Hydrogen ideally polarizable electrodes

Despite of this charge transfer many of these systems may be treated as ideally polarizable electrodes, if the adsorbed species are not transformed into a different component present inside the bulk phase. The latter condition is violated, for example, in the hydrogen adsorption at metals of the platinum group in which the adsorbed hydrogen atoms can be in equilibrium with protons in solution and hydrogen molecules in gas phase or hydrogen dissolved inside the metal. The latter system corresponds to perfectly polarizable electrodes, see Ref. [13] for further discussion. 

The surface of the platinum electrode can be studied conveniently in the potential range between 0 and 1.7 V (RHE), where in inert solutions (not containing substances able to be oxidized or reduced), the surface is ideally polarizable. At a more negative potential, cathodic hydrogen evolution starts, whereas at more positive potentials, oxygen is evolved anodicaUy. 

Hydrogen electrodes are approximately non-polarizable, which implies that the solution and the interface are in equilibrium. This simplifies the task of maintaining a constant reference potential. In an ideally non-polarizable electrode, the electrode... 

Any double layer model has to explain experimental results, for example in Fig. 3.4 for sodium fluoride at a mercury electrode. Until the 1960s measurements were made almost exclusively at mercury electrodes and models were developed for this electrode. The fact that mercury is an ideally polarizable liquid in the zone negative to the hydrogen electrode means that its behaviour is often different from solid electrodes (monocrystalline and polycrystalline). These models are, therefore, of a predominantly electrostatic nature. 

Surface studies are difficult in the case of many metal electrodes since their regions of ideal or perfect polarizability are very narrow that is, the potentials of anodic dissolution (or oxidation) of the metal and of cathodic hydrogen evolution are close... 

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