The potential difference at each pole of the BPE (which may or may not be enough for electrochemical reactions).
Note that the solution potential is not directly controlled by a power source (e.g. potentiostat) because it depends also on the solution composition. Therefore, for electrons to transfer to reduce species in the solution, the potential of the working electrode need to be set to a value more negative than of an electroactive molecule in the solution, and then – depending on the kinetics – electrons may transfer. In similar fashion, oxidation reactions occur. Also, according to Ohm’s law, the electric field and solution potential (V) will increase with increasing solution resistivity and the applied current at the outer-circuit.