H 2 O Air oxidation is promoted by acids, heat, and light. 32

33 Potassium Bromate Primarystandard potassium bromate (KBrO 3 ) 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: BrO 3 + 5Br + 6H + > 3Br 2 + 3H 2 O standard solution excess The primary use of standard KBrO 3 is for the determination of organic compounds that react with Br 2. To determine the excess bromine, an excess of KI is introduced: 2I + Br 2 > I 2 + 2Br The liberated iodine is then titrated with standard sodium thiosulfate (Na 2 S 2 O 3 ). Br 2 is incorporated into an organic molecule either by substitution or by addition. 33

34 Potassium Bromate as a Source of Bromine 34

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36 Potassium Bromate as a Source of Bromine Example of substitution reaction: the use of a bromine substitution reaction to determine 8hydroxyquinoline 8hydroxyquinoline is an excellent precipitating reagent for cations: Al HOC 9 H 6 N (ph 49)> Al(OC 9 H 6 N) 3(s) + 3H + Al(OC 9 H 6 N) 3(s) (hot 4M HCl)> 3HOC 9 H 6 N + Al 3+ 3HOC 9 H 6 N + 6Br 2 > 3HOC 9 H 4 NBr 2 + 6HBr stoichiometric relationship: 1 mol Al 3+ = 3mol HOC 9 H 6 N = 6mol Br 2 = 2mol KBrO 3 Example of Addition Reaction: 36

37 Determining Water with the Karl Fischer Reagent It is used for the determination of water in various types of solids and organic liquids. This important titrimetric method is based on an oxidation/reduction reaction that is relatively specific for water. In an aprotic solvent (neither acidic nor basic), the reaction is: I 2 + SO 2 + 2H 2 O 2HI + H 2 SO 4 The stoichiometry can vary from 2:1 to 1:1 depending on the presence of acids and bases in the solution. Pyridine (C 5 H 5 N) was added in an anhydrous methanol as the solvent. 37

38 Determining Water with the Karl Fischer Reagent To stabilize the stoichiometry and shift the equilibrium further to the right, pyridine (C 5 H 5 N) is added and anhydrous methanol is used as the solvent. A large excess of pyridine was used to complex I 2 and SO 2. C 5 H 5 N I 2 + C 5 H 5 N SO 2 + C 5 H 5 N + H 2 O 2C 5 H 5 N HI + C 5 H 5 N SO 3 C 5 H 5 N + SO 3 + CH 3 OH C 5 H 5 N(H)SO 4 CH 3 Then the 2nd step is pyridinium sulfite consume water: C 5 H 5 N + SO 3 + H 2 O C 5 H 5 NH + SO 4 H The 2nd step is (1) not as specific for water and (2) can be prevent by having a large excess of methanol. The stoichiometry is: 1 mol I 2 = 1 mol H 2 O For volumetric analysis, the classical Karl Fischer reagent consistes of I 2, SO 2, pyridine and anhydrous MeOH. 38

39 Determining Water with the Karl Fischer Reagent PyridineFree chemistry: Pyridine has objectionable odor. Other amines such as imidazole have replaced pyridine. The reaction is now believed to occur as follows: 1. Solvolysis: 2ROH + SO 2 RSO 3 + ROH Buffering: B + RSO 3 + ROH 2 + BH + SO 3 R + ROH 3. Redox: B I 2 + BH + SO 3 R + B + H 2 O BH + SO 4 R + 2BH + I The stoichiometry: 1 mol I 2 = 1 mol H 2 O 39

40 Determining Water with the Karl Fischer Reagent Interfering reactions Oxidizing agents such as Cu(II), Fe(III), nitrite, Br 2, Cl 2, or quinones produce I 2, which can react with H 2 O and cause determinations that are too low. The carbonyl groups on aldehydes and ketones can react with SO 2 and H 2 O to form bisulfite complexes. Oxidizable species such as ascorbic acid, ammonia, thiols, Tl +, Sn 2+, In +, hydroxyl amines, and thiosulfite can reduce iodine and cause water determinations that are too high. Some interfering compounds react to produce H 2 O carboxylic acid and alcohol produce ester and water Ketone and aldehyde react with alcoholic solvents to form ketals and acetals R 2 C=O + 2CH 3 OH R 2 C(OCH 3 ) 2 + H 2 O Phenolic derivatives and bicarbonates also cause reduction of I 2. 40

41 Determining Water with the Karl Fischer Reagent Detecting the End Point A Karl Fischer titration can be observed visually based on the brown color of the excess reagent (C 5 H 5 N I 2 ) or by electroanalytical measurements. Reagent Properties Karl Fischer reagent decomposes on standing and should be prepared a day or two before use. Applications The Karl Fischer reagent can be used in the determination of water in many organic acids, alcohols, esters, ethers, anhydrides, and halides. The hydrated salts of most organic acids, as well as the hydrates of a number of inorganic salts that are soluble in methanol, can also be determined by direct titration. 41

Oxidation/reduction reaction and titration

Oxidation/reduction reaction and titration Dr. Mohammad Khanfar Oxidation/reduction reaction In an oxidation/reduction reaction, electrons are transferred from one reactant to another. An example is the