Realization of rhodium metal-oxide electrode in indifferent electrolytes

The pH-dependence of the stationary open-circuit potential E-i=0(st) 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 H-2 and O-2 evolution. It has been shown that rhodium electrode with a layer of surface compounds formed anodically at E much less than 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 + 3H(2)O double left right arrow Rh(OH)(3) + 3H(+) + 3e(-). The pH-dependence of the reversible potential of E-Rh/Rh(OH)3 electrode has been found to be: E-Rh/Rh(OH)3 E-i=0(st) = 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 Rh-2(SO4)(3), The solubility product of Rh(OH)(3), estimated from the experimental E-Rh/Rh(OH)3 - pH dependence obtained, is ca. 1.0 x 10(-48), which is close to the value given in literature.