The turn-on bias, VBPM, required to drive current through the BPM can be understood from calculating the cell potential required to drive the ionization of water molecules. The autoionization equilibrium constant for water at 25 ºC is KW = 1.0 x 10 -14 . Hence, the reaction is not spontaneous at the conditions of interest in this work. The standard Gibbs free energy for this reaction is: ΔG = -RT ln KW = 79.9 kJ mol -1 .