The quasi-equilibrium assumption is often used to describe the kinetics of multi-step reactions in terms of a single rate-determining step (RDS), but this assumption is only valid when the rate constant (i.e., I0) for the RDS is at least 100 times lower than that for all the preceding steps, a condition which cannot be generally assumed . Alternatively, the steady-state assumption can be adopted: under constant current, it can be assumed that the effective concentrations of all intermediate species in the multi-step reaction approach steady-state values at sufficiently long timescales (i.e., at low frequencies) . Under this assumption, the current must be equal for each reaction step (I = In), so the total charge-transfer resistance for a multi-step reaction with n steps in series is equal to the sum of the resistances of each step: