Sodium thiosulfate is a colorless, crystalline solid with a melting point of 48 degrees C. It is freely soluble in water. It contains five molecules of water as water crystallization. At 48 degrees C, the sodium thiosulfate melts; at 215 degrees C, it loses all its five molecules of water; and above 220 degrees C, it is converted into sodium tetrasulfide.
In analytical chemistry, sodium thiosulfate is used for the determination of the strength of a given solution of iodine. Sodium thiosulfate is preferred in iodometric analysis due to the fact that sodium thiosulfate is oxidized by iodine. It is also used to determine the strength of many oxidizing agents, such potassium dichromate, potassium permanganate and copper sulfate, which, on reacting with excess KI (potassium iodide) solution, liberate iodine. The liberated iodine is titrated with a standard solution of sodium thiosulfate.
Sodium thiosulfate is also used as a fixer in photography. The negative plate or film obtained at the end of photographic development with a reducing agent (i.e., potassium ferrous oxalate, pyrogallol) contains AgBr which has not been light-activated and reduced to Ag metal. This is treated with sodium thiosulfate solution so that AgBr is removed as a soluble complex compound. This process is known as "fixing" of the negative plate or film.
Sodium thiosulfate is used to treat arsenic poisoning, primarily because it is relatively inexpensive and non-toxic. Sodium thiosulfate is proven to be effective in the treatment of copper poisoning when it is applied together with sodium molybdate. It has been also recommended in the treatment of cyanide poisoning.
Sodium thiosulfate is employed in the extraction of gold and silver. Sodium thiosulfate forms a strong complex compound during reactions with gold and silver. The newly formed substances are highly soluble in water, so the target metals (gold and silver) can be leached by thiosulfate.
Sodium thiosulfate is used as a dechlorination agent in neutralizing chlorine and iodine solutions at aquaculture or hatchery facilities. Chlorine is a toxic element, and excess chlorine could be harmful to fish and crustaceans, so it is important to remove chlorine by disinfecting tanks and equipment using sodium thiosulfate before introducing the stock.
Sodium thiosulfate is applied as an antichlor to remove excess chlorine from bleached articles in the textile industry.