Sulphites, salts formed by the union of sulphurous acid with bases, or more strictly speaking by the action of sulphurous acid on bases; the two atoms of hydrogen in the molecule of the acid being partially or wholly replaced by a metallic basyle or a radical, forming normal and acid salts. (See Salts.) Thus, normal sulphite of potassium, K2S03, may be formed by wholly replacing the hydrogen in the acid. H2SO3; or the acid sulphite, KHSO3, may be formed by employing half the quantity of acid. The sulphites of the metals are usually formed by passing sulphurous anhydride, S02, through aqueous solutions or mixtures of hydrates or carbonates, the anhydride being first converted into the acid by combining with the elements of water, whereupon double decomposition immediately takes place, with an exchange between the metallic basyle and the hydrogen constituent of the acid. Sulphurous acid combines with nearly all the metals, the most important sulphites being those of calcium and sodium. Sulphite of calcium may be precipitated in an anhydrous state by passing sulphurous anhydride through water having hydrate of lime in suspension, or through a solution of chloride of calcium. It is manufactured on a large scale and used in bleaching, and for protecting organic substances from decay.
The sulphites of sodium comprise a normal salt, Na2S03, and an acid salt, NaHS03. The normal salt is prepared by saturating a solution of carbonate of soda with sulphurous acid and adding to it as much carbonate of soda as it originally contained while warm. On cooling, the salt separates in monoclinic crystals, having seven molecules of water of crystallization (Na2S03 + 7H20). Another hydrate was formed by Muspratt, containing tun molecules of water. The acid sulphite is prepared by supersaturating the solution of sodic carbonate with sulphurous acid. Both of these salts have been used to remove the traces of chlorine in paper pulp, under the name of anti-chlor, but they have been partially superseded by hyposulphite of sodium, or this salt is used in connection with them. (See Paper.) - The two atoms of hydrogen in the molecule of sulphurous acid may be partly or wholly replaced by monatomic alcohol radicals, forming acid and neutral sulphurous ethers, the acid ethers being sometimes called sulpho-acids. Ethyl-sulphurous acid, (C2H5)HSO3, is formed by the action of nitric acid on sulphydrate of ethyle or mercaptan, (C2H5)HS. Neutral sulphurous ether, (C2H5)2SO3, may be formed, among other methods, by the action of absolute alcohol on disulphide of chlorine.
The alcoholic sulphites and other sulpho-ethers and compounds have been carefully studied by Muspratt, Gerhardt, Rammelsberg, Kolbe, and others. They form an extensive and interesting series.