Sodium thiosulfate ($\mathrm{N}{\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3$)

Sodium thiosulfate is the salt $\mathrm{N}{\mathrm{a}}_2{\mathrm{S}}_2{\mathrm{O}}_3$, often encountered as a hydrate. Its thiosulfate ion can participate in both oxidation-reduction reactions and metal-ion complex formation.

In photographic fixing, thiosulfate binds silver and helps dissolve residual silver halide. In analytical chemistry, reaction with iodine supports quantitative titration. These applications use different aspects of the same anion's chemistry.