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 H_{3}AsO_{4} + 5 H_{2}S ⥂ As_{2}S_{5} ↓ + 8 H_{2}O (1)[504] H_{3}AsO_{4} + H_{2}S ⥂ H_{3}AsO_{3} + S ↓ + H_{2}O (2)[505] 2 H_{3}AsO_{3} + 6 HCl + 3 H_{2}S ⇄ 2 AsCl_{3} + 3 H_{2}O + 3 H_{2}S ⥂ As_{2}S_{3} ↓ + 6 HCl + 3 H_{2}O (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.›). The acceleration of the precipitation of the pentasulphide by the presence of a large excess of hydrochloric acid forms a problem of peculiar interest and importance, and no complete explanation of it has yet been offered.[509] The following considerations lead to one explanation, that has been suggested. Arsenic acid, by virtue of its close relations to antimonic, stannic and arsenious acids, may be assumed to have extremely weak basic, as well as pronounced acid, properties. For its ionization, we would have 3 H^{+} + AsO_{4}^{3−} ⇄ H_{3}AsO_{4} (+ H_{2}O) ⇄ As(OH)_{5} ⇄ As^{5+} + 5 HO^{−}. Further, the precipitation of As_{2}S_{5} may be assumed to result, ultimately,[510] from the action of the sulphide-ion S^{2−} on the positive ion As^{5+} (2 As^{5+} + 5 S^{2−} ⇄ As_{2}S_{5} ↓). The favorable action of the hydrochloric acid might, consequently, be thought to result from the fact, that it ‹facilitates› the ‹ionization of arsenic acid› as a base and the formation of a salt[511] AsCl_{5}. It could thus greatly increase the concentration of the ion As^{5+} and facilitate its combination with the sulphide-ion.