Silver Thioarsenates

Silver Orthothioarsenate, Ag₃AsS₄, is formed as a dark brown precipitate when an aqueous solution of silver nitrate is treated with sodium orthothioarsenate. If, however, silver nitrate is added to a solution of sodium orthothioarsenate, silver orthothioarsenate is not formed but silver sulphide is precipitated and arsenious acid, nitric acid and sodium nitrate are formed in solution. The reactions involved probably take the following course:

2Na₃AsS₄ + 6ArNO₃ = 3Ag₂S + As₂S₅ + 6NaNO₃
As₂S₅ + 10ArNO₃ + 8H₂O = 5 Ag₂S + 2H₃AsO₄ + 10HNO₃

the nitric acid then liberates hydrogen sulphide from the thioarsenate and this reduces the arsenic acid: -

2Na₃AsS₄ + 6HNO₃ = 3H₂S + As₂S₅ + 6NaNO₃
H₃AsO₄ + H₂S = H₃NaO₃ + H₂O + S

the sulphur being partly oxidised to sulphuric acid, which is also found in the solution. The presence of mineral acid in the thioarsenate solution favours the above decomposition; in ammoniacal solution arsenic acid remains in the solution and no arsenious acid is formed.

Silver orthothioarsenate readily decomposes on heating, yielding sulphides of silver and arsenic and free sulphur. It is found in Nature as a yellow or orange-red mineral, generally in the form of powder, known as xanthoeonite, and sometimes associated with selenium as rittingerite.