sodium thiosulfate, Na2S2O3, colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3·5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or "hypo." Sodium thiosulfate is readily soluble in water and is a mild reducing agent. Because it dissolves silver salts, its major use is in photography for developing film. It is also used in chrome-tanning leather and in chemical manufacture. Sodium thiosulfate is produced chiefly from liquid waste products of sodium sulfide or sulfur dye manufacture. It is also produced from sodium carbonate, sulfur dioxide, and sulfur by a process that involves several steps.