A detailed analysis of the electronic structure of three different electrochemical interfaces as a function of the chemical potential (μ) is performed using the grand canonical density functional theory in the joint density functional theory formulation. Changes in the average number of electrons and the density of states are also described. The evaluation of the global softness, which measures the tendency of the system to gain or lose electrons, is straightforward under this formalism. The observed behavior of these quantities depends on the electronic nature of the electrochemical interfaces.