The Effect of Metal Dissolution on the Rest Potential in the Palladium-Oxygen-Acid System

The rest potential of palladium electrodes in oxygen-saturated sulfuric acid solutions was studied as a function of pH and the partial pressure of oxy-gen. In the absence of anodic polarization, a rest potential of 870 _ 10 mv is obtained which is a mixed potential, nd this system is referred to as a Pd/PdO electrode. After anodic polarization but in the absence of peroxide, a rest po-tential of 1470 _+ 10 mv is obtained which is an equil ibrium metal-metal oxide potential, and this system is referred to as a Pd/PdO2 electrode. With time, a Pd/PdO2 electrode is transformed to a Pd/PdO electrode because PdO2 is unstable in acid solutions. In a certain potential region, Pd dissolves in acid solutions to form Pd + + ions which greatly influence the rest potential. Evi-dence is presented which indicates that, of all the noble metals, possibly Pd is the only one for which the evolution of oxygen under anodic polarization proceeds through an oxide mechanism. F rom thermodynamic onsiderations, an iner