The Use of Thiosulfate

The Use of Thiosulfate

Most of thiosulfate's usefulness stems from its ability to convert

certain insoluble metal compounds into soluble complexes, and its

ability to act as a mild reducing agent.

· Photographic fixing agent. An emulsion of sodium thiosulfate (called

hypo by photographers) is used to stop development of exposed film.

Thiosulfate converts undeveloped silver bromide grains in the film

into water-soluble silver thiosulfate complexes that can be removed

when the film is washed.

S2O32- + AgBr(s) rightarrowAgS2O3- + Br-

S2O32- + AgS2O3- doublearrowAg(S2O3)3-

· Extracting silver from ores. Thiosulfate's ability to convert silver

compounds into soluble silver complexes also makes it useful in ore

processing.

· Paper manufacture and dechlorination of water. Thiosulfate ion is

used to remove chlorine (hypo chlorite) from water or from solutions

used to bleach paper pulp:

HOCl + 2 S2O32- [IMAGE]Cl- + S4O62- + OH-

· Dissolved oxygen testing and chemical analysis of oxidants.

Thiosulfate is a reducing titrant used for determining many oxidizing

agents. For example, hypo chlorite concentrations in bleach can be

determined using the reaction above. Thiosulfate is also used as a

titrant in determining biochemical oxygen demand (the extent to which

oxygen within a sample can support microbial life) [RPI].

· Whitening cotton fabrics, bone, straw, and ivory. Some materials are

weakened or yellowed when bleached. Thiosulfate solutions can

sometimes be used to whiten these materials. See Garments:

Discoloration/Yellowing from Age [NCSU]

Where does thiosulfate ion come from?

Thiosulfate is produced industrially by several different reactions:

· Reaction of sulfur with sodium sulfite in boiling aqueous solution,

S8 + 8 SO32- [IMAGE]8 S2O32-

· Reaction of SO2 gas with sulfide/carbonate liquor (a byproduct of

paper manufacturing).

· As a byproduct in the manufacture of sulfur dyes.

What is thiosulfate ion?

Thiosulfate (S2O32-) is an oxyanion of sulfur produced by the reaction

of sulfite ions with elemental sulfur in boiling water,

S(s) + SO32-(aq) rightarrowS2O3-2(aq)

Thiosulfate occurs naturally in hot springs and geysers, and is