Different methods of deducing the basic equation of the thermodynamic theory of eleo trocapillarity for hydrogen-adsorbing metals are compared. The relations that determine the value of the isoelectric potential shift under different conditions, as well as the value of the dependence of the potential on the solution pH, the free charge remaining constant, have been deduced and experimentally verified. Quantitative formulae are given for the contributions of atoms and ions to the setting up of the pd at the metal/electrolyte-solution interface. Rbnn~n compare les diffbrentes m&hodes qui permettent de dCduire l'equation fondamentale de la th4orie de l'&ctrocapillarit8 pour les mktaux adsorbant I'hydrogkne. On tire des conclusions de cette &quation wncemant pfusieurs cas de changement du potentiel sous les conditions iso&ctriques, aussi bien que la variation du potentiel avec le pH de la solution B charge libre constante, Les relations obtenues ont CtB verif&s par l'exp&ience. On donne des expressions quantitatives pour les contributions des ions et des atomes it l'ktablissement de la diffkrence des potentiels B l'interface m&al/ solution. Zusammenfassung-Verschiedene Ableitungen der Grundgleichung der ElektrokapillaritQtstheorie der wasserstoffadsorbierender Metalle wurden verglichen. Aus dieser Gleichung wurden Ausdriicke fti die Potentialabhiingigkeit der Elektrode von der Zusan-unensetzung der Lijsung unter isoelektrischen Bedingungen, sowie fiir die Abhgngigkeit des Potentials vom pH der LSsung bei konstanter freier Ladung, abgeleitet. Die erhaltenen Beziehungen wurden experimentell gepriift. Quantitative Ausdriicke fir die Beteiligung von Atomen und Ionen an der Ausbildung der Potentialdifferenz an der Tremmngsfllche Metall/Lbsung wurden abgeleitet. So far the thermodynamics of surface phenomena has been developed mainly for mercury and amalgam electrodes. For the mercury electrode Frumkinl and Parsons* independently deduced equations that can be presented in the form ($3= -2 (%), (1) where qo is the potential measured against a reference electrode reversible with respect to the cation, ITa-the amount of adsorbed anion, ,xu, the mean chemical potential of the electrolyte ions and e the charge density of the metal side of the electrical double layer. For the case when a surface active ion i is substituted for a non-surface-active ion, with the total concentration remaining constant, Dutkiewicz and Parson9 deduced an equation that can be written? (2) Similar equations can be obtained also for electrodes from the Pt-group metals. * Manuscript received 31 October 1968. t The quantity I', characterizes" the adsorption in the inner part of the double layer, rather than the total adsorption. This difference, due to the chosen conditions of concentration variation, is however of no importance for our further treatment. 391 * The conditions of perfect pokrizability can be realized also with a smooth Pt electrode if the volume of the solution in contact with the electrode is very ~rnall.~~ It would be, however, hardly possible to carry out under these conditions the measurements described below.