Oxytrithioarsenic Acid, H₃AsOS₃

Oxytrithioarsenic Acid, H₃AsOS₃, has not been prepared. The sodium and potassium salts have been obtained by the action of magnesia on freshly prepared arsenic pentasulphide, suspended in water, and removing the magnesium by means of the alkali hydroxide, when the oxytrithioarsenate remains in solution. By adding alcohol to the cold solution, the salt slowly separates. The sodium salt can be recrystallised from water containing a little alkali, when it yields feathery crystals of composition Na₃AsOS₃.11H₂O. It is unstable and is acted on by light at the ordinary temperature with the probable formation of monothio- and dithio-arsenates. It is decomposed by hydrochloric acid with evolution of hydrogen sulphide. Silver nitrate gives a black precipitate. The potassium salt, K₃AsOS₃.7H₂O, separates as a yellow oil which crystallises at -20° C.

By the action of alkaline earth chlorides on the sodium and potassium salts, the following have been obtained:

Ca₃(AsOS₃)₂.20H₂O, unstable needles.
NaSrAsOS₃.10H₂O, white crystals gradually turning yellow.
Na₂Ba₇As₅O₇S₁₄.12H₂O(?).
KBaAsOS₃.7H₂O, yellow crystals.

A series of complex arsenothiomolybdates has been described.