Mohr's salt

Mohr's salt, also known as ammonium iron(II) sulfate, is a double salt of ammonium sulfate and iron(II) sulfate with the chemical formula (NH₄)₂Fe(SO₄)₂·6H₂O. It is named after the German chemist Karl Friedrich Mohr, who made significant contributions to the field of analytical chemistry in the 19th century. Mohr's salt is an important compound in analytical chemistry, particularly in titration techniques where it serves as a primary standard for the determination of oxidizing agents due to its high purity and stability.

Properties

Mohr's salt appears as a pale blue-green or white crystalline solid that is highly soluble in water. Its stability against oxidation by air makes it a preferred source of ferrous ions (Fe²⁺) in the preparation of solutions for redox titration. This stability is attributed to the presence of the ammonium ion, which prevents the oxidation of ferrous (Fe²⁺) to ferric (Fe³⁺) ions by atmospheric oxygen.

Preparation

Mohr's salt is prepared by dissolving an equimolar mixture of hydrated iron(II) sulfate (FeSO₄·7H₂O) and ammonium sulfate ((NH₄)₂SO₄) in water and then crystallizing the product. The reaction can be represented by the equation:

FeSO₄·7H₂O + (NH₄)₂SO₄ → (NH₄)₂Fe(SO₄)₂·6H₂O + H₂O

The crystallization process involves cooling the solution, after which the crystals of Mohr's salt are formed, separated, and dried.

Applications

Mohr's salt is widely used in quantitative analysis and titration methods in chemistry laboratories. Its primary use is as a standard in the titration of oxidizing agents in a process known as redox titration. The ferrous ion in Mohr's salt reacts with oxidizing agents, and the endpoint of the titration can be determined using an appropriate indicator. Mohr's salt is also used in the preparation of other iron compounds and as a fertilizer due to its iron content.

Safety

Handling Mohr's salt requires standard laboratory precautions. It is advisable to wear protective clothing, gloves, and eye protection to avoid contact with skin and eyes. Mohr's salt is not considered highly toxic, but ingestion or inhalation of dust should be avoided.