4. Why it is necessary to add few drops of cone. H2,So,4in the preparation of F.A.S. solution?
Ans.
Answers
Theory
The formula for ferrous ammonium sulfate is FeSO4.(NH4)2SO4.6H2O. It is prepared by dissolving equimolar mixture of hydrated ferrous sulfate and ammonium sulfate in water containing a little sulfuric acid. The solution is subjected to crystallization, ferrous ammonium sulfate separates out from the solution.
The chemical reaction is given below.
FeSO4 + (NH4)2SO4 + 6H2O → FeSO4.(NH4)2SO4.6H2O (Mohr’s salt)
The addition of sulfuric acid in this experiment prevents the hydrolysis of this salt. Ferrous ammonium sulfate is a pale green crystalline compound which does not effloresce like ferrous sulfate. It is less readily oxidised than FeSO4 and therefore, a better volumetric reagent in preference to ferrous sulfate.
Mohr’s salt is also called as double salt which contain more than one simple salt. It undergoes complete dissociation in aqueous solutions. Mohr’s salt dissociates into Fe2+, NH4+, SO42- ions and give their individual chemical test.
Mohr’s salt is light green transparent octahedral crystals. It is soluble in water giving acidic solution which turns blue litmus paper red and gives effervescence with sodium bicarbonate evolving carbon dioxide.
Preparation of Mohr's Salt
Preparation of Mohr’s Salt
Ferrous Ammonium Sulphate also called mohr’s salt 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 crystallization.
Materials Required
Conical flask
Tripod stand
Burner
Funnel
Watch glass
Glass rod
Filter paper
Wire gauze
China dish
Ferrous sulfate
Ammonium sulfate
Dilute sulphuric acid
Apparatus Setup
Preparation of Ferrous Ammonium Sulphate (Mohr's Salt)
Laboratory Preparation of Ferrous Ammonium Sulphate – Mohr’s Salt
Procedure
Weigh 7g of ferrous sulfate and 3.5g of ammonium sulfate separately.
Mix ferrous sulfate and ammonium sulfate in water in a beaker containing dilute sulphuric acid.
Gently warm the solution in order to get a clear solution.
Filter the solution in order to remove suspended impurities and concentrate the clear filtrate by heating it china dish over a sand bath till the crystallization point is reached.
Stir the solution occasionally during heating.
Keep the solution undisturbed for slow cooling. After some time crystal of ferrous ammonium sulfate will get separated from the solution.
Separate the crystals by decantation from the mother liquor and wash the crystals with cold water.
Dry the crystals between the folds of filter paper or by spreading on a porous plate.
Observations
Colour of the crystal Light green
Shape of the crystal Octahedral
Solubility in water Soluble
Action of blue litmus paper Blue litmus turns red