Mohr’s Salt

It should be mentioned that Mohr’s salt is named after the German chemist Karl Friedrich Mohr, who discovered the element. When dissolved in water, Mohr’s salt (as well as most other salts of ferrous sulphate) dissolves to create an aquo complex with the chemical formula [Fe(H2O)6]2+. It should also be noted that the molecular geometry of this aquo complex is octahedral in nature. Mohrite is the mineral form of Mohr’s salt that is most typically seen in nature.

The Mohr’s salt crystal structure

Tutton’s salts, which include Mohr’s salt, are a group of double sulphates that are collectively referred to as Tutton’s salts (or, in some cases, Schonites). All members of this family (including Mohr’s salt) are known to form crystals with a monoclinic geometry, which is the most common type. The bonding patterns of Mohr’s salt’s molecular structure are characterised by the presence of octahedral centres composed of [Fe(H2O)6]2+ centres. Furthermore, it has been seen that these centres create hydrogen bonds with the ammonium and sulphate ions, respectively.

Making Mohr’s salt

Making Mohr’s Salt is a simple process.

The dissolving of ammonium sulphate and hydrated ferrous sulphate (combined in an equimolar ratio) in water containing a tiny amount of sulfuric acid is the most common method of preparing Mohr’s salt in practice. In order to obtain light green crystals of Mohr’s salt, this resulting solution is treated to a crystallisation process after which it is evaporated. It should be noted that when the salt is heated, it undergoes ionisation, causing all of the cations and anions to be released from the salt. Nickel, magnesium, zinc, lead, and manganese are all common contaminants that can be found in Mohr’s salt, as well as other elements. The majority of these contaminants have been shown to create isomorphous salts in the past.

In order to prevent the hydrolysis of ferrous sulphate, dilute sulphuric acid is commonly used as a preservative. Excessive heating is normally avoided when dissolving the salt mixture in water, unless absolutely necessary. This is done in order to prevent the conversion of Fe2+ ions (which are light green in colour) to Fe3+ ions, which would otherwise occur (that are usually yellow in colour). In the event that a yellow-colored solution is obtained, the procedure must be repeated.. It is possible to add a few crystals of mohr’s salt to the concentrated solution to encourage crystal formation if the crystals do not separate after cooling. Generally speaking, this is referred to as “seeding.”

Applications 

Mohr’s salt has a variety of applications.

In the realm of analytical chemistry, Mohr’s salt is one of the most widely used compounds as a source of Fe2+ ions because of its versatility (or ferrous ions). The advantage of employing this solid as a source of ferrous ions is that it has a relatively long shelf life and is resistant to oxidation as a result of exposure to the environment. When the pH of the surrounding environment is high, it is known that the oxidation of this substance occurs more quickly than when it is low (the medium is somewhat basic). It is crucial to note that the solutions of Mohr’s salt are typically slightly acidic, which can be attributed to the presence of ammonium cations in them, as previously stated. Another thing to keep in mind is 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 from occurring. Using Mohr’s salt in Fricke’s dosimeter, which is used to monitor high doses of gamma radiation, is another notable application of the compound.

Conclusion 

NH4+ and Fe2+ are the principal cations that are known to exist in this compound, namely the ammonium cation (denoted by NH4+) and the ferrous cation (denoted by Fe2+). So Mohr’s salt can be thought of as a double salt, consisting of ammonium sulphate and ferrous sulphate in equal amounts. Important to note is the fact that, in the laboratory, Mohr’s salt is a common reagent due to the ease with which crystallisation occurs and the fact that the crystals generated by this reagent are extremely resistant to oxidation when exposed to air.