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A comparative study of the electrochemical behaviour of Ru and RuO 2 electrodes in solutions of 0?5M H 2 SO 4 , 1M HCl and 0?1M KOH has been performed with a view to elucidate in more detail the nature of the processes taking place on the surface of these electrodes within the range of potentials from 0?2 to 1?45 V (reversible hydrogen electrode, RHE). Cyclic voltammetry, chronopotentiometry and spectrophotometric analysis techniques have been employed. In accordance with thermodynamic data, it has been shown that amorphous layers of slightly soluble Ru(OH) 3 and RuO 2 .nH 2 O form on the surface of an active Ru electrode at 0?2,E,0?8 V and 0?8,E,1?1 V respectively, as a result of ruthenium anodic oxidation. Cathodic reduction of these layers proceeds reversibly within the indicated ranges of potential. It has been demonstrated that in the acidic medium, passivation of ruthenium electrode takes place at 1?1,E,1?35 V due to further oxidation of the ruthenium phase and formation of a more compact layer of anhydrous RuO 2 . The latter can be reduced only at potentials more negative than 0?2 V. In the case of alkaline medium, no passivation of the Ru electrode is observed within this particular range of potentials. At E.1?35 V, transpassive anodic dissolution of ruthenium electrode takes place in both acid and alkaline electrolytes concomitantly with oxygen evolution. The RuO 2 electrode demonstrates capacitor type voltammetric response at 0?4,E,1?4 V in both acid and alkaline medium. It has been proposed that such behaviour can be accounted for by a non-faradaic double layer process involving reversible adsorption/desorption of OH 2 ions on the surface of RuO 2 electrode as described by the following equation RuO 2 zH 2 O < Q ad RuO 2 :(OH À ) ad zH z where Q ad is the charge related to this process.

The present study represents comparative analysis of voltammetric and microgravimetric behavior of active ruthenium (Ru), electrochemically passivated ruthenium (Ru/RuO 2 ) and thermally formed RuO 2 electrodes in the solutions of 0.5 M H 2 SO 4 and 0.1 M KOH. It has been found that cycling the potential of active Ru electrode within E ranges 0 V -0.8 V and 0 V -1.2 V in 0.5 M H 2 SO 4 and 0.1 M KOH solutions, respectively, leads to continuous electrode mass increase, while mass changes observed in alkaline medium are considerably smaller than those in acidic one. Microgravimetric response of active Ru electrode in 0.5 M H 2 SO 4 within 0.2 V -0.8 V has revealed reversible character of anodic and cathodic processes. The experimentally found anodic mass gain and cathodic mass loss within 0.2 -0.8 V make 2.2 -2.7 g F À1 , instead of 17 g F À1, which is the theoretically predicted value for Ru(OH) 3 formation according to equationIn the case of Ru/RuO 2 electrode relatively small changes in mass have been found to accompany the anodic and cathodic processes within E range between 0.4 V and 1.2 V in the solution of 0.5 M H 2 SO 4 . Meanwhile cycling the potential of thermally formed RuO 2 electrode under the same conditions has lead to continuous decrease in electrode mass, which has been attributed to irreversible dehydration of RuO 2 layer. On the basis of microgravimetric and voltammetric study as well as the coulometric analysis of the results conclusions are presented regarding the nature of surface processes taking place on Ru and RuO 2 electrodes.