Note 513 — Chapter XIII · THE ARSENIC GROUP. SULPHO-ACIDS AND SULPHO-SALTS

To a certain extent, the effect of the acid may be to coagulate and precipitate the colloidal sulphide. Possibly, also, the concentrated acid renders inactive a considerable portion of the water present (forming oxonium salts OH3Cl, etc., see p. 238), which tends, by hydrolysis, to reverse the formation of the chloride As(OH)5 + 5 HCl ⇄ AsCl5 + 5 H2O. Possibly, the formation of the pentasulphide is not wholly an ionic reaction, its precipitation being always a more or less slow process, and there may be intermediate products whose formation could be accelerated by the presence of acids (see Bredig and Walton, Z. Elektrochem., 9, 114 (1903) for the study of a simple inorganic action involving such catalytic effects of acids).