Question

How we can know that the ferric iron is estimated by ptassiun dichromate method?

Answer 1

As an oxidant, dichromate has some advantages over permanganate, but, as it is less powerful, its use is much more limited. It is obtainable in a state of high purity and can be used as a primary standard. Solutions of dichromate in water are stable indefinitely. The half reaction for the dichromate system is: 

Cr2O72- + 14H+ + 6e- → 2Cr3+ + 7H2O     E° = 1.33 V 

The most important application of dichromate is in its reaction with iron(II) in which it is often preferred to permanganate. 

The relevant half reaction is : 

Fe2+ → Fe3+ + e-     E° = -0.77 V 

and the total reaction is: 

Cr2O72- + 6 Fe2+ + 14H+ → 2Cr3+ + 6 Fe3+ + 7H2O     E° = 0.56 V 

Unlike permanganate, dichromate titrations require an indicator. There are three indicators that may be used for the titration of Fe2+ with K2Cr2O7. These are diphenylamine, diphenylbenzidine and diphenylamine sulfonate. The colour change for all three indicators is green to violet and the standard electrode potentials are all ca 0.78 V. According to Kolthoff and Sandell, this should lie between the electrode potentials of the two reduction reactions. This not being the case, phosphoric acid is added to reduce the electrode potential for the Fe3+ → Fe2+ reaction by stabilising the ferric ion. 

Method 
Prepare a standard dichromate solution by dissolving an accurately weighed sample of about 0.4 g in water and make up to 100 cm3 in a volumetric flask. Into flasks or beakers weigh out accurately duplicate portions of about 0.7 g of the iron(II) solid `M' provided. Add 30 cm3 of dil. sulfuric acid, 100 cm3 of water, 7 cm3 of 85% phosphoric acid and 5 drops of diphenylamine sulfonate indicator. Titrate with dichromate to a purple colour.