POLAROGRAPHY AND PULSE VOLTAMMETRY

Because these methods are so deeply rooted in the polarographic tradition and even now are frequently used with polarographic electrodes, we begin with a discussion of phenomena at dropping mercury electrodes and then develop the subject through conventional polarography and into various forms of pulse voltammetry. 

The classical DME has two principal disadvantages. First, it has a constantly changing area, which complicates the treatment of diffusion and creates a continuous background current from double-layer charging. Second, its time scale is controlled by the lifetime of the drop, which cannot be varied conveniently outside the range of 0.5-10 s. 

In the sections below, we will discuss polarographic concepts first in the context of the DME, then with reference to the SMDE. 

Additional fine points about behavior at the DME are discussed in Section 5.3 of the first edition. 

The typical values of drop lifetime and drop diameter at maturity ensure that linear diffusion holds at a DME to a good approximation [Section 5.2.2(c)]. Thus, we begin by invoking the Cottrell relation, (5.2.11), while remembering that for the moment we are considering electrolysis only at potentials on the diffusion-limited portion of the voltammetric response curve. Since the drop area is a function of time, we must determine A t) explicitly. If the rate of mercury flow from the DME capillary (mass/time) is m and the density of mercury is then the weight of the drop at time t is 

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