Mohr's Salt

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 also be noted that Mohr's Salt is named after the German scientist named "Karl Friedrich Mohr."

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What is Mohr’s Salt?

Mohr's Salt, also known as ammonium iron (II) sulfate, is an inorganic compound having the chemical formula or the mohr salt formula or the ferrous ammonium sulphate formula as (NH4)2Fe(SO4)2(H2O)6. This compound is well-known to contain two primary cations, namely the ferrous cation (denoted by Fe₂⁺) and ammonium cation (denoted by NH4+). Thus, Mohr's Salt can be categorized as a double salt of ferrous sulfate and ferrous ammonium sulphate.

When dissolved in water, Mohr's Salt (like most of the other salts of ferrous sulfate) dissolves to yield an aquo complex having the chemical formula or the mohr salt formula or the ferrous ammonium sulphate formula as [Fe(H2O)6]²⁺. This is also called the chemical formula of Mohr Salt. Also, it is to note that this aquo complex has an octahedral molecular geometry and commonly, the mineral form of Mohr's Salt is referred to as mohrite.

Structure of Mohr's Salt

Let us look at the structure of Mohr's Salt.

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

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Preparation of Mohr's Salt

Typically, the preparation of the Mohr's Salt involves the dissolution of hydrated ferrous sulfate (mixed in an equimolar ratio) and ferrous ammonium sulphate in water, which contains a small amount of sulfuric acid.

After that, the resultant solution goes through a crystallization process to produce bright green Mohr's Salt crystals. Also, it can be noted that the Salt undergoes ionization to release the entire cations and anions present in it when it is heated. Common impurities, which may be present in Mohr's Salt, are given as magnesium, nickel, lead, zinc and manganese. The majority of these impurities are known to form isomorphous salts.

Typically, dilute sulphuric acid is added in order to prevent the hydrolysis of the ferrous sulphate compound. Generally, while dissolving the salt mixture in water, excessive heating is avoided. This is completed in order to prevent the conversion of Fe2+ ions (which are light green in color) to Fe3+ ions (which are usually yellow in color). If in case a yellow-colored solution is obtained, the process should be repeated. Whereas, if the crystals do not separate on cooling, some crystals of the mohr's Salt can be added to the concentrated solution in order to promote the crystal growth. In general, this is referred to as "seeding'.

Properties of Mohr's Salt

Let us look at the Properties of Mohr's Salt in detail.

• The molar mass of the Mohr's Salt or the molecular mass of Mohr Salt (in its anhydrous state) is given as 284.05 grams per mole. The hexahydrate of Mohr's Salt is known to have a molar mass of 392.13 grams per one mole.

• The chemical formula of anhydrous Mohr's Salt is given as Fe(SO4)(NH4)2(SO4) and the hexahydrate form can be represented with the chemical formula Fe(SO4)(NH4)2(SO4).6H2O.

• The density of the Mohr's Salt under standard conditions is given as 1.86 grams per cubic centimetre.

• Under the standard conditions for both temperature and pressure (which is often abbreviated to STP), the appearance of Mohr's Salt is bluish-green in color. And it exists as a crystalline solid under these specified conditions.

• The solubility of hexahydrate of the Mohr's Salt in water is roughly about 269 grams per litre at STP.

• The molar mass of FeSO4.(NH4)2SO4.6H2O or the molecular weight of ferrous ammonium sulphate is given as 392.1388 g/mol. This is also called either molar mass of ferrous ammonium sulphate or molecular formula of mohr salt.

Double Salt

As we all know, salts usually differ. The orientation and size of the crystal structure can impact the transparency or opacity, flavor, texture, properties and color of the Salt.

Double Salt is defined as the mixture of two various simple crystalline salts. By different here, it means that every Salt in the mixture has its own and unique crystal structure. Therefore, the property of the double Salt is changed from the properties of every Salt in the mixture.

A few examples of double salts are given as Tutton's Salts that are used for chemical analysis and in the understanding of light dispersion in chemicals. Further, Alum is also a double salt, which is popular for a wide range of uses, where a few of them include:

• Fire resistance for fabrics, cloths and wood

• Culinary - as a preservative in pickles

• Medicine that contains drying properties and can treat such things as the canker sores

• Fire extinguishers, which are helpful to either chemical or oil fires

Some Alternatives of Salt

Moderate amounts of Salt are not bad for humans. Sodium is an essential nutrient for the human body and fewer amounts help to maintain a good balance of both minerals and water.

Still, it is much better if you avoid the iodized table salt that is highly refined and most commonly used in fast-foods, stripped from most of the nutrient content.

You can use the 7 alternatives to table Salt, which are given below:

Complex Salt

• The simple Salt, which it is formed from, may or may not be in equimolar proportions.

• It exists in the solid-state and in an aqueous solution as well. This is due to the reason, even in the solution, the complex ion does not dissociate into the ions.

• A coordination compound will retain its identity in the aqueous solution.

• It may or may not be ionic. However, the complex part always contains coordinate bonds.

• In a coordination compound, the metal ion satisfies its two types of valencies, which are the primary and secondary valencies.

• For example, here, the K2[NiCl4] complex is always enclosed in square braskets.

Applications of Mohr's Salt

In the field of analytical chemistry, Mohr's Salt is defined as one of the most preferred compounds, which acts as a source of Fe²⁺ ions (or as ferrous ions). The advantage of using Mohr's Salt as a source of ferrous ions is that it contains a relatively long shelf life and is also resistant to oxidation due to atmospheric exposure.

The oxidation of this compound is well-known to take place relatively quickly when the pH of the environment is high (the medium is somewhat basic). And, it is most important to note that, usually, the solutions of the Mohr's Salt are slightly acidic, where it can be explained by the presence of ammonium cations in them. Also, It can be noted that sulfuric acid can be added to Mohr's Salt's solutions in order to prevent the oxidation of the ferrous ion to the ferric ion. The other important application of Mohr's Salt in Fricke's dosimetre is, it helps to measure high doses of gamma radiation.

Conclusion

Ferrous ammonium sulphate is the chemical term for Mohr's salt. Mohr's salt is made by combining equimolar amounts of ferrous sulphate and ammonium sulphate. The preparation of mohr's salt is based on the principle of crystallisation. Because it helps prevent salt hydrolysis, the preparation must be done in the presence of dilute sulphuric acid.

Color, shape, litmus, and solubility can all be used to determine that the crystals are those of mohr's salt. Mohr's salt crystals are light green in hue and octahedral in shape. Mohr's solution changes the colour of blue litmus paper to red. Mohr's salt crystals are easily soluble in water.