| Abstract: | The pH‐dependence of the stationary open‐circuit potential Ei=0st of rhodium electrode with a surface layer of anodically formed insoluble compounds has been studied in sulfate and phosphate solutions by means of cyclic voltammetry and chronopotentiometry. The range of potentials of the investigations performed has been confined to the region of rhodium electrochemical oxidation/reduction, i.e., 0.2<E<1.2 V (RHE) in order to prevent any possible interference of other reactions such as H2 and O2 evolution. It has been shown that rhodium electrode with a layer of surface compounds formed anodically at E<<1.23 V (RHE) behaves like a reversible metal‐oxide electrode within the range of pH values from ca. 1.0 to ca. 8.0. It has been presumed that the stationary potential of such electrode is determined by the equilibrium of the following electrochemical reaction: Rh+3H2O??Rh(OH)3+3H++3e?. The pH‐dependence of the reversible potential of Eequation/tex2gif-inf-6.gif electrode has been found to be: Eequation/tex2gif-inf-8.gif=Ei=0st=0.69?0.059 pH, V. In acid solutions (pH<2.0) rhodium hydroxide dissolves into the electrolyte, therefore, to reach equilibrium, the solution must be saturated with Rh(OH)3. This has been achieved by adding Rh3+ ions in the form of Rh2(SO4)3. The solubility product of Rh(OH)3, estimated from the experimental Eequation/tex2gif-inf-16.gif?pH dependence obtained, is ca. 1.0×10?48, which is close to the value given in literature. |
