Module 9 : Experiments in Chemistry
Lecture 38 : Titrations : Acid-Base, Redox and Complexometric
 

38.7 Experiment 6: Determination of ferrous ions using potassium dichromate by internal indicator method

Theory :

Iron(II) is oxidized in the presence of acid by dichromate.

6Fe2+ + Cr2O72- + 14H+ 6Fe3+ + 2Cr3+ + 7H2 O (38.7)

This disappearance of the colour of dichromate cannot be used to locate the end point because the Cr3+ ions have green colour that makes the Cr2O72- colour indistinguishable. The end point is conveniently detected by using a redox indicator N-phenyl anthranilic acid. The dichromate solution could be used as a primary standard in this determination but it is better to standardize using a standard solution of ferrous ammonium sulphate; this obviates the need for using extra oven dried sample of K2Cr2O7 .
Procedure :

Prepare 100 mL of an approximately N/30 solution of ferrous ammonium sulphate, FeSO4 (NH4)2SO4.6H2O, adding ~10 mL of dilute 2N H2SO4 solution during the dissolution (why?). 10 mL of this solution is pipetted out into a conical flask, 10 mL of dilute H2SO4 solution and three to five drops of N-phenyl anthranilic acid solution are added, and titrated against the supplied potassium dichromate solution. The solution acquires a pale green colour due to the Cr3+ ions; at the end point, the colour changes sharply to red-purple.

The given solution of ferrous ammonium sulphate of unknown concentration is also titrated similarly against the same solution of K2Cr2O7. From the titre values, calculate and report:
 
(a) The normality of the given ferrous ammonium sulphate solution.
(b) The strength of iron in gm/ litre.
(c) its strength in gms of ferrous ammonium sulphate/ litre.
 

In your report, explain the following points:
(a) What are the equivalent weights of K2Cr2O7 and FeSO4(NH4 )2SO4.6H2O?
(b) How does a redox indicator function?
(c) What is the function of a dilute sulphuric acid in this titration? Can you use dilute hydrochloric acid?
(d) What is the structure of N-phenyl anthranilic acid?
(e) An oxidation-reduction which has Eo nearly equal to the potential of the equivalence-point is suitable for that reaction. Explain.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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