Antichlor

ANTICHLOR, in the broad sense of the word, denotes substances capable of reducing free chlorine to a chloride ion and used for this purpose in technology; in the direct sense of the word, A. is a synonym for sodium thiosulfate Na₂S₂O₃ · 5H₂O (hyposulfite, sodium thiosulfate). Crystals of sodium thiosulfate do not change in air, do not effloresce, and dissolve at 0° in 1 part of water, and at 20° in 0.6 parts of water. Aqueous solutions are stable upon brief boiling, but upon prolonged boiling, they release sulfur. Upon acidification, aqueous solutions—slowly when standing in the cold, and rapidly when heated—yield colloidal sulfur solutions, which then release sulfur in the form of a precipitate: H₂S₂O₃ = H₂O + S + SO₂. In analytical chemistry, the reaction of sodium thiosulfate with iodine, 2Na₂S₂O₃ + I₂ = Na₂S₄O₆ + 2NaI, is the basis for an extremely precise volumetric analytical method—iodometry. Sodium thiosulfate finds extensive application in photography for dissolving silver chloride and silver bromide, whereby a double salt of silver and sodium is formed: AgCl + Na₂S₂O₃ = NaCl + AgNaS₂O₃. In the textile industry, sodium thiosulfate is used in calico printing as a resist for aniline black and in the dyeing of wool with methyl green and malachite green. The use of sodium thiosulfate as an A. is based on the rapid oxidation of sodium thiosulfate by chlorine to sulfuric acid: Na₂S₂O₃ + 4Cl₂ + 5H₂O = 2NaCl + 2H₂SO₄ + 6HCl. In this process, the destructively acting free chlorine is converted into harmless chloride ions. Besides sodium thiosulfate, other substances are used as A. in various fields of technology. In the bleaching of fabrics and paper, sodium bisulfite, which is oxidized by chlorine to acid sulfate (NaHSO₃ + Cl₂ + H₂O = NaHSO₄ + 2HCl), and ammonia are also used. In water supply, the need to use A. arises during the chlorination of water when there is an excess of active chlorine. The most common A. in this case is sodium thiosulfate; sodium bisulfite is also used. Both the addition of chlorine in the form of liquid chlorine, bleaching powder, or sodium hypochlorite, and the addition of A. are usually performed after the water has passed through filters and is already purified. The amount of added A. depends on the residual excess of active chlorine. The addition of A. immediately terminates the bactericidal action of chlorine, as a result of which A. is introduced no earlier than 15–30 minutes after the addition of chlorine. In anti-gas defense, the following agents are used as A.: reducing agents—sodium thiosulfate, urotropine (hexamethylenetetramine); alkalis—caustic soda, soda lime; and, finally, substances possessing strong adsorption capacity, of which activated charcoal (see) has gained the greatest prevalence.

S. Bruyevich.