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Chemistry

Mohr's Salt

Mohr's Salt

Q: 2. Why is Mohr's salt classified as a double salt and not a complex salt?

Mohr's salt is classified as a double salt because when dissolved in water, it completely dissociates into its constituent simple ions: Fe²⁺, NH₄⁺, and SO₄²⁻. In contrast, a complex salt contains a complex ion that remains intact in solution and does not break down into smaller ions. Since Mohr's salt fully ionises, it fits the definition of a double salt.

Q: 3. What are the key physical and chemical properties of Mohr's salt?

The key properties of Mohr's salt are:Appearance: It is a light green or bluish-green crystalline solid at room temperature.Structure: It forms crystals with a monoclinic geometry.Molar Mass: The hexahydrate form has a molar mass of approximately 392.14 g/mol.Solubility: It is soluble in water, forming an acidic solution that turns blue litmus paper red.Stability: It is highly stable and resistant to oxidation by air, unlike ferrous sulfate.

Q: 7. What are the primary applications of Mohr's salt in analytical chemistry?

The main application of Mohr's salt is as a primary standard in volumetric analysis, specifically in redox titrations. Its high purity and exceptional stability in air make it a preferred source of ferrous ions (Fe²⁺) for titrating against oxidising agents like potassium permanganate (KMnO₄). It is also used in the Fricke's dosimeter to measure high doses of gamma radiation.

Reviewed by:

Ritika Singla

Introduction

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:

S. No	Name of the Alternative	Definition	Taste Profile	Benefits
1	Sea Salt	Sea salt is defined as the term salt produced by evaporating water from an ocean. It is one of the oldest methods of conciliating Salt.	Flavour is stronger than table salt, so use it less to create a similar taste profile in the dishes. It holds a brighter and cleaner favour than table salt as it does not contain bitter iodine and it tastes great with steaks.	Because generally, less is more when it comes to sea salt, it delivers less sodium level than the common table salt. Sea salt can be unprocessed and delivers a plethora of minerals from the location where it was harvested.
2	Himalayan Salt	Himalayan Salt arrives from the world's oldest salt mine, which is located in Pakistan (the great Salt Range). The pink color of Himalayan Salt is defined to come from the remains of sea life, which existed in an evaporated ocean.	It holds a more subtle taste when compared to sea salt. It tastes great with delicate dishes with fish, poultry and vegetables.	It has around 84 different minerals, including iron, sodium, phosphorus and magnesium. It does not contain iodine that is difficult to digest.
3	Indian Black Salt (Kala Namak)	It looks like a rock salt made out of South Asian condiments. The magical black colour comes from the iron sulphide.	Black Salt contains a savoury taste which is the same as an egg yolk. It mixes well with chutneys, yogurts and cooling drinks.	It has 3% of iron and is used for anemiatreatments. Studies have linked daily consumption of Black Salt with constipation reduction.
4	Coconut aminos	Coconut aminos is a condiment that is similar to soya sauce. It is made out of the coconut blossom by tapping the unopened flowers and fermenting them with a bit of added Salt.	Dark, sweet and salty. It has umami qualities. Used in fish marinades and sauces, it goes well with Japanese cuisine.	It is packed with 17 different amino acids, potassium vitamins - C & B.
5	Tamari	Tamari is the other soya sauce substitute. It can be pressed from the liquid which is drained from the fermented soybeans. Tamari has much fewer ingredients compared to the soya sauce (it does not contain wheat) and it is a less processed condiment.	Tamari contains a much richer and less salty flavor compared to the soya sauce. It is packed with umami and is used to add 'meatiness' to all the vegetarian dishes. It tastes great in stir-fries, marinades and Asian sauces and on its own as a dipping sauce.	It contains different vitamins - B2, B3, B6, and minerals as well: Phosphorus, iron and manganese. Due to the content of nutrients also present in the fermented foods (oligosaccharides and peptides), it helps for good digestion.
6	Nutritional Yeast	It is prepared with 'Saccharomyces cerevisiae' baking yeast. The nutritional yeast is deactivated by the heating and drying process, followed by it.	It has a cheesy, nutty and salty/umami flavor, which is similar to Parmesan cheese.	It is a good vitamin source with the vitamins - B1, B2, B3, zinc and potassium. Mostly the nutritional yeast is additionally fortified with folate and vitamin B12.
7	Kelp	Kelp is given as a large and brown type of seaweed, which grows in shallow, salty seawater across the world.	It contains a natural sea-salty, umami and savoury flavor. It could also be used as a meat substitute. It is great in Asian cuisine, which is added to soups and blended into smoothies.	A natural source of iodine, Vitamin A (eye vitamin) and Vitamin K1, magnesium, folate, calcium and iron. Kelp has been linked to several health benefits due to its weight-loss, disease-fighting and anemia-preventing properties.

