(2)
The value of the constant KHO−, Oxygen is 8.2E49 at 18°, and oxygen of atmospheric pressure at this temperature should be in equilibrium with solutions containing hydroxide-ion at a concentration[556] of 1.36E12.
An electrode of platinum gauze, charged with oxygen under atmospheric pressure, when dipped into the solution of a neutral salt, acquires a very strong positive charge, the minute concentration of hydroxide-ion, 0.9E−7, being very much smaller than the value required by the constant, and the oxygen ionizing very much more rapidly, in consequence, than it is formed by the discharge of hydroxide ions (see p. [259]). [p280]
When we combine the hydrogen and the oxygen electrodes, dipping into a solution of sodium chloride, we find a current is, in fact, established (the apparatus discussed on p. [281] is used), and it flows in the direction anticipated from the above development, the positive current entering the voltmeter from the oxygen electrode.[557]
The potential of the hydrogen electrode, for a constant pressure of hydrogen, is dependent on the concentration of hydrogen-ion in the solution surrounding the electrode, exactly as the potential of a copper plate, against a solution of cupric-ion, depends on the concentration, or osmotic pressure, of cupric-ion in the solution in which the plate is immersed. The concentration of hydrogen-ion, in the present instance, is very small (0.9E−7, at 18°), the solution being practically neutral; but the addition of an alkali must reduce its concentration far below even this value, since for water the product of the concentrations of hydrogen-ion and hydroxide-ion is a constant (p. [176]) and the increase in the concentration of hydroxide-ion, produced by the addition of alkali, must decrease the concentration of the hydrogen-ion proportionally. We would expect, then, that the potential of the hydrogen electrode must increase, when we add alkali to the solution surrounding it, the hydrogen now ionizing against a much smaller concentration of its ion. Such is in fact the case (exp.), and the increase is found to be subject to a logarithmic function for the relation between potential and the concentration of the ion, similar to that found to hold for copper and its ion.[558] In the same way, the potential of the oxygen electrode must depend on the concentration[559] of the hydroxide-ion in the solution bathing it. The addition of a strong acid, like sulphuric acid, to this solution, by suppressing the hydroxide-ion, small as its concentration is, should increase the potential of the [p281] electrode and the total potential of the cell. This, in fact, is the case (exp.); the cell working under these conditions shows us the largest potential yet observed.[560]
Fig. 14.
The arrangement of the apparatus and the course of the current are shown in Fig. 14. The glass tube of the hydrogen electrode is connected with a hydrogen generator, the tube of the oxygen electrode with a cylinder or gasometer filled with oxygen. The hydrogen electrode is connected with the negative post of the voltmeter, the oxygen electrode with the positive post. Since the hydrogen ionizes, under the conditions used, more rapidly than it is formed from the small concentration of the hydrogen ions surrounding the hydrogen electrode, hydrogen ions pass from the electrode into solution A, leaving a negative charge on the electrode; there is a migration of sodium ions through the salt bridge (see p. [254]) to solution B, and the hydrogen ions formed combine with hydroxide ions and produce water. In a similar way, oxygen passes into solution B in the form of hydroxide ions and these combine with hydrogen ions of the sulphuric acid, forming water; SO42− ions migrate from the solution B through the salt bridge toward solution A and thus prevent polarization (p. [254]). While water is an actual product of the action of the cell, working under these conditions, the essential feature of the oxidation of hydrogen is its ionization—H2 → 2 H+; it would be in the same condition of oxidation if the hydrogen ions combined with any negative ions other than HO−, or if they remained ions (as they would, if sodium chloride surrounded the hydrogen electrode). Similarly, the essential feature of the reduction of oxygen is its ionization in the form of HO− ions; in the present [p282] instance, these actually combine with hydrogen ions and form water, but the reduction of oxygen would also be accomplished, if the hydroxide ions remained ionized (as they would, if sodium chloride bathed the oxygen electrode). The formation of water is the result of a union of ions, following the oxidation-reduction reaction, which may be expressed in the following condensed form:
2 H2 ⥂ 4 H+
2 HOH + O2 ⥂ 4 HO−
4 HO− ⥂ 4 H2O