Preparation of Ferrous Ammonium Sulphate

Ferrous Ammonium Sulphate is also called Mohr’s salt a light green color sand like crystalline solid. It is a hazardous substance and preparation of ferrous ammonium sulfate is carried out from equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water; containing a little of sulphuric acid.

Aim

To prepare Mohr’s salt: ferrous ammonium sulphate from ferrous sulfate and ammonium sulfate in the presence of acid.

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.

Materials Required

  1. Conical flask
  2. Tripod stand
  3. Burner
  4. Funnel
  5. Watch glass
  6. Glass rod
  7. Filter paper
  8. Wire gauze
  9. China dish
  10. Ferrous sulfate
  11. Ammonium sulfate
  12. Dilute sulfuric acid

Apparatus Setup

Preparation of Ferrous Ammonium Sulphate (Mohr's Salt)

Laboratory Preparation of Ferrous Ammonium Sulphate – Mohr’s Salt

Procedure

  1. Weigh 7g of ferrous sulfate and 3.5g of ammonium sulfate separately.
  2. Mix ferrous sulfate and ammonium sulfate in water in a beaker containing dilute sulfuric acid.
  3. Gently warm the solution in order to get a clear solution.
  4. 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.
  5. Stir the solution occasionally during heating.
  6. Keep the solution undisturbed for slow cooling. After some time crystal of ferrous ammonium sulfate will get separated from the solution.
  7. Separate the crystals by decantation from the mother liquor and wash the crystals with cold water.
  8. 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

Results and Discussion

The yield of ferrous ammonium sulfate or Mohr’s salt is _______ gm.

Precautions

  • Allow slow cooling and do not disturb the solution during cooling in order to get good quality crystals.
  • During the process heating of the solution should be done in a short time only. Because prolonged heating forms ferric ions along with ferrous ammonium sulfate.
  • Suppose if the solution is yellow in colour instead of green the experiment should be repeated.

Viva Questions

1. What are double salts? Give two examples

Ans: A combination of two different salts taken together as a single substance in a definite molecular proportion is known as double salt.

Eg:- Mohr’s salt and potash alum.

2. What is the oxidation state of Fe in Mohr’s salt?

Ans: Iron has a +2 oxidation state in Mohr’s salt.

3. If a saturated solution is evaporated quickly then what will happen?

Ans: We will not get pure and good crystals.

4. What will happen when concentrated sulfuric acid is added instead of dilute sulfuric acid?

Ans: Concentrated sulfuric acid will oxidize Fe2+ to Fe3+.

5. What is the difference between double salt and complex salt?

Ans: Double salt is simple salt which dissociates in aqueous solutions. Complex salt may or may not be simple but does not dissociate in aqueous solution.

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