Action of alkali and acid on K2Cr2O7:
i. In neutral solution, water soluble orange red coloured potassium dichromate is in equilibrium with yellow coloured potassium chromate. In acid medium, chromate ion changes to dichromate ion and in basic medium, dichromate ion changes to chromate ion.
ii. Addition of alkali shifts the equilibrium to right as OH- combines with H+ to form water. Thus, chromate ions are formed which turn the solution yellow.
iii. On addition of an acid, the H+ ion concentration increases and thereby the equilibrium shifts to the left, thus resulting in the formation of dichromate ions, which turn the solution orange-red.
iv. On addition of potassium hydroxide solution to the orange red coloured solution of potassium dichromate, yellow coloured solution of potassium chromate is obtained.
v. On addition of hydrochloric acid to the yellow coloured solution of potassium chromate, orange-red coloured solution of potassium dichromate is obtained.
