The formula of this substance is NaHSO , as distinguished from what is commonly known as hyposulphite of soda, Na S O . It is produced by the action of zinc-dust on the acid sulphite of soda.

But we must observe that with these dibasic acids it is possible, by adding insufficient alkali to completely saturate them, to obtain salts in which only one hydrogen atom of the acid is replaced by the metal of the base. Thus sulphuric and carbonic acids yield NaHSO , acid sulphate or bisulphate of soda, and NaHCO , bicarbonate of soda, respectively.

The zinc may be supposed to remove oxygen from the acid sulphite, NaHSO , giving hyposulphite, NaHS0 . The reduction of the acid sulphite is best performed in a cask, which can be closed at the top, so as to avoid entrance of air. The liquid is then mixed with milk of lime, and after again thoroughly stirring, the liquid is allowed to settle, and the clear is decanted into the dyeing-copper.