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.

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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

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

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 sulphuric acid

Apparatus Setup

Laboratory Preparation of Ferrous Ammonium Sulphate – Mohr’s Salt

Procedure

1. Weigh 7 g of ferrous sulfate and 3.5 g of ammonium sulfate separately.
2. Mix ferrous sulfate and ammonium sulfate in water in a beaker containing dilute sulphuric 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

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 instead of green the experiment should be repeated.

Notes on Mohr’s Salt

• Dilute sulphuric acid is added to prevent the hydrolysis of ferrous sulphate.
• For dissolving the mixture of salts in water excessive heating is avoided. This is for the prevention of Fe2+ ions (light green) to Fe3+ ions (yellow). In case a yellow coloured solution is obtained, the experiment should be repeated.
• In case the crystals do not separate on cooling the concentrated solution a few crystals of mohr’s salt are added. This is called “seeding/’.

Applications of Mohr’s Salt

• Ferrous Ammonium Sulfate is in the manufacture of iron blue; in the metal industry as an ingredient of brass colouring baths and iron plating solutions; and as a substitute for ferrous sulfate in a number of fields.
• Correction of an iron deficiency can be accomplished by including iron sulfate, chelated iron, or ferrous ammonium sulfate with pesticide spray applications or by use of complete analysis fertilizers, which contain iron.
• Ferrous ammonium sulfate, the most common and usually the least expensive, contains 21 percent iron. Ferrous sulfate is the quickest-acting material; it improves turf colour within a couple of days, but the improvement does not last long.

Mohr’s Salt Known as Double Salt

The salts which contain more than one simple salt are called double salts. These salts undergo complete dissociation in the aqueous solutions. They give reactions of all ions present in both the simple salts of all the ions present in both the simple salts.

The name alum is given to the special series of double salts, which correspond to a general formula X2SO4.Y2(SO4)3.24H2O where X stands for monovalent metal ion while Y stands for trivalent metal ion. X can be either Na, K or NH4 while Y can be Al, Fe or Cr etc.

• Potash alum – K2SO4.Al2(SO4)3.24H2O
• Soda alum – Na2SO4.Al2(SO4)3.24H2O
• Chrome alum – K2SO4.Cr2(SO4)3.24H2O

However, it may be mentioned here that a universal or monovalent element, Lithium does not form any alum because of its small size and thus cannot be accommodated without the loss of stability of structure.