Electrochemical, spectroscopic and structural investigations of the Cd/Cd(II) system in alkaline media - 2. Concentration effects

Ex-situ grazing angle X-ray diffraction and direct reflectance FTIR spectro scopy were used to characterise the composition of the passive layers forme d during anodic oxidation of cadmium in alkaline solutions. Measurements we re made at different hydroxide concentrations in order to assess the import ance of the dissolution/precipitation path with increasing concentrations. The composition of the passive layer was found to depend on the electrolyte cation and on the hydroxide concentration. In LiOH the only product formed was beta-Cd(OH)(2), irrespective of the concentration (0.1-3.7 M) and pote ntial. In NaOH an increasing amount of gamma-Cd(OH)(2) was detected as the concentration increased, from pure beta-Cd(OH)(2) in a 0.1 M solution to ma inly gamma-Cd(OH)(2) in a 7 M solution. For KOH, beta-Cd(OH)(2) was primari ly formed in the concentration range 0.1-7 M. These data are discussed in t he context of surface complexation with emphasis on the effect on crystal g rowth. Carbon paste electroactive electrodes (CPEE) were used to investigat e the reduction of synthesised oxidation products, CdO, beta- and gamma-Cd( OH)(2). The results show that the reduction process is facilitated by the p resence of cadmium metal, as revealed from the difference between the first and second reduction scans. These results are interpreted by a re-proporti onation path in the presence of cadmium metal. (C) 1999 Elsevier Science S. A. All rights reserved.