Kinetics of the anodic reaction of molten nitrites on platinum

The kinetics of the anodic reaction occurring on platinum electrodes when molten nitrites are electrolysed has been studied from 240 to 340°C. The following electrolytes were used (i) molten sodium nitrite (ii) mixtures of molten sodium and potassium nitrites (iii) solutions of nitrites in the sodium-nitrate-potassium-nitrate eutectic, covering the concentration range from 10 to 10−1 M. The overall anodic reaction is NO2− → NO2 + e. Current/voltages curves, and decay and build-up of electrode overpotential were measured and the results analysed in the usual way. After the interruption of the electrolysis current a steady residual potential was established on the working electrode. Current/voltage curves fit a Tafel plot in two regions of current density. At lower current density the slope approaches 2·3 (RT/F), whereas at higher ones it becomes 2·3 (RT/2F). Two slopes are also obtained from the logarithmic plot of overpotential decay for the pure nitrites at high current density; the results are satisfactorily coincident with those already mentioned. The transition from the higher to the lower Tafel line occurs at an overpotiential of about 300 mV. From the decay curves the apparent electrode capacitance and its dependence on potential within a certain range of the latter was calculated. The interpretation of results is based upon a simple reaction scheme involving a fast discharge of nitrite ion followed by two alternative reactions for the intermediate formed in the initial discharge, finally yielding nitrogen dioxide. Possibilities related to a low or an intermediate degree of coverage of the electrode surface are discussed.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada