T H E POTENTIALS OF PLATINUM ELECTRODES IN PHOTOGRAPHIC DEVELOPERS' A. E. CAMEROS
Kodak Research Laboratories, Rochester, New York Received February 4, 1958
In a recent communication from these Laboratories, R. M. Evans and W. T. Hanson (2) have described the measurement of potentials of blank platinum electrodes immersed in solutions of photographic organic developers and discussed the photographic significance of the potentials so measured. They regarded these potentials as belonging t o a possible oxidation-reduction system present, and postulated the existence of a small and essentially constant amount of the oxidized form of the system in equilibrium with sodium sulfite and with the reduced form of the monosulfonate of the developing chemical. Their measurements were made in a vessel exposed to the air, and an investigation of the part which oxygen might play in the production of the potentials was undertaken by the writer in an effort to clarify the precise origin of the observed potentials. EXPERIMENTAL
Measurements were made in a 250-ml. water-jacketed beaker which was fitted with a glass cap carrying the necessary electrodes, salt bridge, stirrer, etc. Any desired atmosphere could be maintained in this reaction vessel by passing gas in through a porous alundum bubble head. Rapid stirring with a motor-driven glass paddle which entered the reaction vessel through a water seal aided in securing equilibrium conditions. Readings of the electrode potentials were made with a Leeds and Northrup student type potentiometer with a current galvanometer of medium sensitivity. Readings were, in general, made with a series resistance of 10,000 ohms to avoid or minimize polarization of the electrodes. S o criterion of correctness of readings was available, so that agreement of two electrodes within 2 t o 3 millivolts was considered satisfactory. Electrodes of virgin platinum foil 1 em. square were used and were stored in a solution of chromic acid in sulfuric acid between readings. With these electrodes it was found that the potentials were stable and reprodurihle under
