3 – Electrode Potentials

Publisher Summary In this chapter, the Nernst potential of a metal using the concentration dependence of the chemical potential of an electrolyte is derived. To measure the potential, the half-cell is connected to a suitable second half-cell. For example, a copper electrode (Cu metal in CuSO4) can be coupled with a zinc electrode (Zn metal in ZnSO4). A diffusion potential Edif usually occurs then at the interface of the two electrolytes. If both half-cells contain sulfuric acid of a concentration much larger than the copper sulfate concentration, the diffusion potential can be neglected. For unknown activities the measurement of the standard electrode potential is more complicated. The standard electrode potential is defined at 1 mol/kg with the hypothetical activity coefficient of 1 (ideal diluted solution). First principal experimental determinations of standard potentials may only be made by extrapolation to this hypothetical value. For measurements, selected cell arrangements are used with complete elimination of the diffusion potential and with diluted electrolytes. Any cell reaction can be considered to be an electron transfer between two coupled half-cells. The measured potential corresponds to the difference of the electron energy. The arbitrary definition of a reference electrode raises the question of whether the electrochemical potential scale can be correlated with energy scales of electrons in surface physics. If measuring work functions or electron affinities, the reference value is the free electron in a vacuum.