To Prepare a Pure Sample of Ferrous Ammonium Sulphate (Mohr’s salt), [FeSO4 . (NH4)2 SO4.6HO20]
Mohr’s salt is prepared by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water containing a little of sulphuric acid, and then subjecting the resulting solution to crystallisation when light green crystals of ferrous ammonium sulphate. FeS04 . (NH4 )2S04.6H20 separate out.
Two beakers (250 ml), china-dish, funnel, funnel-stand, glass-rod, wash-bottle, tripod stand and wire-gauze. Ferrous sulphate crystals, ammonium sulphate crystals, dilute sulphuric acid and ethyl alcohol.
- Take a 250 ml beaker and wash it with water. Transfer 7.0 g ferrous sulphate and 3.5 g ammonium sulphate crystals to it. Add about 2-3 ml of dilute sulphuric acid to prevent the hydrolysis of ferrous sulphate.
- In another beaker boil about 20 ml of water for about 5 minutes to expel dissolved air.
- Add the boiling hot water to the contents in the first beaker in small instalments at a time. Stir with a glass rod until the salts have completely dissolved.
- Filter the solution to remove undissolved impurities and transfer the filtrate to a china-dish.
- Heat the solution in the china-dish for some time to concentrate it to the crystallisation point.
- Place the china-dish containing saturated solution over a beaker full of cold water. On cooling crystals of Mohr’s salt separate out.
- Decant off the mother liquor quickly. Wash the crystals in the china-dish with a small quantity of alcohol to remove any sulphuric acid sticking to the crystals.
- Dry the crystals by placing them between filter paper pads.
Weight of crystals obtained =……….. g
Expected yield = ………..g
Colour of the crystals = …………..
Shape of the crystals =…………
Note: The crystals of Mohr’s salt are monoclinic in shape.
- Cool the solution slowly to get good crystals.
- Do not disturb the solution while it is being cooled.
- Do not heat the solution for a long time as it may oxidize ferrous ions to ferric ions.