To Prepare a Pure Sample of Ferrous Ammonium Sulphate

Ferrous Ammonium Sulphate is used in the production of iron blue, as an ingredient in brass colouring baths and iron plating solutions, and as a substitute for ferrous sulphate in a variety of applications.

Iron deficiency can be corrected by incorporating iron sulphate, chelated iron, or ferrous ammonium sulphate into pesticide spray applications or using iron-containing complete analysis fertilisers. The most common and usually least expensive is ferrous ammonium sulphate, which contains 21% iron. The quickest-acting material is ferrous sulphate, which improves turf colour in a matter of days but does not last long.

Table of Contents

• Aim of the Experiment

• Apparatus Required

• Theory

• Procedure

• Observations

• Result

• Precautions

Aim of the Experiment

To prepare ferrous ammonium sulphate and submit it for inspection.

Apparatus Required

• Conical flask

• Tripod stand

• Burner

• Funnel

• Watch glass

• Glass rod

• Filter paper

• Wire gauze

• China dish

• Ferrous sulphate

• Ammonium sulphate

• Dilute sulphuric acid

Theory

${{Fe}{S}{O}_{4}{.}{(}{N}{H}_{4}{)}_{2}{S}{O}_{4}{.}{6}{H}_{2}{O}}$ is the formula for ferrous ammonium sulphate. It is made by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in sulfuric acid-containing water. When the solution is crystallised, ferrous ammonium sulphate separates from the solution.

Given below is the chemical reaction

${{Fe}{S}{O}_{4} {+} {(}{N}{H}_{4}{)}_{2}{S}{O}{4} {+} {6}{H}_{2}{O}}$ $\to$ ${{Fe}{S}{O}_{4}{.}{(}{N}{H}_{4}{)}_{2}{S}{O}_{4}{.}{6}{H}_{2}{O} {(Mohr's salt)}}$

In this experiment, the addition of sulfuric acid prevents the hydrolysis of this salt. Ferrous ammonium sulphate is a pale green crystalline compound that, unlike ferrous sulphate, does not effloresce. It is less easily oxidised than FeSO4 and, thus, a better volumetric reagent than ferrous sulphate.

Ferrous Ammonium Sulphate, also known as Mohr's salt, is made by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water with a trace of sulfuric acid. Crystallisation occurs in the solution.

Procedure

• Separately weigh 7 g ferrous sulphate and 3.5 g ammonium sulphate.

• In a beaker containing dilute sulphuric acid, dissolve ferrous sulphate and ammonium sulphate in water.

• Warm the solution gently to obtain a clear solution.

• Filter the solution to remove suspended impurities, then concentrate the clear filtrate by heating it in a china dish over a sand bath until crystallisation occurs.

• Occasionally stir the solution while it is heating.

• Allow the solution to cool slowly and quietly. Crystals of ferrous ammonium sulphate will form in the solution after some time.

• Decant the crystals from the mother liquor and wash the crystals with cold water.

• Dry the crystals by spreading them on a porous plate or between the folds of filter paper.

Observations

Result

The yield of ferrous ammonium sulphate, known as Mohr's salt, is measured in grams.

Precautions

• Allow for slow cooling and avoid disturbing the solution during cooling to obtain high-quality crystals.

• The heating of the solution should be done quickly during the process. Because prolonged heating produces ferric ions in addition to ferrous ammonium sulphate.

• If the solution is yellow rather than green, the experiment should be repeated.

Lab Manual Questions

1. What are double salts? 

Ans. Double salts are ionic compounds that are formed by the combination of two different salt compounds. Complex salts are ionic compounds containing a central metal atom surrounded by ligands linked to it via covalent bonds. Double salts completely dissociate in an aqueous medium.

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

Ans. The oxidation state of Fe is +2.

3. Why is dilute sulphuric acid used instead of concentrated sulphuric acid?

Ans. Dilute sulphuric acid is used because the reaction occurs in water and not sulfuric acid thus amount of sulfuric acid should be lower than in water.

4. Why is Mohr’s salt preferred over its counterparts?

Ans. To reduce the oxidation of ferric ions Mohr's salt is preferred.

