Preparation of Pure Sample of Ferrous Ammonium
Sulphate Ferrous ammonium sulphate is a double salt
of ferrous sulphate and ammonium sulphate. It has the formula FeSO4.(NH4)2SO4.6H2O. It
contains two different cations Fe2+ and NH4+. It 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 crystallization when pale green crystals
of ferrous ammonium sulphate separate out. Our aim here is to prepare a pure sample of
ferrous ammonium sulphate. Materials Required: 7 grams Ferrous sulphate, 3.5 grams ammonium
sulphate, dilute Sulphuric acid, alcohol, distilled water, cold water, china dish, funnel,
spatula, glass rod, beakers, measuring jars, crystallising dish, trough, watch glass, filter
papers, stand with clamp and hot plate. Procedure: Take about 7 grams of ferrous sulphate and
3.5 grams of ammonium sulphate in a clean 250 milliliters beaker.
Measure about 3 milliliters of dilute sulphuric acid in a measuring jar.
Add it into the beaker containing ferrous sulphate and ammonium sulphate to prevent
the hydrolysis of ferrous sulphate. Now measure about 20 milliliters of distilled
water in another measuring jar. Transfer it into another 250 milliliters beaker.
Boil the water in the beaker for about 5 minutes over the hot plate to expel the dissolved
air. Add the boiling hot water to the contents
in the first beaker in small installments at a time.
Stir the contents of the beaker with a glass rod until the salts have completely dissolved.
Then filter the solution into the china dish using the funnel and the filter paper.
The stem of the funnel should touch the wall of the china dish to avoid the solution splashing
out. After filtration the insoluble impurities
are left behind on the filter paper. Heat the solution in the china dish over the
hot plate. To avoid crust formation on the side of the
dish, stir the solution continuously with a glass rod.
When the volume of the solution is reduced to about one-half of its original volume,
take out a drop of the concentrated solution at the end of the glass rod and cool it by
blowing. Formation of thin crust indicates that crystallisation point has reached.
Now transfer the hot saturated solution into the crystallising dish.
Place the crystallising dish in a trough containing cold water.
Cover the dish with the watch glass and allow it to cool slowly for some time.
On cooling, pale green crystals of ferrous ammonium sulphate separate out.
Decant the mother liquor carefully into a beaker.
Wash the crystals 2 or 3 times with a little alcohol.
Using the spatula, remove the crystals on a filter paper which soaks the solution.
Transfer the crystals on another filter paper and dry them by pressing gently between the
folds of the filter paper. Now, transfer the crystals into a watch glass.
The crystals of pure ferrous ammonium sulpahte are pale green in colour and are monoclinic
in shape. Precautions: Cool the solution slowly to get good crystals.
Do not heat the solution for a long time as it may oxidize ferrous ions to ferric ions.