Applications of Redox Titrations - Reon Sylvester

Applications of Redox Titrations

1. Applications of redox titrations REON SYLVESTER D CUNHA Reg no: 199326

2. Introduction Redox titration is a titration in which the reaction between analyte and the titrant is an oxidation/Reduction reaction..

3. Applications in Organic analysis Determination of COD in natural wastewaters. COD(Chemical Oxidation Demand) is the quantity of oxygen necessary to oxidize all the organic matter in a sample to CO 2 to H2O. Important in managing industrial wastewaters.

4. Procedure Sample is refluxed in excess of K 2 Cr 2 O 7 , acidified with H 2 SO 4 and Ag 2 SO 4 is added as a catalyst . HgSO 4 is added to prevent precipitation of Ag catalyst as AgCl. After refluxing for 2 hours, Solution is cooled to a room temperature and excess Cr 2 O 7 2- is determined by back titration with FAS as titrant and ferroin as the indicator. Blank titration is also performed . By knowing the difference in amount of potassium dichromate used, amount of oxygen consumed can be calculated.

5. Applications in inorganic analysis Determination of water in Non aqueous solvents Karl Fischer reagent is used as a titrant . Karl Fischer reagent is a mixture of Iodine , sulphur dioxide, Pyridine and Methanol. Iodine and SO 2 are basic components. Pyridine forms complex with I 2 and SO 2 . It stabilises the stoichiometry and shifts the reaction to further right. Methanol prevents side reactions and buffer maintains the P H

6. Stoichiometry I 2 + SO 2 +2H 2 O 2HI + H 2 SO 4 Pyridine Is used to stabilize the stoichiometry and forms complex with I 2 and SO 2 . Stoichiometry is 1:1 because 1 mole of iodine is consumed per 1 mole of water. Formation of Pyridinium Iodide and Pyridinium Sulphite. H 2 O + I 2 + SO 2 + 3C 6 H 5 N 2C 5 H 5 NHI + C 5 H 5 NSO 3 Reaction of Pyridinium Sulphite with Methanol C 5 H 5 NSO 3 + CH 3 OH C 5 H 5 NH SO 4 CH 3 Second step prevents Pyridinium Sulphite to react with water. End point is found when colour changes from yellow to brown of Karl Fischer reagent.

7. Determination of dissolved oxygen Necessary to support aquatic life. Most readily available oxidant for biological oxidation of inorganic and organic pollutants. Anaerobic oxidation of waste in treatment plants. Dissolved oxygen is determined by Winkler’s method

8. Winkler’s method Water sample is collected and treated with MnSo 4 solution and then with solution of KI and NaOH. Under these conditions, Mn 2+ is oxidised to MnO 2 by dissolved oxygen. 2Mn 2+ (aq)+ 4OH – (aq) + O 2 (aq) 2MnO 2 (s) + 2H 2 O (l) The Solution is acidified with. H 2 SO 4 and under acidic conditions I – is oxidised to I 3 - by MnO 2. MnO 2 (s) + 3I – (aq) +4H 3 O + Mn 2+ (aq) + I - 3 (aq) + 6H 2 0(l) Liberated iodine is titrated against sodium thiosulphate using starch as the indicator.

9. Iodometric and IodImetric titrations Indirect titration To determine oxidizing agents. I – is added to oxidizing agent. Liberated iodine is titrated against Na 2 S 2 O 3 Indicator at the end of titration Disappearance of blue colour Direct titration To determine Reducing agents. Iodine is the titrating agent. Indicator at the beginning of titration Permanent blue colour

10. Chlorination of public water supplies The total chlorine residual is determined by using the oxidizing power of chlorine to convert I – to I - 3 . The amount of iodine liberated is redox titrated using sodium thiosulphate using starch as the indicator. Total chlorine residual is reported as ppm of Cl. This is a type of indirect iodometric titration.

11. Procedure:- When the Sample of iodide free chlorinated water is mixed with excess of DPD indicator The free chlorine oxidizes a portion of DPD to its Red coloured form. Oxidized DPD is Back titrated to its colourless form with Ferrous Ammonium Sulfate. Volume of titrant is proportional to the amount of free residual chlorine.