Mass and charge transfer kinetics and coulometric current efficiencies. Part X. An examination of the tin(IV)-tin(II)-tin(0) systems at platinum and gold electrodes

Abstract

Tin(II) is more readily handled coulometrically than volumetrically, and the electrode processes are critically evaluated. Zero-current tin(IV)-tin(II) potentials cannot be measured or derived from Lewartowicz plots, and charge-transfer kinetic parameters must therefore be referred to 0 V. Gold cathodes are deactivated towards reduction of hydrogen ion by specific adsorption of bromide ion, and give well separated waves for the reduction of tin(IV) to tin(II) and to tin metal, with limiting currents proportional to the tin(IV) concentration in 3·0 M bromide plus 0·4 M perchloric or hydrobromic acid medium. Mass and charge transfer kinetic parameters are derived from the voltammograms. Platinum cathodes become filmed and theories of the nature of the film are reviewed in the light of new work, which shows that the film is difficult to remove and was not completely removed in earlier work. The generation of tin(II) in bromide medium is superimposed on the film-suppressed hydrogen-ion wave. Chloride media are unsuitable. Kinetic parameters must be derived from anodic voltammograms of tin(II), which are uncomplicated in the bromide medium. At low bromide concentrations, the platinum anode is filmed with a species such as [SnBr₅OH]^2–, which blocks the reaction. The electrode mechanism is discussed. Charge-transfer parameters vary with potential or current, but synthesised computer-plotted voltammograms give a good fit with experimental curves. From the kinetic parameters the current efficiency for tin(II) generation is computed, and for 0·2 M tin(IV) the efficiency is better than 99·98 per cent. over the current density range 100 to 300 mA cm^–2 but decreases rapidly outside this range. Sample concentrations must be chosen so as to maintain the intermediate current within range throughout the determination, and platinum cathodes must be filmed quickly, but not aged for too long.