Mohr’s Salt

What is Mohr’s Salt?

Mohr’s Salt, also referred to as ammonium iron(II) sulfate, is an inorganic compound whose chemical formula is represented as FeSO₄(NH₄)₂SO₄.6H₂O. This compound is known to contain two primary cations, namely the ammonium cation (denoted by NH₄⁺) and the ferrous cation (denoted by Fe²⁺). Therefore, Mohr’s salt can be categorized as a double salt of ammonium sulfate and ferrous sulfate. It is important to note that Mohr’s salt is a common laboratory reagent since it readily undergoes crystallization and the crystals formed by it are quite resistant to oxidation in the presence of air.

It can be noted that Mohr’s salt is named after the German scientist Karl Friedrich Mohr. When dissolved in water, Mohr’s salt (like most other salts of ferrous sulfate) dissolves to yield an aqua complex with the chemical formula [Fe(H₂O)₆]²⁺. It can also be noted that this aqua complex has an octahedral molecular geometry. The mineral form of Mohr’s salt is commonly referred to as mohrite.

Structure of Mohr’s Salt

Mohr’s salt is known to belong to a family of double sulfates that are collectively called Tutton’s salts (or, in some cases, Schonites). All members of this family (Mohr’s salt included) are known to form crystals with a monoclinic geometry. The bonding patterns in the molecular structure of Mohr’s salt feature octahedral centres made up of [Fe(H₂O)₆]²⁺ centers. Furthermore, these centres are known to form hydrogen bonds with the ammonium and sulfate ions.

Preparation of Mohr’s Salt

The preparation of Mohr’s salt typically involves the dissolution of ammonium sulfate and hydrated ferrous sulfate (mixed in an equimolar ratio) in water that contains a small amount of sulfuric acid. This resulting solution is then subjected to a crystallization process in order to obtain light green crystals of Mohr’s salt. It can be noted that the salt undergoes ionization to release all the cations and anions present in it when it is heated. Common impurities that may be present in Mohr’s salt include nickel, magnesium, zinc, lead, and manganese. Most of these impurities are known to form isomorphous salts.

Dilute sulphuric acid is typically added in order to prevent the hydrolysis of ferrous sulphate. While dissolving the mixture of salts in water, excessive heating is generally avoided. This is done in order to prevent the conversion of Fe²⁺ ions (that are light green in colour) to Fe³⁺ ions (that are usually yellow in colour). In case a yellow coloured solution is obtained, the process must be repeated. In case the crystals do not separate on cooling, a few crystals of mohr’s salt can be added to the concentrated solution in order to promote crystal growth. This is generally referred to as “seeding’.

Properties of Mohr’s Salt

• The chemical formula of anhydrous Mohr’s salt is Fe(SO₄)(NH₄)₂(SO₄). The hexahydrate form can be represented by the chemical formula Fe(SO₄)(NH₄)₂(SO₄).6H₂O.
• The molar mass of Mohr’s salt (in its anhydrous state) is equal to 284.05 grams per mole. The hexahydrate of Mohr’s salt is known to have a molar mass of 392.13 grams per mole.
• Under standard conditions for temperature and pressure (often abbreviated to STP), the appearance of Mohr’s salt is bluish-green in colour. It exists as a crystalline solid under these conditions.
• The density of Mohr’s salt under standard conditions is equal to 1.86 grams per cubic centimetre.
• The solubility of the hexahydrate of Mohr’s salt in water is roughly equal to 269 grams per litre at STP.

Applications of Mohr’s Salt

In the field of analytical chemistry, Mohr’s salt is one of the most preferred compounds that act as a source of Fe²⁺ ions (or ferrous ions). The advantage of using this solid as a source of ferrous ions is that it has a relatively long shelf life and is also resistant to oxidation due to atmospheric exposure. The oxidation of this compound is known to occur relatively quicker when the pH of the environment is high (the medium is somewhat basic). It is important to note that the solutions of Mohr’s salt are usually slightly acidic, which can be explained by the presence of ammonium cations in them. It can also be noted that sulfuric acid can be added to solutions of Mohr’s salt in order to prevent the oxidation of the ferrous ion to the ferric ion. Another important application of Mohr’s salt is in Fricke’s dosemeter, where it helps measure high doses of gamma radiation.

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