Tafel analysis irreversible

Fig. 2.3 The cyclic voltammetric response for an irreversible one-electron reduction process, with the region required for Tafel analysis highlighted. The inset shows the Tafel plot for the forward scan highlighted is the required linear region. The voltage scan starts at 0.0 V and sweeps negativelyto —0.5 Vbefore returningto 0.0 V (small arrows indicate scan direction).

Tafel analysis of the voltammetric wave will provide the same information. From Problems 2.3 and 2.4, we know that for a one-electron irreversible wave, Tafel analysis yields a line of gradient equal to aP/RT, whereas for a reversible one-electron wave the gradient is equal to F/RT, such that the exponential portion (Tafel region) of the voltammogram is steeper in the reversible case. 

Hence, for a reversible system, the well-known linear relation is obtained between the potential E and log (/iim -///). Other equations have been derived for those reversible systems that involve semiquinone formation, dimerization, or the formation of complex compounds with mercury. Logarithmic analysis of the polarographic wave is often the only proof of reversibility which is considered but recently several authors, in particular Zuman and Delahay, " have pointed out that it is inadequate to assume that an electrode process is reversible on this evidence alone. For a reversible reaction, plots of E vs. In (/lim - ///) give the electron number z from the slope of the plot, RT/zF, A clearer indication of irreversibility is the evaluation of slopes of log i-E curves for higher concentrations (for i < /lim). Irreversible processes will give Tafel behavior. 

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