The electrode responses of a tungsten bronze electrode differ in potentiometry and voltammetry and give access to the individual contributions of electron and proton transfer

• A pH sensitive tungsten bronze electrode has two different response mechanisms, depending on the measuring technique
• The open-circuit potentials are determined by a protonation/deprotonation equilibrium at the surface
• Potentials in CV are determined by an electron transfer centers and by a protonation/deprotonation equilibrium
• A thermodynamic analysis allows separating the individual contributions of electron and proton transfer

A tungsten wire covered with Na0.75WO3 acts in potentiometry as a reversible pH electrode having a pH dependent open-circuit potential Eocp with nernstian slope. The mid-peak potential Emp of cyclic voltammograms also depends on pH. At low pH (e.g., pH 2) and slow scan rates (e.g., 2 mV s–1) the voltammetric response is almost completely reversible. At higher pH and faster scan rates, the voltammetric systems exhibit features of increasing irreversibility. Under the conditions of reversibility, the Eocp and Emp differ significantly. Eocp is determined by the proton transfer at the electrode surface; whereas Emp is determined by the electron transfer equilibrium tungsten(VI)/tungsten(V) and the proton transfer at the electrode surface. The difference between Eocp and Emp provides the individual thermodynamic contributions of electron and proton transfer to the overall pH dependent redox electrode. This is the first time that both contributions can be separated for an insertion electrochemical system (thin surface layer). It is also shown for the first time that the mechanism of an ion-sensitive electrode can differ in potentiometry and voltammetry.

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