Sodium sulphate (Glauber's salt) (Na2SO4·10H2O). This salt is prepared by the action of sulphuric acid upon sodium chloride, hydrochloric acid being formed at the same time:
2NaCl + H2SO4 = Na2SO4 + 2HCl.
Some sodium sulphate is prepared by the reaction represented in the equation
MgSO4 + 2NaCl = Na2SO4 + MgCl2.
The magnesium sulphate required for this reaction is obtained in large quantities in the manufacture of potassium chloride, and being of little value for any other purpose is used in this way. The reaction depends upon the fact that sodium sulphate is the least soluble of any of the four factors in the equation, and therefore crystallizes out when hot, saturated solutions of magnesium sulphate and sodium chloride are mixed together and the resulting mixture cooled.
Sodium sulphate forms large efflorescent crystals. The salt is extensively used in the manufacture of sodium carbonate and glass. Small quantities are used in medicine.
Sodium sulphite (Na2SO3·7H2O). Sodium sulphite is prepared by the action of sulphur dioxide upon solutions of sodium hydroxide, the reaction being analogous to the action of carbon dioxide upon sodium hydroxide. Like the carbonate, the sulphite is readily decomposed by acids:
Na2SO3 + 2HCl = 2NaCl + H2O + SO2.
Because of this reaction sodium sulphite is used as a convenient source of sulphur dioxide. It is also used as a disinfectant and a preservative.
Sodium thiosulphate (hyposulphite of soda or "hypo") (Na2S2O3·5H2O). This salt, commonly called sodium hyposulphite, or merely hypo, is made by boiling a solution of sodium sulphite with sulphur: