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 CO2 to H2O.
• Important in managing industrial wastewaters.

4. PROCEDURE
Sample is refluxed in excess of K2Cr2O7 , acidified with H 2 SO 4 and Ag 2SO 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
2O7
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.

6. 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

7. 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 2O + 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 3OH 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.

8. 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

9. 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 3O + Mn 2+(aq) + I -
3 (aq) + 6H2 0(l)
Liberated iodine is titrated against sodium thiosulphate using starch as the indicator.

10. 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

11. 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.

12. 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.