Mohr’s Salt Formula, Properties and Applications Explained

Mohr’s Salt is an important inorganic compound widely used in chemistry laboratories and educational institutions. It is especially valued for its stability, purity and resistance to oxidation, which makes it a reliable source of ferrous (Fe²⁺) ions. Mohr’s Salt helps us to understand double salts, crystallisation, coordination compounds and redox reactions.This perfect guide is about Mohr's salt, how it is formed, how it behaves chemically and why it is so important in laboratory work. 

Table of Contents 

• What is Mohr’s Salt
• Properties of Mohr’s Salt
• Preparation of Mohr’s Salt
• Applications of Mohr’s Salt

What is Mohrs’s Salt

Mohr’s Salt, also known as ammonium iron(II) sulphate, is an inorganic double salt formed by the combination of ferrous sulphate and ammonium sulphate. 

Do you know? It is named after the German scientist Karl Friedrich Mohr, who contributed significantly to analytical chemistry. In nature, its mineral form is known as mohrite.

Its Mohr's Salt chemical formula is written as:

FeSO4⋅(NH4)2SO4⋅6H2O

It contains two main positive ions, the ammonium ion (NH₄⁺) and the ferrous ion (Fe²⁺). Because it is formed from two different salts that crystallise together, it is called a double salt. When dissolved in water, Mohr’s Salt breaks into its individual ions rather than remaining as a single complex compound.

When it comes to appearance Mohr’s Salt is well known for its pale green crystals and high stability in air. Unlike normal ferrous sulphate, it does not oxidise easily, which makes it very useful in laboratory experiments. 

What about the structure of Mohr’s salt?

Mohr’s Salt belongs to a special group of double salts called Tutton’s salts. These salts share a similar crystal structure and form well-defined, stable crystals. The structure of Mohr’s Salt is based on hydrated ferrous ions.

At the centre of the structure is the [Fe(H₂O)₆]²⁺ unit, where one Fe²⁺ ion is surrounded by six water molecules in an octahedral arrangement.

What's interesting is that this compound has a stable and symmetrical structure. These hydrated iron units are connected to sulphate ions and ammonium ions through hydrogen bonding.

The presence of water molecules plays a major role in stabilising the crystal structure. This organised arrangement of ions and water molecules gives Mohr’s Salt its strength, stability and resistance to oxidation.

Properties of Mohr’s Salt

Physical Properties 

Property	Details
Appearance	Pale green or bluish-green crystalline solid
Stability	Stable in air and resists oxidation
Solubility	Soluble in water
Formula (Anhydrous)	FeSO₄·(NH₄)₂SO₄
Formula (Hexahydrate)	FeSO₄·(NH₄)₂SO₄·6H₂O
Molar Mass (Anhydrous)	284.05 g/mol
Molar Mass (Hexahydrate)	392.13 g/mol

Chemical Properties

• It behaves as a source of Fe²⁺ ions in solutions.
• It forms slightly acidic solutions due to ammonium ions.
• It undergoes oxidation in a basic medium.
• It shows typical reactions of ferrous salts.
• Under standard temperature and pressure (STP), Mohr’s salt appears as a bluish-green crystalline solid.
• The density of Mohr’s salt at STP is 1.86 g/cm³.
• The solubility of hexahydrate Mohr’s salt in water is approximately 269 g/L at STP, showing that it is highly soluble in water.

Preparation of Mohr’s Salt

Mohr’s salt is prepared in the laboratory in following way:

• By dissolving hydrated ferrous sulphate (FeSO₄·7H₂O) and ammonium sulphate ((NH₄)₂SO₄) in water in an equimolar ratio. 
• The water used for dissolving the salts always contain a small amount of dilute sulphuric acid (H₂SO₄). 
• This acidic medium is very important because it prevents the hydrolysis of ferrous sulphate and protects ferrous ions (Fe²⁺) from oxidation.
• During preparation, the mixture is dissolved gently in water and excessive heating is strictly avoided. Heating can cause Fe²⁺ ions (which are light green in colour) to oxidise into Fe³⁺ ions (which produce a yellow colour), leading to impurity formation and failure of proper crystal formation.
• If the solution becomes yellow, the preparation process must be repeated to obtain pure Mohr’s salt.Once the salts are completely dissolved, the clear solution is concentrated carefully and then allowed to cool slowly.
• On cooling, light green crystalline solids of Mohr’s salt begin to separate out due to crystallisation. These crystals are then filtered, washed with a small amount of cold water and dried.If crystals do not separate easily during cooling, a few small crystals of already prepared Mohr’s salt are added to the solution. This process is known as seeding and it helps in initiating and promoting crystal growth.
• When Mohr’s salt is heated strongly, it undergoes ionisation, releasing all the cations and anions present in its structure.
• Common impurities that may be present during preparation include nickel, magnesium, zinc, lead and manganese, most of which form isomorphous salts (salts having similar crystal structures), making purification important.

Applications of Mohr’s Salt

Mohr’s Salt has wide importance in both education and scientific research.

• It is widely used in analytical chemistry as a standard source of Fe²⁺ ions.
• It is used in redox titrations because of its stability and accuracy.
• It is preferred in laboratories because it does not oxidise easily in air.
• It is used in Fricke’s dosimeter to measure gamma radiation.
• It helps study double salts, crystallisation and coordination compounds.
• It is used in research laboratories for controlled iron reactions.

Because of its stable nature and simple structure, it is ideal for school and college level experiments.

Read More: Acid-Base Titration and Acid Test

As we have learned, Mohr's Salt is a scientifically important compound that connects theory with practical laboratory chemistry. From analytical chemistry to radiation measurement and educational demonstrations, Mohr's salt plays a vital role in science and learning.