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.

FAQs on Mohr's Salt

1. What is Mohr's salt, and what is its chemical formula?

Mohr's salt is an inorganic double salt with the chemical name ammonium iron(II) sulfate. Its chemical formula is (NH₄)₂Fe(SO₄)₂(H₂O)₆. It is composed of two different cations, the ferrous ion (Fe²⁺) and the ammonium ion (NH₄⁺), along with the sulfate anion (SO₄²⁻), making it a stable crystalline compound.

2. Why is Mohr's salt classified as a double salt and not a complex salt?

Mohr's salt is classified as a double salt because when dissolved in water, it completely dissociates into its constituent simple ions: Fe²⁺, NH₄⁺, and SO₄²⁻. In contrast, a complex salt contains a complex ion that remains intact in solution and does not break down into smaller ions. Since Mohr's salt fully ionises, it fits the definition of a double salt.

3. What are the key physical and chemical properties of Mohr's salt?

The key properties of Mohr's salt are:

Appearance: It is a light green or bluish-green crystalline solid at room temperature.
Structure: It forms crystals with a monoclinic geometry.
Molar Mass: The hexahydrate form has a molar mass of approximately 392.14 g/mol.
Solubility: It is soluble in water, forming an acidic solution that turns blue litmus paper red.
Stability: It is highly stable and resistant to oxidation by air, unlike ferrous sulfate.

4. How are crystals of Mohr's salt prepared in a laboratory?

The preparation of Mohr's salt involves dissolving equimolar quantities of hydrated ferrous sulfate (FeSO₄·7H₂O) and ammonium sulfate ((NH₄)₂SO₄) in water that contains a small amount of dilute sulfuric acid. The resulting solution is then gently heated to concentrate it, followed by slow cooling. This process, known as crystallisation, yields light green, octahedral crystals of Mohr's salt.

5. What is the purpose of adding dilute sulfuric acid during the preparation of Mohr's salt?

Dilute sulfuric acid is added during the preparation of Mohr's salt primarily to prevent the hydrolysis of the ferrous sulfate. Without the acid, the ferrous ions (Fe²⁺) would react with water, forming unwanted precipitates. The acidic medium also suppresses the oxidation of the Fe²⁺ ions to ferric ions (Fe³⁺) by atmospheric oxygen, ensuring the purity of the final product.

6. What is unique about the crystal structure of Mohr's salt?

The unique aspect of Mohr's salt's structure is that it belongs to a family of isomorphous compounds called Tutton's salts. Its monoclinic crystal lattice is highly organised, featuring central octahedral complexes of [Fe(H₂O)₆]²⁺. These centres are interconnected through strong hydrogen bonds with the surrounding sulfate and ammonium ions, contributing to its remarkable stability.

7. What are the primary applications of Mohr's salt in analytical chemistry?

The main application of Mohr's salt is as a primary standard in volumetric analysis, specifically in redox titrations. Its high purity and exceptional stability in air make it a preferred source of ferrous ions (Fe²⁺) for titrating against oxidising agents like potassium permanganate (KMnO₄). It is also used in the Fricke's dosimeter to measure high doses of gamma radiation.

8. Why is Mohr's salt preferred over ferrous sulfate for use in titrations?

Mohr's salt is preferred over ferrous sulfate primarily due to its superior stability against oxidation. Ferrous sulfate solutions are notoriously unstable and are easily oxidised by air, converting the ferrous (Fe²⁺) ions to ferric (Fe³⁺) ions. Mohr's salt is much more resistant to this atmospheric oxidation, meaning its solutions maintain a consistent and accurate concentration for longer, making it a more reliable reagent in quantitative analysis.

9. How is the n-factor of Mohr's salt determined for redox reactions?

The n-factor (or valency factor) in a redox reaction represents the number of electrons lost or gained per molecule. In a typical titration with an oxidising agent, only the ferrous ion (Fe²⁺) in Mohr's salt participates in the reaction. It gets oxidised to the ferric ion (Fe³⁺) by losing exactly one electron. Since only one electron is transferred per formula unit of Mohr's salt, its n-factor is 1.