Sodium Thiosulphate

About Sodium Thiosulphate

Sodium thiosulfate is a white powder and has a formula of Na₂S₂O₃. It exists mainly in the form of pentahydrate and is called sodium hyposulfite. It is a stable inorganic compound, but decomposes at high temperature to sodium sulfite and sulfur.
The thiosulfate anion has a tetrahedral geometry and is notionally derived by replacing one of the oxygen atom with sulfur atom. It reacts with dilute acids to produce sulfur, sulfur dioxide and water. Sodium thiosulfate is soluble in water and together with sodium nitrite, they are used to treat cyanide poisoning.
The phosphate anion reacts with iodine stiochiometrically, reducing it to iodide as it is oxidized to tetrathionate. Due to the quantitative nature of this reaction, sodium thiosulfate is often used as a titrant in iodometry.

Manufacturing Process

Industrial Method

Sodium thiosulfate can be manufactured industrially as a by-product using the Effluent from the manufacturing process of sodium sulfide or sulfur dye. By reacting the liquid affluent with sulfur dioxide, and boiling nitro compounds with sodium polysulfide solution, yield a leach liquor of sodium thiosulfate. This is then purified, processed and standardized for industrial consumption.

Laboratory Method

Sodium thiosulfate can be manufactured using sodium sulfite or sodium hydroxide and sulfur. By boiling and heating them, both chemicals dissolve and sodium thiosulfate crystallizes out of solution upon cooling. Alternatively, sodium thiosulfate can also be manufactured using liquid affluent obtained in the production of sodium sulphide.

Textile Industry

Sodium thiosulfate is used as a mordant in dyeing and printing textiles, fixing and setting dye onto cellulosic. In addition, it serves to remove excess chlorine from bleached textiles. After being a dechlorination agent, it is used as a bleaching agent as well. Sodium thiosulfate is also used in sulfur dyes.

Paper Industry

It is used as an antichlor agent in the bleaching of paper pulp to manufacture white paper. Also known as "Bleach Stop", it is used to neutralize residual chlorine and chlorine based compounds such as hypochlorite after chlorine-based bleaching, to prevent potential reactions that continue which can result in damage of the bleached paper.

Other Applications

Sodium thiosulfate is also used in pharmaceutical industry as detergents and disinfectants, photography industry as a filter and metallurgy industry to extract silver metal from its ore.