Electrooxidation of the hypophosphite ion on a palladium electrode

Anodic oxidation of sodium hypophosphite on smooth Pd and Pd/Pt electrodes and a Pd membrane is studied. The i vs. E curves for the Pd electrode exhib it two anodic current peaks. One is caused by oxidation of H2PO2- and the o ther, by simultaneous ionization of Pd and oxidation of H2PO2-. The hypopho sphite ion adsorbed on the Pd surface hinders the formation of the passive film. This brings about a rapid dissolution of Pd in the oxygen region and its subsequent deposition with the formation of palladium black. The oxidat ion probably includes a slow heterogeneous chemical reaction, specifically, a cleavage of the P-H bond of the hypophosphite ion. The change in the rea ction stoichiometry following an increase in solution pH and in anodic pola rization is probably due to changing conditions of the H2PO2- adsorption an d the number of adsorption sites occupied by H2PO2- on the surface. Followi ng an increase in polarization, the phosphite ion may undergo oxidation to phosphate.