[506] Bunsen, Ann. (Liebig), 192, 305 (1878). Brauner and Tomicek, J. Chem. Soc. (London), 53, 145 (1888). Usher and Travers, ibid., 87, 1370 (1905).
[507] Neher, Z. anal. Chem., 32, 45 (1893).
[508] Neher, loc. cit.
[509] The theory of the relations favoring the precipitation expressed in equation (1) as against the reduction expressed in equation (2), forms a second interesting problem.
[510] Intermediate derivatives, such as H3AsSO3 (p. [246]), could be the result of the ionization of As(OH)5, or of AsCl5, in stages (see p. [106]). Neher, loc. cit., McCay, loc. cit.
[511] Neher (loc. cit.) suggested that the favorable action of the large excess of hydrochloric acid might well be due to the formation of AsCl5. McCay, J. Am. Chem. Soc., 24, 661 (1902), discusses the ionization of arsenic acid as a base, in connection with the precipitation of As2S5.
[512] As(OH)5 is considered to be an extremely weak base and AsCl5 to be an ionizable salt.
[513] 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).