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Ferrous Ammonium Sulphate is also referred to as Mohr’s salt, named after German chemist Karl Fredrich Mohr. It is light green coloured sand like crystalline solid. It is a harmful substance, an inorganic compound and a double salt with the formula (NH4)2Fe (SO4)2(H2O)6. It is also a common laboratory reagent ammonium iron (II) sulphate. Its preparation is conducted with an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water containing a little of sulphuric acid. Here, we will learn more about the preparation of ferrous ammonium sulphate and discuss some important questions.
| Table of Contents |
Key Terms: double salt, equimolar, oxidation, crystallization, hydrolysis, yield.
Aim
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To prepare Ferrous Ammonium Sulphate: Mohr’s salt from ferrous sulphate and ammonium sulphate in the presence of acid.
Theory
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- Mohr’s salt is a double salt: a salt that contains an equimolar mixture of two salts with different cations but one anion.
- It is of the formula: FeSO4.(NH4)2SO4.6H2O.
- It is prepared by dissolving an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate in water containing a little sulphuric acid.
- The resulting solution is then subjected to crystallization.
- Ferrous ammonium sulphate separates from the solution in the form of crystals
- Adding sulphuric acid prevents the hydrolysis of the iron (II) cations.
FeSO4 + (NH4)2SO4 + 6H2O à FeSO4.(NH4)2SO4.6H2O
Apparatus Required
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- Beaker
- Conical flask
- Trough
- Glass rod
- Tripod stand
- Funnel
- Wire gauze
- Weighing machine
- Ferrous sulphate
- Ammonium sulphate
- Distilled water
- Dilute sulphuric acid
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Procedure
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- Weigh 3.5 g of ferrous sulphate and keep it away.
- Weigh 1.7 g of ammonium sulphate and keep it away.
- Take a conical flask of 50 ml and wash it with the distilled water.
- Add the ferrous sulphate and ammonium sulphate weighed early in 5 ml of distilled water in a conical flask.
- Add 0.5 ml of dilute hydrochloric acid in the flask to prevent hydrolysis of the iron (II) cations.
- Gently warm it to get a clear solution.
- Filter the solution obtained to clear the impurities.
- Concentrate the filtrate by heating till the concentration point is reached.
- Keep stirring the solution while heating.
- Let the mixture cool down to room temperature.
- After cooling down, we get green colored crystals of ferrous ammonium sulphate.
- Separate the crystals from the mother liquor via decantation.
- Wash them with a 1:1 mixture of water and alcohol.
- Dry the crystals, either by spreading on a porous plate or between the folds of the filter paper.
Also check: Rate of Reaction Between Sodium Thiosulphate and Hydrochloric Acid
Observation Table
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| Colour of the crystal | Light green. |
| Shape of the crystal | Octahedral. |
| Solubility in water | Soluble. |
| Action of blue litmus paper | Blue litmus turns into red. |
Results
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The yield of ferrous ammonium sulphate is ______ g.
Precautions
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- The solution should be allowed to cool slowly for obtaining good quality crystals.
- The solution should not be disturbed during cooling.
- Do not overheat the solution, as it may lead to the formation of ferric ions along with ferrous ammonium sulphate.
- Make sure the solution is green and not yellow.
- Inhalation of Mohr’s salt can cause irritation in the nose and throat, and over exposure can cause diarrhoea. Keep utmost care while conducting the experiment.
Applications of Mohr’s salt
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- It is used in the manufacture of iron blue.
- Substitute for ferrous sulphate in various fields.
- It is also used in the metal industry in iron plating solutions and brass colouring baths.
- It is used to correct iron deficiency with pesticide sprays and complete analysis of fertilizers having iron.
- It is the quickest acting material.
- It is a strong reducing agent.
- It is also used in titration.
Also read: Preparation of standard solution of oxalic acid
Things to Remember
- Mohr’s salt is a double salt.
- Dilute sulphuric acid is added to prevent hydrolysis of Iron (II) cations.
- If the crystals do not separate on cooling, a few crystals of mohr’s salt are added. This is known as seeding.
- Excessive heating is avoided.
- Experiment must be repeated if yellow solution is obtained.
- It is a common laboratory reagent.
- It is a strong reducing agent.
Sample Questions
Ques: What are double salts? Give examples. (2 marks)
Ans: Double salts are those salts that are a combination of two different salts taken together as a single substance in a definite molecular proportion. Eg: Mohr’s salt and Potash alum.
Ques: What’s the difference between a double salt and a complex salt? (2 marks)
Ans: Double salts are an equimolar mixture of two salts that have two cations but one anion and dissociates in aqueous solution whereas complex salts have one or more complex ions with metal centers and different molecules attached and do not dissociate in aqueous solution.
Ques: What is the oxidation state of Fe in Mohr’s salt? (1 mark)
Ans: Iron has a +2 oxidation state in Mohr’s salt.
Ques: Why is dilute sulphuric acid used instead of concentrated sulphuric acid? (2 marks)
Ans: Dilute sulphuric acid prevents the hydrolysis of Iron (II) cations and if we use concentrated sulphuric acid, Fe2+ will be oxidised to Fe3+. Hence, we use dilute sulphuric acid.
Ques: Why is Mohr’s salt preferred over its counterparts? (2 marks)
Ans: Mohr’s salt is the preferred source of Ferrous ions in analytical chemistry, due to its long shelf life and resistance to oxidation.
Ques: What is mother liquor? (2 marks)
Ans: The liquid remaining after the separation of the crystals from a saturated solution is known as mother liquor. It contains soluble impurities.
Ques: Why is the hot saturated solution not subjected to sudden cooling? (2 marks)
Ans: If the solution is subjected to sudden cooling down, crystals of a smaller size are formed. Allowing the solution to cool down slowly helps the crystals grow in size.
Ques: What do you understand by the term ‘seeding?’ (3 marks)
Ans: At times crystallization does not occur even after slowed cooling. To overcome this, a crystal of the same substance is placed (here: Mohr’s salt) in the saturated solution which in turn induces crystallization. It helps in the quick separation of crystals from the saturated solution. This process is known as seeding.
Ques: What is the use of Mohr’s salt? (3 marks)
Ans: The double swab of Ferrous Ammonium Sulphate is a laboratory reagent. It is used in the logical chemistry as the preferred source of Ferrous ions due to its extended shelf life and resistance to oxidation. Sulphuric acid is added to it to reduce the oxidation of ferric ions.
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