Preparation of Ferrous Ammonium Sulphate

Introduction

Ferrous Ammonium Sulphate is an inorganic compound. It is mostly used in Analytical Chemistry. Its inhalation causes body irritation, harmful to nose and throat, as well as causes diarrhoea. It weighs around 1.86 g/cm3 in standard conditions. Mohr’s Salt solutions are slightly acidic in nature. Sulphuric acid prevents oxidation of Ferrous ions in the solution of Mohr’s Salt.

What is Ferrous Ammonium Sulphate?

Ferrous Ammonium Sulphate is also called Mohr’s Salt. It is used as a common laboratory reagent since it easily undergoes Crystallisation. Mohr’s Salt ' is commonly extracted from Mohrite mineral. Mohr’s salt belongs to the family of double Sulphates known as Tutton’s salts'. Mohr’s Salt including other family members of the family forms crystals of monoclinic geometry. The octahedral centres of Mohr’s salt forms Hydrogen bonds with Ammonium and Sulphate ions

Figure 1 − Ionic structure of Ammonium Sulphate

Ferrous Ammonium Sulphate or Mohr’s Salt is obtained as transparent octahedral crystal form normally in light green colour. It is acidic in aqueous solution hence, shows change in colour of blue litmus paper into red. It also gives effervescence (which means its forms bubbles in liquid form) when mixed with Sodium Bicarbonate.

Formation of Ferrous Ammonium Sulphate

• Ferrous Ammonium Sulphate' or Mohr’s Salt is prepared by dissolving equal moles of $\mathrm{FeSO_{4}.7H_{2}O}$ and Ammonium Sulphate into water in presence of Sulphuric acid. The mixture result in octahedral crystals of Mohr’s salt in the aqueous medium

• A clean tea cup of 250 mL is required to add solutions of 7 g of $\mathrm{FeSO_{4}}$ and 3.5 g of Ammonium Sulphate

• For hydrolysis, approximately, 2.3 mL of dilute Sulphuric acid is added to the former solution of salt mixture.

• The mixture is warmed up to obtain a clear solution.

• The solution is filtered for separation of impurities like, Magnesium, Zinc, Nickel, Manganese etc. to get clear solution of salt mixture.

• The filtrate part is now heated to get concentrated portion of salt mixture until it reaches to its crystallisation point

• The process needs a well stirring system. After crystallization of salt mixture, the solution is set to cool down at room temperature.

• The crystals of Ferrous Ammonium Sulphate salt appear in green colour

• The remain liquor portion is poured and the crystals are separated

• The green crystals are washed with alcoholic water mixture of 1:1 volume ratio. The crystals are kept for 12 hours in a sludge paper to dry

The crystals are kept for 12 hours in a sludge paper to dry

Precautions

• For the process of crystallisation, the salt mixture solution should be cooled down with extreme precautions and ease for crystallization process.

• The solution cannot be messed up during crystallisation process is going on

• One has to be careful about overheating and stoichiometric changes of the solution.

• Inhaling Mohr’s Salt may cause throat and nose infections and diarrhea.

Properties of Ammonium Sulphate

• $\mathrm{Fe(SO_{4})(NH_{4})_{2}(SO_{4})}$ is the formula of Anhydrous Mohr’s salt.

• The molar mass of Mohr’s Salt is approximately 284 g/moles in anhydrous state.

• The crystallised solid appears in bluish green colour under room temperature and standard pressure.

• The approximate density of the crystallised solid is 1.86 g/cm3.

• The solubility of Mohr’s salt is around 270 g/L.

Applications of Ammonium Sulphate

Mohr’s Salt is mainly used in analytical chemistry. It is one of the most stable compounds of $\mathrm{(Fe^{2+})}$ ions. Sulfuric acid is added to Mohr’s Salt to reduce oxidation of Ferrous ions which extends shelf life of the compound. It is used to measure gamma radiation through Fricke’s dosimeter. It is also used for qualitative analysis of unknown concentration of salts in given solution. Ferrous Ammonium Sulphate is used in the production of Iron blue and as a regular element of brass colouring baths and iron plating in the metal industry. Mohr’s Salt also helps in corrections of iron deficiency in the applications of pesticide spray or in analysis fertilizers which contain iron as one of element.

Conclusion

Ferrous Ammonium Sulphate is also called Mohr’s Salt mostly used in analytical chemistry. It is a very common laboratory reagent as it proceeds to crystallisation process very easily than any other reagents. It contains two cations, one is Ammonium ion $\mathrm{(NH_{4}^{+})}$ and the other one is Ferrous ion $\mathrm{(Fe^{2+})}$. It possesses the properties of double salt. It is mainly used as oxidation resistance agent.

FAQs

1. What is Ferrous Ammonium Sulphate?

Ferrous Ammonium Sulphate is commonly known as Mohr’s Salt mostly used in analytical chemistry. It is a double salt as it contains both Ammonium $\mathrm{(NH_{4}^{+})}$ and Ferrous ion $\mathrm{(Fe^{2+})}$. Its formula is $\mathrm{(NH_{4})_{2}Fe(SO_{4})_{2}(H_{2}O)_{6}}$.

2. What are the properties of Ferrous Ammonium Sulphate or Mohr’s Salt ?

In anhydrous state of Mohr’s Salt contains molar mass of 284 g/mole. In room temperature and under standard pressure, the double salt appears in bluish-green colour and in a crystalline solid state. The approximate density of is 1.86 g/cm3.

3. What do you know about double salt?

Double salt means it contains two cations in a single molecule of salt.Ammonium $\mathrm{(NH_{4}^{+})}$ and Ferrous ions $\mathrm{(Fe^{2+})}$ are two ions of Mohr’s salt. It is basically an admixture of two crystalline salts.

4. Mention some applications for Ferrous Ammonium Sulphate or Mohr’s Salt?

Mohr’s Salt is mainly used in analytical chemistry and is one of the most stable compounds of $\mathrm{Fe^{2+}}$ ions. It reduces oxidation and helps in shelf life. Sulfuric acid can be added to Mohr’s Salt to avoid oxidation of Ferrous ions. It is also used to measure gamma radiation through Fricke’s dosimeter.