Chemistry of Zinc Sulphate and White Vitriol

Zinc Sulphate is one of the most important compound of zinc. It has wide application in chemistry. We can find zinc sulphate mainly in two forms. It’s anhydrous  form is called zinc sulphate (ZnSO4) and hydrous form is called white vitriol. (ZnSO4.7H2O).

Preparation

a. From dilute sulphuric acid: When zinc is reacted with dilute sulphuric acid, zinc sulphate is formed.
$$\ce{Zn + dil. H2SO4->ZnSO4 + H2}$$
When the product is further heated slowly at 39 degree, it crystallizes as ZnSO4.7H2O which is called called white vitriol.

b. Roasting of zinc blende: Zinc sulphate is prepared by roasting the zinc blende as follows:
$$\ce{ZnS + 2O2->ZnSO4 }$$
$$\ce{ZnS + 3O2->2ZnO + 2SO2}$$

Physical Properties:

  • It is colorless, crystalline and efflorescent substance.
  • It is highly soluble in water.

Chemical properties

a. Action of heat on white vitrol:
It loses six molecule of water at 100ºC and anhydrous salt is formed at 300ºC. When it is further heated to above 800ºC, it decomposes to give zinc oxide, sulphur dioxide and oxygen.

$$\ce{ZnSO4.7H2O->[{100^{\circ}C}]ZnSO4.6H2O->[{300^{\circ}C}]ZnSO4->[{800^{\circ}C}]ZnO + SO2 + O2}$$

b. Action with NaOH:

When 1-2 drops of NaOH is added to the zinc sulphate solution, at first gelatinous white ppt is obtained but when excess NaOH is added to the ZnSO4 solution, a soluble complex called sodium zincate is formed.
$$\ce{ZnSO4 + 2NaOH->Zn(OH)2 v + Na2SO4}$$
$$\ce{Zn(OH)2 + 2NaOH ->Na2ZnO2 + H2O}$$

c. Action with barium sulphate:
Zinc sulphate reacts with barium sulphate to give lithophone which is used as a white paint.
$$\ce{ZnSO4 + BaS->BaSO4 + ZnS}$$

The solution of BaSO4 and ZnS is called lithophone.

Uses of Zinc Sulphate

  • It is used in eye lotion.
  • It is used in dye industry.
  • It is used for the electrolytic refining of zinc.

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