Aurous sulphide, Au₂S

The pure sulphide can be prepared by saturation of a solution of potassium aurocyanide with hydrogen sulphide, and precipitation of the salt by acidifying the solution with hydrochloric acid. It is not produced by the action of hydrogen sulphide on a hot solution of auric chloride, as supposed by Berzelius. The moist substance has a steel-grey colour; when dried, it becomes brownish black. When freshly prepared, the sulphide dissolves in water to a colloidal solution, from which it is reprecipitated by addition of hydrochloric acid. The sulphide is unaffected by dilute acids, but is decomposed by powerful oxidizers such as aqua regia and chlorine. It is readily dissolved by solutions of polysulphides, and less readily by those of monosulphides. It also dissolves in a solution of potassium cyanide. With sodium sulphide and with potassium sulphide, aurous sulphide forms double sulphides of the type NaAuS.

Although aurous sulphite itself has not been isolated, double sulphites of aurous gold with sodium, potassium, ammonium, and barium have been obtained. The sodium salt is formed by the interaction of auric chloride and sodium sulphite in alkaline solution, or by the action of sodium hydrogen sulphite on a boiling solution of a gold salt, or by that of sulphurous acid on a similar solution at 30° to 50° C. It has the formula Au₂SO₃,3Na₂SO₃,3H₂O or Na₃Au(SO₃)₂,1½H₂O, containing the complex anion Au(SO₃)₂''', since it does not display the reactions characteristic of sulphites. It is readily soluble in water.

The corresponding potassium salt forms white needles, and the barium salt is a purple-red, amorphous substance. The constitution of the ammonium salt produced by the interaction of auric chloride and an ammomacal solution of ammonium sulphite is uncertain, but is given as

2Au₂SO₃,(NH₄)₂SO₃,6NH₃,4H₂O,
or
5Au₂SO₃,2(NH₄)₂SO₃,1ONH₃,4H₂O.