The Elements of Qualitative Chemical Analysis, vol. 1, parts 1 and 2. / With Special Consideration of the Application of the Laws of Equilibrium and of the Modern Theories of Solution.

by Julius Stieglitz

Behavior of Arsenic Acid toward Hydrogen Sulphide.

When a solution of arsenic acid, containing the usual small amount of hydrochloric acid (0.3 molar), is treated with hydrogen sulphide at ordinary temperatures, the following three reactions take place, but exceedingly slowly:

2 H₃AsO₄ + 5 H₂S ⥂ As₂S₅ ↓ + 8 H₂O

(1)[504]

H₃AsO₄ + H₂S ⥂ H₃AsO₃ + S ↓ + H₂O

(2)[505]

2 H₃AsO₃ + 6 HCl + 3 H₂S ⇄
2 AsCl₃ + 3 H₂O + 3 H₂S ⥂
As₂S₃ ↓ + 6 HCl + 3 H₂O

(3)

Even in the presence of a considerable amount of arsenic acid, precipitation, either of the trisulphide or of the pentasulphide, may not occur for some time, and, unless one takes account of that fact, the dangerous element, arsenic, would easily be overlooked. Heat accelerates both the precipitation of the pentasulphide and the reduction of arsenic acid and the subsequent precipitation of arsenic trisulphide.[506]

The interesting observation has also been made that, in the presence of an unusually large excess of hydrochloric acid and of a rapid stream of hydrogen sulphide, the precipitation of the pentasulphide (equation (1)) is favored and accelerated.[507] For instance, if 100 c.c. of concentrated hydrochloric acid (sp. gr. 1.2) are added to 50 c.c. of a 0.1 molar solution of potassium arseniate and a rapid stream of hydrogen sulphide is passed through the mixture at the ordinary temperature, a copious precipitate is formed within a minute (exp.). The precipitate formed under these conditions [p249] is the pentasulphide.[508] On the other hand, a mixture of 5 c.c. of hexanormal hydrochloric acid and 50 c.c. of 0.1 molar potassium arseniate fails, for a long time, to give a precipitate when treated in the same way (exp.).