Reduction Potentials of Organic Substances by the Lead Peroxide Electrode.

It has been pointed out frequently that the potentials measured in a solution of pure oxidant or pure reductant have a questionable meaning. Moreover such potentials are difficult to measure because they fall in a region of a curve approaching the potential axis asymptotically, and thus a slight change of conditions in the solution causes a wide fluctuation of potential. In such a solution an “inert” electrode, being unpoised, is subject to further fluctuation due to its own surface activity.

The same considerations obviously apply to solutions of those reductants which tend to transform irreversibly into products not active at an electrode, such as most organic compounds. Various schemes have been proposed for the measurement of such potentials, which depend in general upon the addition of some other component which poises the system.

It occurred to us that an oxidizing electrode might in itself have a poising action on an otherwise irreversible system. We have tried manganese dioxide and lead peroxide electrodes. In the following we report preliminary determinations with lead peroxide electrode on solutions of certain organic reductants. The electrodes were prepared by plating out lead peroxide on platinum electrodes, made anodal in a solution of lead nitrate acidified with eight per cent nitric acid, under voltage sufficient to cause evolution of a fine stream of gas bubbles. These electrodes read in buffer solutions far in the the oxygen overvoltage region. On long standing they approach the oxygen electrode potential, but still read sluggishly in a reducing solution. If placed in a solution of a reducer until equilibrated, then washed in buffer until they again approach the oxygen electrode potential, when next placed in a reducing solution, equilibrium is attained very promptly.