Mohr salt is a double salt containing ferrous sulphate and ammonium sulphate in equimolar amounts. Hence, when these two salts are mixed in the ratio of their molecular masses and then a hot saturated solution prepared, filtered and the hot filtrate cooled, light blue or green crystals of Mohr salt (octahedral in shape) are obtained.
FeSO4.7H2O + (NH4)2.SO4 → FeSO4(NH4)2SO4.6H2O + H2O
How To Perform Experiment
(i) Weigh 10 g of ferrous sulphate crystals and 5 g of ammonium sulphate crystals on two separate pieces of paper on a rough balances.
(ii) Transfer the amounts thus weighed into a 250 ml beaker.
(iii) Boil about 20-30 ml of distilled water in the second beaker for about 5 minutes to remove the dissolved air (as otherwise, it may oxidize ferrous sulphate to ferric sulphate in presence of sulphuric acid).
(iv) Add the boiling water to the contents of the first beaker. Immediately add 2-3 ml of dil HASO4 (to check the hydrolysis of ferrous sulphate by the boiling water).
(v) Stir the contents of the above beaker till the salts dissolve.
(vi) Filter the above solution directly into a china dish to remove the suspended or the undissolved impurities.
(vii) Heat the china dish containing the solution on a wire gauge or a sand bath to concentrate it till the crystallization point is reached. This is tested by dipping an end of a glass rod into the hot solution and then blowing air over it from the mouth to cool it. If a solid crust is formed on the rod, the crystallization point is reached.
(viii) Allow the china dish to cool either directly in the air (for slow cooling to get big sized crystals) or by placing it over the mouth of a beaker filled with water (for fast cooling if small sized crystals are required). In the latter case, the water of the beaker has to be replaced 2-3 times. However in either case, the china dish must be kept undisturbed during cooling. Light green or blue coloured crystals will be obtained.
(ix) Separate the crystals from the mother liquor by decantation.
(x) Wash the crystals with a little alcohol to remove the sulphuric acid sticking to the surface of the crystals.
(xi) Transfer the crystals on a pad of filter papers or a clean porous plate and dry them by pressing them gently with filter papers to avoid cracks in crystals.
(xii) Weigh the crystals obtained on the rough balance. Compare it with the theoretically calculated value.