Ion adsorption on mercury electrodes

The interfacial charge ojj = - a is given by the interfacial excess of adsorbed anions r. and cations r., in Eqn. 5-19  

Chemisorption of anions at the electrode interface involves dehydration of hydrated anions followed by adsorption of dehydrated anions which, then, penetrate into the compact double layer to contact the interface directly, this result is called the contact adsorption or specific adsorption. The plane of the contact adsorption of dehydrated anions is occasionally called the inner Helmholtz plane 

Many anions are chemisorbed at the interface of metallic electrode whereas, most cations are not prone to contact adsorption because the energy of hydration-dehydration of cations is usually greater than that of anions cations are usually smaller in size and greater in the hydration energy than anions. 

The shift of the potential of zero charge toward the negative direction induced by the contact adsorption of chloride ions has been found not only with liquid mercury electrodes but also with solid metal electrodes such as gold [Jiang-Seo-Sato, 1990]. 

For ion adsoiption in equilibrium on the electrode interface, the electrochemical potential Pi.., of hydrated adsorbate ions in aqueous solution equals the electrochemical potential Pi..d of adsorbed ions as shown in Eqn. 5-20  

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