CHARACTERIZATION OF QUASI-REVERSIBLE SURFACE PROCESSES BY SQUARE-WAVEVOLTAMMETRY

Quasi-reversible redox reactions of species bound to the electrode sur face are characterized by standard potential, transfer coefficient, an d rate constant. Numerical calculations show that these nonlinear para meters affect the square-wave voltammetric response in complex ways yi elding a variety of peak shapes. For typical analytical conditions [st ep height (nDELTAE(s)) = 10 mV, amplitude (nE(sw)) = 50 mV], voltammog rams fall into three types depending upon the dimensionless rate const ant 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 asympt otically approaches the standard potential and passes through maximum height at about log kappa-degrees = 0.4. For kappa-degrees greater tha n 10(0), the single peak splits into two and the peak heights approach zero. Other values of square-wave amplitude change the response in pr edictable ways which can be visualized in the two dimensions of base s taircase potential and square-wave amplitude. The practical characteri zation of a surface redox reaction by square-wave voltammetry is demon strated with azobenzene adsorbed on mercury. Adsorbate coverage, stand ard potential, transfer coefficient, and rate constant are obtained fr om square-wave voltammograms using nonlinear least squares analysis (C OOL). At a surface concentration of 26 pmol cm-2 in 0.5 M acetate buff er, 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 co efficient of 0.51.