NH42FeSO42SO4

Ferrous ammonium sulfate, commonly known as Mohr's salt, is a double salt of ferrous sulfate and ammonium sulfate. Its usual formulation is the hexahydrate, with the chemical formula (NH4)2Fe(SO4)2·6H2O. It is widely used in analytical chemistry as a standard reducing agent and as a stable source of Fe2+ ions for redox titrations.

The compound appears as pale green, odorless crystals that are highly soluble in water. In aqueous solution it provides Fe2+ ions that are relatively stable against hydrolysis compared with ferrous sulfate, especially under mildly acidic conditions created by the ammonium and sulfate environments. However, Fe2+ can still oxidize to Fe3+ upon exposure to air, so solutions are typically prepared and used fresh or under controlled conditions.

Ferrous ammonium sulfate is prepared by crystallizing the hexahydrate from aqueous solutions containing Fe2+ ions and both ammonium and sulfate ions, often by combining ferrous sulfate and ammonium sulfate and allowing the mixture to crystallize. The presence of ammonium sulfate helps stabilize the Fe2+ state and promotes formation of the well-defined Mohr’s salt crystals.

In practice, Mohr's salt serves as a primary standard for redox determinations and is used to standardize oxidizing agents such as potassium dichromate or potassium permanganate. Its known stoichiometry and stability in acidic aqueous solution make it convenient for preparing accurate Fe2+ solutions.

Safety considerations are typical for iron(II) salts: handle with standard laboratory precautions to avoid ingestion or inhalation of dust, and store away from strong oxidizers and at moderate temperatures. The compound is relatively low in toxicity but should be used with normal chemical hygiene.