Sodium Thiosulfate

OVERVIEW

Sodium thiosulfate (SO-dee-um THYE-oh-SUL-fate) is a colorless to white crystalline solid or powder with no odor and a cooling, bitter taste. The compound usually occurs in the form of the pentahydrate (Na₂S₂O₃·5H₂O). Sodium thiosulfate is an antichlor, a compound that reacts with and neutralizes excess chlorine used in some industrial, commercial, or other applications. Two other popular antichlors are sodium bisulfite (NaHSO₃) and sodium sulfite (Na₂SO₃). Some of the most important applications of antichlors are in the production of pulp and paper and in the textile industry. After pulp, paper, or a textile has been treated with chlorine to bleach the material, an antichlor such as sodium thiosulfate is added to remove any remaining chlorine from the reaction vat. The antichlor itself is then removed by washing with water.

KEY FACTS

OTHER NAMES:

Sodium hyposulfite; thiosulfic acid sodium salt

FORMULA:

Na₂S₂O₃

ELEMENTS:

Sodium, sulfur, oxygen

COMPOUND TYPE:

Salt (inorganic)

STATE:

Solid

MOLECULAR WEIGHT:

158.11 g/mol

MELTING POINT:

Decomposes at about 100°C (200°F)

BOILING POINT:

Not applicable

SOLUBILITY:

Soluble in water; insoluble in ethyl alcohol

HOW IT IS MADE

Large amounts of sodium thiosulfate are obtained as the byproducts of other industrial reactions, such as the production of sodium sulfide (NaS₂) and dyes that contain sulfur. The compound can also be produced directly by adding powdered sulfur to a solution of sodium sulfite and heating the reactants:

Na₂SO₃ + S → Na₂S₂O₃

COMMON USES AND POTENTIAL HAZARDS

In addition to its use as an antichlor, sodium thiosulfate can, itself, also be used as a bleach for paper, pulp, bone, straw, ivory, and other materials. Its other major application is in photography, where it is used as a fixing agent. A fixing agent is a chemical that reacts with silver bromide and silver chloride on a photographic film that has not been exposed. The silver bromide and silver chloride are then washed away, leaving behind and "fixed in place" the silver that has been produced by exposure to light. Smaller amounts of sodium thiosulfate are used for other purposes, such as:

• A food additive for the purpose of maintaining the proper of acidity of a food product or sequestering (capturing and holding) unwanted materials in the food;
• For certain medical purposes, such as the treatment of fungal infections of the skin in humans and, specifically, for the treatment of ringworm in animals;
• In hide tanning and dyeing procedures that use compounds of the element chromium;
• For the extraction of silver metal from its ores; and
• In the analysis of the composition of chemical mixtures.

Interesting Facts

• An important, but seldom used, application of sodium thiosulfate is as an antidote for cyanide poisoning.

Words to Know

ANTICHLOR

A chemical that reacts with excess chlorine used for purification, disinfecting, or some other purpose.

Sodium thiosulfate is an irritant to the skin, eyes, and respiratory tract, although such effects tend to be mild unless one is exposed to large quantities of sodium thiosulfate dust, mist, or solutions. The compound can also be toxic if ingested, causing irritation of the gastrointestinal tract. The amount of sodium thiosulfate permitted in foods is 0.1 percent. No serious long-term effects of exposure to the compound are known.

FOR FURTHER INFORMATION

"Sodium Thiosulfate." Esseco General Chemistry. http://www.genchemcorp.com/pdf/msds/Sodium%20Thiosulfate,%20EGC%20-%204-03.pdf (accessed on November 12, 2005).

"Sodium Thiosulfate, Pentahydrate." Boehringer Ingelheim. http://www.boehringer-ingelheim.ca/vetmedica/msds_sodium.asp (accessed on November 12, 2005).

"Sodium Thiosulfate (Systemic)." Drugs.com. http://www.drugs.com/cons/Sodium_Thiosulfate.html (accessed on November 12, 2005).

See AlsoSodium Sulfite