Question

Discuss the use of potassium dichromate K2 cr2 O7 in the volumetric analysis

Answer 1

Explanation:

Potassium dichromate is also a very strong oxidizing agent (E°red = +1.33V) . However it is not as strong oxidizing agent as permanganate is (E°red = +1.51V).Still it is widely used in redox titrations because of several advantages over permanganate. Unlike potassium permanganate, potassium dichromate is available in high purity and is highly stable upto its melting point. Its aqueous solutions are not attacked by oxidisable impurities like rubber or any other organic matter and thus composition of aqueous solution does not change on keeping. The aqueous solutions are quite stable towards light. It is thus an excellent primary standard and its standard solutions can be prepared by direct weighing of an amount of it and dissolving in a known volume of distilled water.

Potassium dichromate acts as oxidizing agent in acidic medium only:

The neutral aqueous solution of Potassium dichromate is 1:1 equilibrium mixture of dichromate and chromate, a consequence of hydrolysis of dichromate ions.

Cr2O7 2– + H2O = 2 CrO4 2– + 2H+

Chromate ions are weaker oxidizing agent than dichromate. Thus oxidizing strength of dichromate is reduced in neutral solution. The above hydrolysis reaction however can be reversed by adding acid to the solution and this explains the necessity of acidic medium for the reaction.

Though the dichromate solutions are intensely orange coloured solutions and a single drop of it imparts yellow colour to a colourless solution, it can’t be used as a self indicator like KMnO4. This is because its reduction product (Cr3+) is green which hinders in the visual detection of end point by observing dichromate colour. Thus an indicator is must in this titration. The indicator should be redox active and must be properly chosen keeping in mind the electrode potential values of the reducing agent being titrated with dichromate. Suitable indicators for dichromate titrations are Diphenylamine (specifically sodium diphenylamine sulphonate)(0.2%aq. Soln) in presence of orthophosphoric acid, and N-phenylanthranilic acid (0.1% soln. in 0.005 M NaOH).