Antichlor, in chemistry, any substance ca-pable of eliminating the excess of chlorine or of free hypochlorous acid left in goods and paper after the process of bleaching by chloride of lime. Several agents can be employed, and they generally act by converting the chlorine into an innocuous salt. 'One of the first substances employed for this purpose was the sulphite and bisulphite of soda patented by Henry Donkin in 1847. In 1853 these salts were superseded by the hyposulphite of soda, which has now become the principal antichlor of commerce. Sulphide of calcium, prepared by boiling sulphur with milk of lime, has also been used as an antichlor; so likewise has a solution of protochloride of tin in hydrochloric acid. In the latter case, however, it is necessary, after the completion cf the bleaching process, to add carbonate of sodium, in order to neutralize the free hydrochloric acid, which would otherwise act as injuriously as the free chlorine itself. The precipitate of oxide of tin thereby produced is quite white and soft, and does not interfere with the subsequent stages of the paper manufacture. Coal gas was also used as an antichlor in paper making as early as 1818, but it is not so convenient as the agents mentioned above.

The products formed by the action of chlorine (or hypochlorous acid) on sulphite or hyposulphite of sodium are sulphate and chloride of sodium, both of which are innocuous and easily removed by washing.