The mechanism of the oxygen electrode

Abstract

It is well known that the so-called "oxygen electrode" does not behave in a thermodynamically reversible manner. The decomposition voltage of water has been calculated thermodynamically from various calorimetric and solubility data by Lewis, Nernst and von Wartenberg, Brönsted and Lewis and Randall. The final critical value given by the last-named authors is 1.227 volt at 25ºC., which should therefore be the e. m. f. of a cell consisting of a reversible hydrogen electrode and a reversible oxygen electrode immersed in the same electrolyte, both gases being at 760 mm. pressure. In practice this value has never been obtained. Smale found that the e. m. f. of the hydrogen-oxygen cell, though independent of the p H of the electrolyte, was only 1.07-1.08 volt. Wilsmore obtained a value of 1.07 volt, rising to 1.12 volt if the cell were allowed to stand for stand for some days, while a similar result, 1.06 volt, was obtained by Crotogino. More recently, Richards has reported 0.979 volt, and Furman also obtains a value of about 0.98 volt. Since it is well established that the hydrogen electrode bahaves in a perfectly reversible manner in accord with thermodynamic laws, the discrepancy between the "theoretical" and experimental e. m. f. of the hydrogen-oxygen cell must have its origin in the oxygen electrode. It is in fact experimentally found that oxygen electrodes, whether set up with bright or plantinized plantium, ( a ) tend to be irreproducible, ( b ) do not obey the thermodynamic relation between electrode potential and partial pressure of oxygen, and ( c ) are readily polarized even by minute currents, thus failing to conform with any of the criteria of reversibility.