Ammonium iron(II) sulfate

Ammonium iron(II) sulfate, also known as Mohr's salt, is a double salt of iron(II) sulfate and ammonium sulfate with the formula (NH₄)₂Fe(SO₄)₂·6H₂O. It is a common laboratory reagent used in various chemical applications.

Chemical Properties

Ammonium iron(II) sulfate is a light green crystalline solid. It is a member of the class of compounds known as double salts, which are formed by the combination of two different salts that crystallize together in a fixed ratio. The compound is stable in air, unlike many other iron(II) salts, due to the presence of ammonium ions which help to stabilize the iron(II) state.

Crystals of Mohr's salt

Structure

The structure of ammonium iron(II) sulfate consists of octahedral [Fe(H₂O)₆]²⁺ ions, sulfate ions, and ammonium ions. The iron(II) ions are coordinated by six water molecules, forming a complex ion. The sulfate ions and ammonium ions are present to balance the charge of the complex ion.

Diagram showing the bonding in Mohr's salt

Preparation

Ammonium iron(II) sulfate can be prepared by dissolving equimolar amounts of iron(II) sulfate and ammonium sulfate in water, followed by crystallization. The solution is typically heated to ensure complete dissolution of the salts, and then allowed to cool slowly to form crystals of the double salt.

Sample of ammonium iron(II) sulfate

Uses

Ammonium iron(II) sulfate is used in various applications:

• As a standard in analytical chemistry for the titration of potassium permanganate solutions.
• In photography as a reducing agent.
• In water treatment processes.

Safety

While ammonium iron(II) sulfate is generally considered safe to handle, it should be treated with care. It can cause irritation to the skin and eyes, and ingestion can lead to iron poisoning.

Related Pages

• Iron(II) sulfate
• Ammonium sulfate
• Double salt