(1)[569]
and AsO43− + 2 H+ + 2 I− ⇄ AsO33− + H2O + I2.
(2)
Both of these forms of expression give the net results of the action correctly. Neither attempts to interpret the interesting and important fact that the reduction of arsenic acid is facilitated by the presence of acids (of hydrogen-ion). It is, at least, also permissible to consider As5+ ions to be present and to express the oxidation-reduction reaction with the aid of this conception,[570] as has been done in the previous discussion. In the final analysis, this method seems to have the advantage of showing directly the changes of the valences[571] (electric charges) of the atoms involved in the oxidation-reduction, and it also expresses, clearly and definitely, the relation of the hydrogen-ion to the action.[571] The following case furnishes an illustration as to how the new point of view works out from the standpoint of a quantitative study of an oxidation-reduction reaction of this type[572]: uranyl salts, such as the sulphate UO2SO4, are oxidizing reagents, which are readily reduced, particularly in acid solutions, to uranous salts (e.g. to the sulphate, U(SO4)2). The potential of a mixture of uranyl and uranous salts is found[573] to depend on the action expressed in the equation UO22+ + 4 H+ + 2 ⊖ ⇄ U4+ + 2 H2O. For the condition of equilibrium (zero potential), it follows that
[UO22+] × [H+]4 / [U4+] = Kequil.
(3)
The value of this constant, at 18°, is found, by calculation,[574] to be approximately 1 / 1024. Now, the uranyl-ion UO22+ may be assumed to have the power of ionizing, with the aid of water, to a very slight degree into ions U6+ and HO−, according to
UO22+ + 2 H2O ⇄ U(OH)42+ ⥃ U6+ + 4 HO−.
(4)
For the ionization of an extremely weak base of this character, we have, further, [U6+] × [HO-]4 / [UO22+] = kbase. And, since [HO−] = KHOH / [H+], we also find, by substitution and by solving for U6+,