Mohr’s Salt, also known as Ferrous Ammonium Sulphate, is a light green crystalline compound commonly used in chemistry labs. It’s especially helpful in titration experiments because it gives accurate and stable results. Mohr’s Salt is made by combining ferrous sulphate and ammonium sulphate in the presence of dilute sulphuric acid. It’s called a double salt because it contains two different salts that work together without reacting with each other. This compound is often used in redox reactions, making it an important part of practical chemistry for students and researchers.

Chemical Formula and Composition of Mohr’s Salt

Mohr’s Salt has the chemical formula \(\left(\mathrm{NH}_4\right)_2 \mathrm{Fe}\left(\mathrm{SO}_4\right)_2 \cdot 6 \mathrm{H}_2 \mathrm{O}\) .This means it is made up of ammonium ions (NH4+), ferrous ions (Fe²⁺), and sulphate ions (SO₄²⁻) along with six water molecules that keep it in crystal form. It’s called a double salt because it is formed by mixing two different salts ferrous sulpahte (FeSO₄) and ammonium sulpahte ((NH₄)₂SO₄). These two work together in the same crystal without reacting with each other. The presence of water molecules makes it a hydrated salt, which gives it a fresh green colour and helps keep it stable.

Required Materials for Mohr’s Salt Preparation

• Conical flask
• Tripod stand
• Burner
• Funnel
• Watch glass
• Glass rod
• Filter paper
• Wire gauze
• China dish
• Ferrous sulfate
• Ammonium sulfate
• Dilute sulphuric acid

Laboratory Preparation of Ferrous Ammonium Sulphate – Mohr’s Salt

Steps:-

• Measure 7 g of ferrous sulfate and 3.5 g of ammonium sulfate separately.
• Combine the ferrous sulfate and ammonium sulfate in a beaker of water containing dilute sulphuric acid.
• Gently warm the solution to obtain a clear solution.
• Filter the solution to remove any suspended impurities and concentrate the clear filtrate by heating it in a china dish over a sand bath until the crystallization point is reached.
• Stir the solution occasionally during heating.
• Leave the solution undisturbed for slow cooling. After a while, crystals of ferrous ammonium sulfate will separate from the solution.
• Separate the crystals from the mother liquor by decantation and rinse the crystals with cold water.
• Dry the crystals between the folds of filter paper or by spreading them on a porous plate.

Observations from Mohr’s Salt Experiment

Precautions to take during Mohr’s Salt Experiment

• Allow for slow cooling and do not disturb the solution during cooling to obtain high-quality crystals.
• Only heat the solution for a short time during the process. Prolonged heating can cause the formation of ferric ions along with ferrous ammonium sulfate.
• If the solution is yellow instead of green, the experiment should be repeated.

Properties of Mohr’s Salt 

Physical properties

• Colour- Light green crystals
• Solubility- Easily dissolves in water
• Stability- More stable than plain ferrous sulphate due to the presence of ammonium sulphate and sulphuric acid, which prevent it from oxidizing quickly in air.

Chemical Properties

• Oxidation State: Iron is in the +2, ferrous state
• Reaction with KMnO4: It reacts with potassium permanganate in acidic medium during redox titrations, where Fe+2 is oxidised to Fe+3

Uses of Mohr’s Salt

Common Lab Applications

1. Used in redox titrations as a standard solution
2. Helps in studying reactions involving iron ions

Industrial or Analytical Uses

1. Used in chemical analysis to determine the strength of oxidizing agents
2. Sometimes used in fertilizer and dye industries for specific chemical processes

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