Thiosulfate

Thiosulfate are derivatives of the unstable in the free state thiosulphuric H2S2O3. Its salts containing the thiosulfate anion - S2O32.

Properties

Aqueous solutions of thiosulfate are stable in basic. In the presence of acids they decay due to the instability of the free thiosulphuric in a redox reaction slowly to sulfur, sulfur dioxide and water:

Iron (II) ion complex formation occurs. This reaction is well suited to demonstrate with the aid of the stopwatch, and sight dependency of the response speed of the temperature and the ( molar ) concentration of the reactants.

Use

The most important in chemistry and technology thiosulfate is the sodium salt and ammonium salt, also known as hypo. It is used in photography for fixing the exposed materials (films, photo paper, color and black and white material ) in the darkroom, as it dissolves the unexposed and undeveloped silver bromide so as soluble Dithiosulfatoargentatkomplex. ( In the coloring material and the reduced metallic silver is removed by bleaching so that the emulsion after processing is free of silver ):

Thiosulfate can and Others ( such as sulfate ) act as monodentate and bidentate - chelating ligand as:

Thiosulfate can be used as reducing agents for halogens.

In the reaction of thiosulfate with the weak oxidizing agent iodine formed under oxidative (SS ) linkage the tetrathionate:

This reaction takes place at the Iodometry, a determination method in analytical chemistry, application.

Proof

In the presence of an excess of silver ions is silver thiosulfate first white precipitated. This disproportionated then to sulfide and sulfate. The sulfide ions form here with the silver ions, a black precipitate, so that discolors the precipitate of silver thiosulfate in a color change from white to yellow / orange and brown to black. This is used for the detection of thiosulfate ions and is also called Abendrotreaktion or sunset reaction.

However, if thiosulfate is added in excess, the silver ions react immediately to the above Dithiosulfatoargentatkomplex.