Rotating pulsed current techniques

As the field of electrochemical kinetics may be relatively unfamiliar to some readers, it is important to realize that the rate of an electrochemical process is the current. In transient techniques such as cyclic and pulse voltammetry, the current typically consists of a nonfaradaic component derived from capacitive charging of the ionic medium near the electrode and a faradaic component that corresponds to electron transfer between the electrode and the reactant. In a steady-state technique such as rotating-disk voltammetry the current is purely faradaic. The faradaic current is often limited by the rate of diffusion of the reactant to the electrode, but it is also possible that electron transfer between the electrode and the molecules at the surface is the slow step. In this latter case one can define the rate constant as ... 

The electrochemical impedance may be obtained from potentiostatic or galvanostatic experiments. Alternating current voltammetric techniques are well documented at the DME, as are various kinds of pulse techniques. The former has also been developed at rotating and tubular/channel electrodes. 

Many of the limitations of traditional linear-scan voltammetry were overcome by the development of pulse methods. We will discuss the two most important pulse techniques, differential-pulse voliammetry and Si/uare-wave voliamme.try. I he idea behind all pulse-voUammetric methods is lo measure the current at a lime when the difference between the desired faradaic curve and the interfering charging current is large. These methods are used with many different types of solid electrodes, the HMDL, and rotating electrodes (Section 25C-4). 

The Cottrell s equation indicates that the current is at any time proportional to the bulk concentration of the electroactive species, which potentially makes chronoamperometry a technique suitable for quantitative determinations. Actually, it is not so often directly used to this purpose, due to low sensitivity at times not short enough and poor selectivity exhibited in many situatimis. However, it is at the basis of a not so often used, maybe underestimated technique, namely the voltammetry with periodical renewal of the diffusion layer. This technique very often furnishes comparable information in respect to the more sophisticated voltammetry making use of the rotating disk electrode, requiring a much less sophisticated experimental setup. Although different initial conditions hold, the responses of pulse techniques described in the following are also based on chronoamperometric decays. 

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