In this paper a model was proposed to calculate the interface potential of a non ideal finite crystal. Most of the research in this issue usually assume ideal conditions to work with infinite perfect crystals. The model includes a perturbative potential to consider an effect associated to finite size crystal and superficial atomic rearrangement. This effect is considered to be in a first order. The model was applied to graphite , as an example, mainly because of its theoretical interest for wastewater electrochemical treatment.