Viva Questions

1. What type of salt is Mohr’s salt?

Ans. Mohr's salt can be categorised as a double salt of ammonium and ferrous sulfate. It is important to note that Mohr's salt is a common laboratory reagent since it readily undergoes crystallisation, and the crystals formed by it are quite resistant to oxidation in the presence of air.

2. What is the formula of ferrous ammonium sulphate?

Ans. $${{(}{N}{H}_{4}{)}{2}{Fe}{(}{S}{O}_{4}{)}_{2}{(}{H}_{2}{O}{)}{6}}$$

3. What is the colour of ferrous ammonium sulphate?

Ans. The colour of ferrous ammonium sulphate is light green.

4. What kind of test method is ferrous ammonium sulfate?

Ans. In common labs, ferrous ammonium sulfate is used to perform a qualitative chemical analysis to identify a solution's unknown concentration.

5. Why ferrous ammonium sulphate is called Mohr's salt?

Ans. It derives its name from the German chemist Karl Friedrich Mohr, a 19th-century pioneer in the development of titration methodology. Mohr's salt is used in analytical chemistry as the preferred source of ferrous ions due to the solid's extended shelf life and resistance to oxidation.

6. What happens when ammonium ferrous sulphate is heated?

Ans. Ferrous ammonium sulfate forms light green crystals. This salt, when heated, ionises to give all cations and anions present in it.

7. Is ferrous ammonium sulphate soluble in water?

Ans. Ferrous ammonium sulphate is dissolved in water.

8. Why mohr’s salt is a double salt?

Ans. This compound is known to contain two primary cations, namely, the ammonium cation and the ferrous cation. Therefore, Mohr's salt can be categorised as a double salt. 

9. Why Mohr's salt is used in titration?

Ans. In this titration, Mohr salt acts as a reducing agent, and potassium permanganate acts as an oxidising agent.

10. What is hydrated ferrous sulphate?

Ans. Iron sulfate heptahydrate is a hydrate that is the heptahydrate form of iron sulfate. It is used as a source of iron in the treatment of iron-deficiency anaemia (generally in liquid-dosage treatments; for solid-dosage treatments, the monohydrate is normally used).

Practical Based Questions

1. Which of the following is a double salt?

(a)Potassium ferrocyanide

(b)Potassium ferricyanide

(c)Ferrous ammonium sulphate

(d)Bleaching powder

Answer: (c)

2. The molar mass of Mohr’s salt is

(a)398 g/mol

(b)276 g/mol

(c)360 g/mol

(d)392 g/mol

Answer: (d)

3.Dilute sulphuric acid is added to the preparation of standard Mohr’s solution

(a)To make the solution acidic

(b)To prevent hydrolysis of Mohr’s salt

(c)To make the solution neutral

(d)To make the solution basic

Answer: (b)

4. The apparatus not used in the preparation of standard Mohr’s salt solution is

(a)Volumetric flask

(b)Burette

(c)Analytical electronic balance

(d)None of these

Answer: (b)

5. Is ammonium sulphate a neutral salt?

(a)True

(b)False

Answer: (b)

6. Is Ferrous ammonium sulphate a primary standard?

(a)True

(b)False

Answer: (a)

7. What is the pH of ammonium sulphate?

(a)7.1

(b)5.6

(c)4

(d)5.5

Answer: (d)

8. Is ferrous ammonium sulfate a reducing agent?

(a)True

(b)False

Answer: (a)

9. Is Ferrous ammonium sulphate harmful?

(a)True

(b)False

Answer: (a)

10. Is Ferrous ammonium sulphate a primary standard?

(a)True

(b)False

Answer: (a)

Conclusion

Ferrous Ammonium Sulphate is known as Mohr’s. It is a light green colour sand-like crystalline solid. It is a hazardous substance, and preparation of ferrous ammonium sulfate is carried out from an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water, containing a little sulphuric acid.

The formula for ferrous ammonium sulfate is $${{Fe}{S}{O}_{4}{.}{(}{N}{H}_{4}{)}_{2}{S}{O}_{4}{.}{6}{H}_{2}{O}}$$. It is made by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in sulfuric acid-containing water. When the solution is crystallised, ferrous ammonium sulphate separates from the solution.