Mohr’s salt

Mohr’s salt, chemically known as ammonium iron(II) sulphate hexahydrate, is an important double salt with the formula (NH₄)₂Fe(SO₄)₂·6H₂O. It is widely used in analytical chemistry as a primary standard for titrations involving oxidising agents, particularly in redox reactions. Named after the German chemist Karl Friedrich Mohr, it has played a significant role in both laboratory practice and educational demonstrations due to its stability and ease of handling.

Chemical Composition and Structure

Mohr’s salt is composed of two simple salts—ammonium sulphate and ferrous sulphate—crystallised together in a 1:1 molar ratio along with six molecules of water of crystallisation. The chemical structure can be represented as:
(NH₄)₂Fe(SO₄)₂·6H₂O
It belongs to the class of ferrous double salts and crystallises in the monoclinic system. The compound contains Fe²⁺ ions (iron in the +2 oxidation state), which are coordinated with sulphate ions and water molecules. The presence of ammonium ions stabilises the ferrous ions, preventing their oxidation to ferric ions (Fe³⁺) when exposed to air.

Preparation of Mohr’s Salt

Mohr’s salt is prepared by the crystallisation method from a mixture of ferrous sulphate and ammonium sulphate solutions. The general laboratory preparation procedure is as follows:

1. Dissolution – A known amount of ferrous sulphate (FeSO₄·7H₂O) and ammonium sulphate ((NH₄)₂SO₄) are dissolved separately in distilled water containing a small amount of dilute sulphuric acid to prevent hydrolysis and oxidation.
2. Mixing – The two solutions are mixed in equimolar proportions under gentle heating.
3. Crystallisation – The solution is then allowed to cool slowly, during which pale green crystals of Mohr’s salt form.
4. Filtration and Drying – The crystals are filtered, washed with small quantities of acidulated water, and dried between filter papers.

The addition of sulphuric acid is essential during preparation as it minimises the oxidation of Fe²⁺ to Fe³⁺ by atmospheric oxygen.

Physical Properties

Mohr’s salt appears as pale green, monoclinic crystals. Its main physical properties are:

• Molecular mass: 392.14 g·mol⁻¹
• Colour: Pale green
• Solubility: Soluble in water but insoluble in alcohol
• Melting point: Loses water of crystallisation around 100°C and decomposes at higher temperatures
• Odour: Odourless
• Taste: Slightly metallic

The solution of Mohr’s salt in water is faintly acidic due to hydrolysis of Fe²⁺ ions. However, the acidity is low enough to maintain the compound’s stability.

Chemical Properties

The key feature of Mohr’s salt is its stability towards oxidation, which distinguishes it from ferrous sulphate. In aqueous solution, it dissociates into its constituent ions:
(NH₄)₂Fe(SO₄)₂·6H₂O → 2NH₄⁺ + Fe²⁺ + 2SO₄²⁻ + 6H₂O
Some important chemical reactions include:

1. Reaction with Potassium Permanganate (KMnO₄): Used in redox titrations, where Fe²⁺ is oxidised to Fe³⁺, and MnO₄⁻ is reduced to Mn²⁺.

   5Fe2++MnO4−+8H+→5Fe3++Mn2++4H2O5Fe^{2+} + MnO_4^- + 8H^+ → 5Fe^{3+} + Mn^{2+} + 4H_2O5Fe2++MnO4−​+8H+→5Fe3++Mn2++4H2​O

2. Reaction with Dichromate (K₂Cr₂O₇): Another redox reaction where ferrous ions are oxidised by dichromate ions in acidic medium.

   6Fe2++Cr2O72−+14H+→6Fe3++2Cr3++7H2O6Fe^{2+} + Cr_2O_7^{2-} + 14H^+ → 6Fe^{3+} + 2Cr^{3+} + 7H_2O6Fe2++Cr2​O72−​+14H+→6Fe3++2Cr3++7H2​O

3. Thermal Decomposition: On heating, Mohr’s salt loses its water of crystallisation and eventually decomposes to give ferric oxide (Fe₂O₃), sulphur dioxide, and ammonium sulphate.

These reactions demonstrate the dual nature of Mohr’s salt as both a stable storage form of Fe²⁺ and a reliable reducing agent in chemical analysis.

Uses and Applications

Mohr’s salt holds extensive importance in chemistry laboratories and industry. The major uses include:

• As a Primary Standard: Mohr’s salt is employed in standardising potassium permanganate and potassium dichromate solutions because it is stable, pure, and non-hygroscopic.
• In Redox Titrations: Commonly used in volumetric analysis to determine the concentration of oxidising agents, including permanganates and dichromates.
• Analytical Chemistry Education: Due to its ease of preparation and predictable behaviour, it serves as a standard reagent in laboratory practicals and educational demonstrations.
• Source of Ferrous Ions: Used in various reactions where a controlled release of Fe²⁺ ions is required.
• Prevention of Oxidation: In some applications, Mohr’s salt is preferred over ferrous sulphate because it resists oxidation and remains stable under ordinary laboratory conditions.

Advantages over Ferrous Sulphate

Mohr’s salt offers several advantages that make it preferable to simple ferrous sulphate in analytical use:

• Greater Stability: The ammonium ions help stabilise Fe²⁺, reducing oxidation.
• Purity: It crystallises easily and can be obtained in a pure form.
• Ease of Handling: Less prone to oxidation and deliquescence, making it easier to weigh accurately.
• Consistency: Solutions maintain a consistent Fe²⁺ concentration over time.

These characteristics make Mohr’s salt a reliable primary standard for quantitative analytical work.

Storage and Handling

To preserve its stability, Mohr’s salt should be stored in airtight containers away from direct sunlight and moisture. Acidified water is often used to prepare its solutions to prevent oxidation. Long exposure to air can gradually oxidise Fe²⁺ to Fe³⁺, leading to the formation of basic ferric sulphate and brown discolouration of the crystals.