Kinetic parameters, migrating species

When mass transport kinetics interferes, the steady-state current-potential curves show a limiting current for the redox process, which brings into play this mass transport. The limiting current is shown on a current-potential curve as a horizontal plateau. It is often also called a limiting diffusion current. It is dependent on the mass transport parameters (migration, diffusion and convection) and is, with a few exceptionsproportional to the concentration of the consumed species, because their mass transport limits the reaction rateand therefore the current flow... 

The experimental technique controls how the mass transport and rate law are combined (and filtered, e.g. by removing convective transport terms in a diffusion-only CV experiment) to form the overall material balance equation. Migration effects may be eliminated by addition of supporting electrolyte steady-state measurements eliminate the need to solve the equation in a time-dependent manner excess substrate can reduce the kinetics from second to pseudo-first order in a mechanism such as EC. The material balance equations (one for each species), with a given set of boundary conditions and parameters (electrode/cell dimensions, flow rate, rate constants, etc.), define an I-E-t surface, which is traversed by the voltammetric technique. 

Helmholtz and diffuse-layer capacitances due to the presence of supporting electrolyte but not electroactive species discharged at e electrode. The bulk-resistance parameter due to migration remains the same in both representations. In the kinetic representation the double-layer capacitance (that is, the capacitance between the electrode and both supporting electrolyte and electroactive species in diffuse and Helmholtz layers) and the charge-transfer resistance (due to electroactive species in the compact Helmholtz layer) are replaced by the reaction capacitance in parallel with the reaction resistance... 

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