[558] Cf. footnote 1, p. [278].
[559] Cf. footnote 1, p. [279].
[560] See Ostwald, Z. phys. Chem., 11, 521 (1893), Arrhenius, ibid., 11, 805 (1893), and Nernst, ibid., 14, 155 (1893), for a detailed discussion of oxygen-hydrogen gas cells. For more recent work, vide G. N. Lewis, J. Am. Chem. Soc., 28, 158 (1905), where references to other recent investigations are given.
[561] Cf. footnote 1, p. [261], and pp. [277]–[279].
[562] See pp. [42], [252], for the expression of the changes as transfers of electrons.
[563] Fredenhagen, Z. anorg. Chem., 29, 424 (1902), has brought interesting experimental evidence of the charging of an electrode with gaseous oxygen, when ferric-ion is the oxidizing agent in aqueous solutions. Whether the oxygen, which is liberated by the action of the ferric-ion on water, 4 Fe3+ + 4 HO− ⇄ 4 Fe2+ + O2 + 2 H2O, is always the intermediate product and the direct oxidizing agent in aqueous solution, can hardly be considered decided by the experiment—it may well be the product of a parallel action, which must take place to a certain extent, according to the laws of equilibrium, in a system containing both Fe3+ and HO− ions. The result hardly proves that oxygen must be the intermediate product in the main action, when ferric ions act as the oxidizing agent. We may consider, for instance, a solution containing an iodide and a ferric salt: iodide ions have a far smaller affinity for their negative charges (electrons) than hydroxide ions have, and, consequently, will transfer their negative charges (electrons) more readily to the ferric ions than the hydroxide ions would. The action, if oxygen were first liberated, would lead to the same ultimate result, but the observation made by Fredenhagen would not prove that the main action would not nevertheless go by the shorter direct path rather than through an intermediate formation of oxygen.
[564] According to the theory, that arsenic acid is an oxidizing agent because it gives up oxygen of a definite pressure, this pressure would be the more effective, the more completely the opposing hydroxide-ion is suppressed by added acid (p. [280]; see also p. [272], on the action of ferro-ion on silver-ion in the presence and in the absence of fluorides).
[565] Only the simplest form of basic ionization of arsenic acid is considered. Intermediate ionization into As(OH)4−, As(OH)32−, etc. (see p. [249]), is, of course, to be assumed in any complete investigation of the subject.
[566] Arsenious acid As(OH)3, or HAsO2, as well as its anions AsO33− and AsO2−, may have their own characteristic tendencies to assume positive charges and be oxidized to arsenic acid and its derivatives (see footnote 1, p. [270]). In alkaline solutions these tendencies, and the potentials corresponding to them, might well be more important factors in determining the course of an action, than the tendency of As3+ to form As5+. The discussion in the text, which deals primarily with acid solutions, does not exclude such relations.
[567] A millivoltmeter is used for this experiment.