Prolonged heating is avoided during the preparation of ferrous...

The formula for ferrous ammonium sulphate is ${FeSO}_{4} . ( {NH}_{4}{)}_{2}{SO}_{4} . 6{H}_{2}O$. It is prepared by dissolving equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water containing a little sulphuric acid. The solution is subjected to crystallisation, ferrous ammonium sulphate separates out from the solution.

The chemical reaction is given below.

${FeSO}_{4} + ( {NH}_{4}{)}_{2}{SO}_{4} + 6{H}_{2}O \to {FeSO}_{4} . ( {NH}_{4}{)}_{2}{SO}_{4} . 6{H}_{2}O ( Mohr ’ ssalt )$

Avoid prolonged heating while preparing crystals of ferrous ammonium sulphate, as it may oxidise ferrous ions to ferric ions and change the stoichiometry of the crystals.