Non-stationary or relaxation techniques

Without enforced mass transport, an excitated system will have the tendency to reach a new (pseudo-) equilibrium situation concordant with the demands of the perturbation. It is said that the system relaxes on perturbation by means of a response that varies with time towards an asymptotic value. 

Theoretically, the perturbation can be an arbitrary, more or less complex, function of time. However, only a limited number of functions have been shown to be of practical importance. These are known as step, pulse, double-step, double-pulse, periodic square wave and periodic sine wave. A survey of the most common techniques is found in Table 2. 

The variable to be perturbed is either the potential, E, or the Current density, j. The response on a potential perturbation, logically, will be the resulting current j (t), but it is additionally useful to measure the integral of j(t), i.e. the charge q (0 that has passed the interface. As a counterpart, a technique is known where the perturbation is a current pulse of infinitely small time duration (delta function) comprising a certain amount of charge the coulostatic impulse technique. 

Finally, a distinction has to be made between techniques employing 

Examples of wave forms in use as perturbation in electrochemical kinetics. 

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