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 vertical bar solution Interface than the canon 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 (C) 2010 Elsevier B V All rights reserved