CHARACTERIZATION OF QUASI-REVERSIBLE SURFACE PROCESSES BY SQUARE-WAVE VOLTAMMETRY

Quasi-reversible redox reactions of species bound to the electrode surface are characterized by standard potential, transfer coefficient, and rate constant. Numerical calculations show that these nonlinear parameters affect the square-wave voltammetric response in complex ways yielding a variety of peak shapes. For typical analytical conditions [step height (nDELTAE(s)) = 10 mV, amplitude (nE(sw)) = 50 mV], voltammograms fall into three types depending upon the dimensionless rate constant kappa-degrees = k-degrees-t(p), where k-degrees is the first-order rate constant (s-1) and t(p) (s) is the pulse width of the waveform. For kappa-degrees less than 10(-2), the net peak is constant in height and width and shifts linearly with the logarithm of the rate constant. For kappa-degrees between 10(-2) and 10(0), the peak position asymptotically approaches the standard potential and passes through maximum height at about log kappa-degrees = 0.4. For kappa-degrees greater than 10(0), the single peak splits into two and the peak heights approach zero. Other values of square-wave amplitude change the response in predictable ways which can be visualized in the two dimensions of base staircase potential and square-wave amplitude. The practical characterization of a surface redox reaction by square-wave voltammetry is demonstrated with azobenzene adsorbed on mercury. Adsorbate coverage, standard potential, transfer coefficient, and rate constant are obtained from square-wave voltammograms using nonlinear least squares analysis (COOL). At a surface concentration of 26 pmol cm-2 in 0.5 M acetate buffer, azobenzene on mercury has a standard potential of -0.239 V vs SCE, a surface reaction rate constant of 160 s-1, and a charge-transfer coefficient of 0.51.