Aurous thiosulphate, Au₂S₂O₃,3H₂S₂O₃,H₂O

This substance is to be regarded as a complex aurothiosulphuric acid. It is prepared by the action of dilute sulphuric acid on the barium salt.

Sodium thiosulphate reacts with a solution of auric chloride to form sodium aurothiosulphate,

Au₂S₂O₃,3Na₂S₂O₃,H₂O, or Na₃Au(S₂O₃)₂,½H₂O,

colourless, acicular crystals. At 150° to 160° C. it loses water, and at higher temperatures it is decomposed. Hydrogen sulphide and ammonium sulphide precipitate aurous sulphide from its solution, and iodine transforms it into sodium tetrathionate and aurous iodide. In accordance with the presence of a complex anion, hydrochloric acid and sulphuric acid do not precipitate sulphur, a contrast to their action on ordinary thiosulphates. The complexity of the anion is further manifested in the inability of the usual gold reducers to precipitate the metal.