Sodium sulphite, Na₂SO₃

The anhydrous sulphite can be prepared by heating equimolecular proportions of sodium hydrogen sulphite and sodium hydrogen carbonate:

NaHSO₃+NaHCO₃=Na₂S₃+H₂O+CO₂.

It is also precipitated by the action of ammonia on a solution containing sodium chloride and ammonium sulphite in equimolecular proportions. Hartley and Barrett have described a method of preparation from sulphur dioxide and sodium carbonate. It is a white, crystalline salt.

For the solubility of this substance in 100 grams of water at 20° C. Kremers gives 28.7 grams; at 40° C., 49.5 grams; and at 100° C. Fourcroy gives 33 grams. At 10° C. the heat of solution of the anhydrous salt is 2.5 Cal.

Hartley and Barrett state that the only stable forms are the anhydrous salt and the heptahydrate, Na₂SO₃,7H₂O. Neither they nor Schultz-Sellack could isolate the decahydrate described by Muspratt. The anhydrous salt belongs to the hexagonal system, and at 15° C. has the density 2.6334. The heptahydrate is monoclinic, its density at 15° C. being 1.5939. At 10° C. the heat of solution of the heptahydrate according to de Forcrand is –11.1 Cal., and the heat of hydration 13.6 Cal.

Dilute solutions readily undergo atmospheric oxidation, but Lumiere and Seyewetz found that a 20 per cent, solution is stable, and Ehrenfeld states that the presence of excess of alkali prevents oxidation, while cuprous oxide exerts a catalytic accelerating effect on the transformation. A similar catalysis is occasioned by cupric sulphate, but substances such as potassium cyanide and mannitol exercise a retarding influence, affording an example of "negative catalysis." Solutions of sodium sulphite are sensitive to light, paper immersed in such a solution being capable of development by silver nitrate and other reagents.

References are appended to investigations of the solubility, formation of hydrates, molecular depression of the freezing-point, and electric conductivity.