Mohr's Salt (original) (raw)

Last Updated : 23 Jul, 2025

**Mohr's Salt is named after the German chemist Karl Friedrich Mohr, who synthesized it in 1855. In analytical chemistry, Mohr's salt is frequently used as a primary standard to standardize solutions of other substances, especially when titrating permanganate solutions. In addition, it finds use as a fertilizer in agriculture and various laboratory applications. In this article, we will discuss the various concepts related to Mohr's Salt.

Mohr's-Salt

Table of Content

What is Mohr’s Salt?

Mohr's salt, also known as Ferrous Ammonium Sulfate, is a chemical compound with the formula (NH4)2Fe(SO4)2·6H2O. It is a double salt of iron(II) sulfate and ammonium sulfate. The chemical structure shows that it contains two ammonium ions (NH4⁺), one iron(II) ion (Fe²⁺), two sulfate ions (SO4²⁻), and six water molecules (H2O) in its hydrated form.

**Note: Mohr's salt is commonly utilized as a laboratory reagent due to its easy crystallization and the production of crystals that exhibit high resistance to oxidation when exposed to air.

Structure of Mohr’s Salt - IIT JEE

The Chemical Formula of Mohr's Salt is ****(NH** 4 ) 2 Fe(SO 4 ) 2 ·6H 2 O. On breaking this structure we get,

These components come together with ions to form the complex structure of Mohr's salt with a monoclinic geometry, that looks like the given below figure.

Mohrs-salt structure

Formula for Mohr's Salt

**The Chemical Formula of Mohr's Salt is (NH 4 ) 2 Fe(SO 4 ) 2 ·6H 2 O.

Preparation of Mohr's Salt

Mohr's salt ((NH4)2Fe(SO4)2·6H2O), can be prepared through a reaction between ferrous sulfate (FeSO4) and ammonium sulfate ((NH4)2SO4) in the presence of water. Here's a simple method for the preparation:

Materials Required

Procedure of Preparation

The steps to prepare Mohr's Salt is mentioned below:

**Step 1: Calculate the necessary quantities of ammonium and ferrous sulfates. These two compounds have a molar ratio of 1:2.

**Step 2: Dissolve the measured amount of ferrous sulfate in a suitable volume of distilled water. Stir the solution until the compound is completely dissolved.

**Step 3: Similarly, dissolve the measured amount of ammonium sulfate in another container with distilled water. Stir until the ammonium sulfate is fully dissolved.

**Step 4: Now, combine the two solutions into a single container, and stir thoroughly. This mixing will lead to the formation of Mohr's salt.

**Step 5: Allow the solution to cool. During cooling, Mohr's salt will begin to crystallize. The crystals can be separated from the solution through filtration.

**Step 6: Wash the crystals with a small amount of cold water to remove any impurities.

**Step 7: Dry the crystals by air-drying. Once dried, Mohr's salt can be stored in a dry place.

Chemical Reaction

The chemical reaction involved in the preparation of Mohr's Salt is as follows:

**FeSO 4 + (NH 4 ) 2 SO 4 + 6H₂O → Fe(NH 4 ) 2 (SO 4 ) 2 .6H 2 O

Properties of Mohr's Salt

Ferrous ammonium sulfate hexahydrate, also known as Mohr's salt ((NH4)2Fe(SO4)2·6H2O), has several interesting physical and chemical characteristics. The properties of Mohr's Salt can be understood easily under the following heads below:

Physical Properties

The physical properties of Mohr's Salt can be understood by the table below:

Properties of Mohr's Salt
Chemical Formula (NH4)2Fe(SO4)2·6H2O
Chemical Name Ferrous ammonium sulfate hexahydrate
Physical State Crystalline solid
Crystal Structure Monoclinic
Color Light Green
Solubility in Water Soluble (increases with temperature)
Magnetic Properties Exhibits magnetic properties due to iron(II) ions
Color Change upon Oxidation May change from green to brown upon exposure to air

Chemical Properties of Mohr's Salt

The chemical properties of Mohr's Salt are as follows:

**1. Reaction with Alkalis: Mohr's salt reacts with alkalis to form insoluble hydroxides. For example:

****(NH** 4 ) 2 Fe(SO 4 ) 2. 6H 2 O + 2NaOH → Fe(OH) 2 **+ 2(NH 4 ​) 2 SO 4 **+ 6H 2 O

**2. Formation of Ferric Salt: Mohr's salt can be oxidized to form ferric sulfate in the presence of oxidizing agents.

****(NH** 4 ) 2 Fe(SO 4 ) 2. 6H 2 O + Br 2 **→ Fe 2 (SO 4 ) 3 **+ 2NH4Br + 6H2O

**3. Decomposition upon Heating: Mohr's salt undergoes thermal decomposition upon heating, releasing ammonia, sulfur dioxide, and water vapor.

****(NH** 4 ) 2 Fe(SO 4 ) 2 .6H 2 O→FeSO 4 **+ (NH 4 ) 4 SO 4 **+ 6H 2 O

**4. Oxidation by Air: In the presence of air, the iron(II) ions in Mohr's salt can oxidize to iron(III) ions (Fe³⁺). This oxidation is often observable as a color change from green to brown.

Double Salt

A double salt is a salt that contains two or more different cations or anions.

**Learn, **Family of Salts

Uses of Mohr’s Salt

The uses of Mohr's Salt are mentioned below:

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