[H+]2 / [H2] = KH+, Hydrogen.
In this equation [H2] represents the concentration of the hydrogen in contact with the electrode (see below) and with the solution, and [H+] represents the concentration of hydrogen-ion in the solution bathing the electrode. The ionization of hydrogen, at a given temperature, depends, according to this equation, on two variables, the concentration, or pressure, of the gas and the concentration, or osmotic pressure, of the hydrogen-ion in a given solution.
If platinum gauze, coated with platinum black, is charged with hydrogen, then, the greater the pressure of the gas, the more soluble the hydrogen will be in the platinum (p. [121]). Such a charged gauze may be used as a hydrogen electrode (Fig. 13, p. [281]), the concentration of the hydrogen in which is proportional to the concentration, or pressure, of the hydrogen gas surrounding it; the platinum will allow of the ready transmission of electric charges from and to the hydrogen dissolved in it. [p278]
The value of the constant[553] KH+, Hydrogen = [H+]2 / [H2], at 18°, is found to be 5.55E−9, and hydrogen, at 18°, under atmospheric pressure, is directly in equilibrium with hydrogen-ion of the concentration [H+] = 1.52E−5.
If such an electrode, in contact with hydrogen of atmospheric pressure, is dipped into the solution of some neutral salt, say sodium chloride, in which the concentration of the hydrogen-ion, formed by the ionization of water, at 18°, is 0.9E−7, which is less than 1.5E−5, the hydrogen in the electrode must tend to ionize more rapidly than it is formed from the ion, and the electrode must receive a negative charge, exactly as in the case of zinc, placed in a zinc sulphate solution.
For oxygen similar relations may be developed.[554] We have: O2 ⇄ 2 O2− and
[O2−]2 / [O2] = K1.
(1)
If we use the relation of the oxide-ion, O2−, to the more stable hydroxide-ion, HO−, we also have:[555] [p279]
[HO−]4 / [O2] = K2 = KHO−, Oxygen