Chemistry of Green Vitriol

Green vitriol is an important compound of iron. It is also called hydrous ferric sulphate and has molecular formula FeSO4.7H2O.

Preparation:

When iron, ferrous oxide and iron carbonate is treated with dilute sulphuric acid, ferric  sulphate solution is obtained. This solution is further heated to crystallization point to obtain the crystals of green vitriol. The reaction proceeds in several steps as shown below.

$$\ce{Fe + dil. H2SO4->FeSO4 + H2 ^ }$$
$$\ce{FeO + dil. H2SO4->FeSO4 + H2O}$$
$$\ce{FeCO3 + dil. H2SO4->FeSO4 + H2O + CO2}$$
$$\ce{FeSO4 ->[\Delta]\underset{Blue vitrol}{ FeSO4.7H2O}}$$

Manufacture in large scale:

Green Vitriol is manufactured in large scale by the slow oxidation of iron pyrites in the presence of air and moisture. The pyrites are exposed to air in big heaps. Ferrous sulphate is heated to its crystallization point to obtain the crystals of green vitriol.
$$\ce{2FeS2 + 2H2O + 7O2->2FeSO4 + 2H2SO4}$$

Physical properties:

  •  It is a green crystalline compound soluble in water.
  •  It is an efflorescent compound, meant loses its water when exposed to air.
  •  when exposed in air for long time is oxidized to give brown color of ferric salt.

Chemical properties:

1. Action with heat: White vitriol when heated to 300oC loses six molecules of water and when when strongly heated gives Fe2O3, SO2 and SO3.

$$\ce{FeSO4.7H2O->[{300^{\circ}C}]2FeSO4->[\Delta]Fe2O3 + SO2 ^ + SO3 ^ }$$

2. Action with water: Green vitriol when reacted with water becomes acidic due to hydrolysis.
$$\ce{FeSO4 + 2H2O->Fe(OH)2 + 2H2SO4}$$

3. Action with Potassium dichromate: Green vitriol turns acidified potassium dichromate solution to green. Thus its acts as a reducing agent.
$$\ce{K2Cr2O7 + 7H2SO4 + 6FeSO4->K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 7H2O}$$

4. Action with Nitric oxide solution: A cold solution of ferrous sulphate solution when reacted with nitric oxide forms brown ring compound.
$$\ce{FeSO4 + NO->$\underset{\text{Nitrosoferrous sulphate}}{\ce{FeSO4.7H2O}}$}$$

Uses of green vitriol:

  • It is used as a mordant in dye industry.
  • It is also used as insecticides in agriculture.
  • It is also used as lab reagent in the preparation of Mohr’s salt.

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