RandlesSevcik

RandlesSevcik, more commonly referred to as the Randles-Sevcik equation, is a fundamental relation in electrochemistry that describes how the peak current in cyclic voltammetry depends on experimental parameters for a reversible, diffusion-controlled redox couple. The equation is named after Randles and Sevcik, who described it for planar electrodes at room temperature. It provides a practical means to extract kinetic and transport information from voltammetric data.

The standard form, valid at 25°C, is Ip = 2.69 × 10^5 n^(3/2) A C* D^(1/2) v^(1/2). In this expression, Ip is the peak current (A), n is the number of electrons transferred in the redox step, A is the electrode area (cm^2), C* is the bulk concentration of the electroactive species (mol cm^-3), D is the diffusion coefficient (cm^2 s^-1), and v is the scan rate (V s^-1). The square-root dependence on both D and v reflects diffusion-controlled transport to a planar electrode during a reversible electron transfer.

Assumptions underlying the equation include semi-infinite linear diffusion to a flat electrode, a reversible redox couple, diffusion-controlled current with negligible migration (due to sufficient supporting electrolyte), and a constant temperature. The expression is a tool for estimating diffusion coefficients, validating reversibility, and analyzing concentration and surface area in electrochemical experiments. Extensions and alternative forms exist for irreversible or quasi-reversible systems, different electrode geometries, or non-25°C conditions.