Sodium Thiosulfate — Frequently Asked Questions

From an old and incorrect name, hyposulphite of soda, which stuck in the trade long after the chemistry was properly understood. Its function was to fix the image: it converts insoluble silver halides to soluble silver thiosulfate complexes, dissolving away the unexposed silver so the image becomes permanent and light-stable.

Because it works rapidly on both free chlorine and chloramine and does so without the sulphur dioxide odour the sulfites produce. It is also effective at low dose and is not itself toxic to fish at the concentrations used, which makes it the standard choice for aquarium, fish farm and pond applications.

It supplies sulphur to the enzyme rhodanese, which transfers that sulphur onto cyanide to form thiocyanate, a far less toxic compound that is excreted in urine. It is a component of standard cyanide poisoning antidote kits, usually given alongside a compound that sequesters cyanide directly, since thiosulfate acts by accelerating the body's own detoxification rather than binding cyanide itself.

It is the standard titrant in iodometry. Iodine liberated by a reaction is titrated against a standardised thiosulfate solution using starch as the indicator, and the endpoint is the sharp disappearance of the blue starch-iodine colour. That method underlies a wide range of quantitative determinations including dissolved oxygen, peroxide value and available chlorine.