Cyclic Voltammetry at Micropipet Electrodes for the Study of Ion-Transfer Kinetics at Liquid/Liquid Interfaces

Cyclic voltammetry at micropipet electrodes is applied to the kinetic study of ion transfer at liquid/liquid interfaces. Simple and facilitated transfer of an ion that is initially present outside a tapered pipet was simulated by the finite element method, enabling complete analysis of the resulting transient cyclic voltammogram (CV) with a sigmoidal forward wave followed by a peak-shaped reverse wave. Without serious effects of uncompensated ohmic resistance and capacitive current, more parameters can be determined from a transient CV than from the steady-state counterpart obtained with a smaller pipet or at a slower scan rate. A single transient CV under kinetic limitation gives all parameters in a Butler−Volmer-type model, i.e., the formal potential, the transfer coefficient, the standard ion-transfer rate constant, k⁰, and the charge of a transferring ion as well as its diffusion coefficients in both phases. Advantages of the transient approach are demonstrated experimentally for reversible, quasi-reversible and irreversible cases. With a multistep transfer mechanism, an irreversible transient CV of facilitated protamine transfer gives an apparent k⁰ value of 3.5 × 10^-5 cm/s, which is the smallest k⁰ value reported so far. With the largest reliable k⁰ value of ∼1 cm/s reported in the literature, an intrinsic rate of the interfacial ion transfer varies by at least 5 orders of magnitude.