Is perchlorate ion contact adsorbed from aqueous solutions at polarizable electrodes?

The potential and differential capacity of a mercury electrode in solutions containing perchlorate anion are examined at the point of zero charge within the context of modified Gouy–Chapman theory. It is shown that the data support a model of the inner layer in which the anion is closer to the electrode|solution interface than the cation without being contact adsorbed. It is also shown that the same model can explain data obtained at constant ionic strength in nitrate solutions. On the other hand, modified Gouy–Chapman theory cannot explain the behaviour of chloride ion at mercury because this anion is contact adsorbed.