Sodium thiosulfate

Sodium thiosulphate (deprecated)

  • 7772-98-7 (anhydrous)
  • 10102-17-7 ( pentahydrate )

V03AB06

Colorless and odorless, salty - bitter-tasting crystals

Fixed

1.67 g · cm -3 ( 20 ° C)

45-50 ° C ( pentahydrate )

Decomposition above 300 ° C.

Well in water (701 g · l-1 at 20 ° C)

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Sodium is the sodium salt of stable unstable in the free state thiosulphuric.

Production and representation

Sodium thiosulfate is produced by stirring sulfur in boiling sodium sulfite:

Properties

Sodium thiosulfate forms colorless crystals which crystallize with 5 mol crystal water and are readily soluble in water; when dissolving the liquid cools down heavily, as the hydration enthalpy is less than the lattice energy and the lack of quantity of heat is removed from the system. This so-called pentahydrate Na2S2O3 · 5H2O is also known under the name of fixer, because it is used in film development for fixation. Under the name of anti- chlorine, it is used after the bleaching of paper and textile fibers in order to remove excess chlorine. The pentahydrate crystals have a melting point of 48.5 ° C, the melt can be cooled and is caused by the solidification of a large amount of a seed of crystallization heat. If acid added to the aqueous sodium thiosulfate solution will separate after a short time sulfur in the form of a yellowish haze from. The liberated, unstable thiosulphuric ( H2S2O3 ) decays rapidly namely sulfur and sulfur dioxide:

The water-insoluble silver halides are dissolved by a Fixiersalzlösung. Due to the formation of water-soluble Natriumdithiosulfatoargentat (I ) complex of the developed film is sensitive to light:

Sodium thiosulfate is a reducing agent and thus reacts readily with the oxidizing agent potassium permanganate.

Use

Sodium thiosulfate is used in photography as a fixer, mining for the extraction of silver chloride from silver ores and electroplating for the production of gold and silver electroplating baths. In medicine it is used as an antidote for cyanide poisoning, while less dangerous thiocyanate is formed.

Sodium acts as an anti-chlorine in bleaching processes by reduction of chlorine to chloride, with hydrogen sulfate and hydrochloric acid is:

In chemistry it is used for determination of the iodine value, in the iodometric thiosulfate is oxidized to tetrathionate:

Further use it finds, for example, in so-called heating pad. By bending a metal plate creates a crystallization seed, which initiates an exothermic reaction. The regeneration of the pad is effected by heating in boiling water.

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