An Elementary Study of Chemistry

by William McPherson; William Edwards Henderson

Na₂SO₃ + S = Na₂S₂O₃.

The substance is called sodium thiosulphate, and is a salt of the easily decomposed acid H₂S₂O₃, called thiosulphuric acid. This reaction is quite similar to the action of oxygen upon sulphites:

Na₂SO₃ + O = Na₂SO₄.

More commonly the salt is called sodium hyposulphite, or merely "hypo." It is a white solid and is extensively used in photography, in the bleaching industry, and as a disinfectant.

Monobasic and dibasic acids. Such acids as hydrochloric and nitric acids, which have only one replaceable hydrogen atom in the molecule, or in other words yield one hydrogen ion in solution, are called monobasic acids. Acids yielding two hydrogen ions in solution are called dibasic acids. Similarly, we may have tribasic and tetrabasic acids. The three acids of sulphur are dibasic acids. It is therefore possible for each of them to form both normal and acid salts. The acid salts can be made in two ways: the acid may be treated with only half enough base to neutralize it,—

NaOH + H₂SO₄ = NaHSO₄ + H₂O;

or a normal salt may be treated with the free acid,—

Na₂SO₄ + H₂SO₄ = 2NaHSO₄.

Acid sulphites and sulphides may be made in the same ways.