sodium thiosulfate

Sodium thiosulfate (Na₂S₂O₃), also known as sodium hyposulfite, is a sodium salt which sees common use in photography. Its natural form is a crystalline solid similar to common table salt (sodium chloride), though most of its uses occur in solution. Most of its useful chemical properties are consequences of its status as a reducing agent, able to alter the structure of other chemicals in a number of useful ways.

In photography, a solution of sodium thiosulfate is often called 'hypo', and is used as a fixer in the process of developing film. A fixer's purpose is to wash all the light-sensitive chemicals off of the film while leaving the developed silver compounds which make up the image. It is so named because the film is no longer light sensitive after treatment in the fixer, and thus the image is 'fixed' and can no longer be changed. The thiosulfate ion S₂O₃²⁺ is the component of sodium thiosulfate responsible for this effect, so other similar compounds, like ammonium thiosulfate, also act as fixers. The fixer solution in a darkroom can be identified by its strong smell of sulfur.

Sodium thiosulfate is useful for other things besides photography, however. In solution, it breaks iodine and chlorine molecules into ions, removing many of their original chemical properties. In the case of iodine, this is useful for both chemistry experiments and removing iodine stains from surfaces. For chlorine this is useful for de-chlorinating water, as chlorine ions are innocuous in solution and do not affect taste or micro-organisms. Water dechlorinated with sodium thiosulfate is safe to drink, as sodium thiosulfate is non-toxic in relatively low concentrations. In fact, it is even used medically as part of a treatment for cyanide poisoning. The reducing properties of thiosulfate are also used in leather tanning, in combination with acids based on chromium.

Though it is completely non-toxic, strong solutions of sodium thiosulfate as used in darkrooms can cause serious chemical burns on skin or eye contact. Thus, when handling darkroom fixer, one should always wear gloves.

Produced as a by-product of sulfur dye manufacture, sodium thiosulfate has found a number of uses both in and out of the world of photography.