Controlled-current techniques reversible waves

Voltammetric techniques that can be applied in the stripping step are staircase, pulse, differential pulse, and square-wave voltammetry. Each of them has been described in detail in previous chapters. Their common characteristic is a bell-shaped form of the response caused by the definite amount of accumulated substance. Staircase voltammetry is provided by computer-controlled instruments as a substitution for the classical linear scan voltammetry [102]. Normal pulse stripping voltammetry is sometimes called reverse pulse voltammetry. Its favorable property is the re-plating of the electroactive substance in between the pulses [103]. Differential pulse voltammetry has the most rigorously discriminating capacitive current, whereas square-wave voltammetry is the fastest stripping technique. All four techniques are insensitive to fast and reversible surface reactions in which both the reactant and product are immobilized on the electrode surface [104,105]. In all techniques mentioned above, the maximum response, or the peak current, depends linearly on the surface, or volume, concentration of the accumulated substance. The factor of this linear proportionality is the amperometric constant of the voltammetric technique. It determines the sensitivity of the method. The lowest detectable concentration of the analyte depends on the smallest peak current that can be reliably measured and on the efficacy of accumulation. For instance, in linear scan voltammetry of the reversible surface reaction i ads + ne Pads, the peak current is [52]... 

Among electrochemical techniques,cyclic voltammetry (CV) utilizes a small stationary electrode, typically platinum, in an unstirred solution. The oxidation products are formed near the anode the bulk of the electrolyte solution remains unchanged. The cyclic voltammogram, showing current as a function of applied potential, differentiates between one- and two-electron redox reactions. For reversible redox reactions, the peak potential reveals the half-wave potential peak potentials of nonreversible redox reactions provide qualitative comparisons. Controlled-potential electrolysis or coulometry can generate radical ions for smdy by optical or ESR spectroscopy. 

Cyclic voltammetry provides a very convenient method for determining the redox potentials of couples as the peak potentials for the cathodic, E, and anodic, pa, processes of a reversible couple are related, at 25 °C, to the redox potential by pa = E /2 = E° + 0.285/n volts and E. = pa/2 = E° — 0.285/n volts. pc/2 and EpJ2 are the potentials at a point half-way up the wave at these points the current is half the maximum value, i.e. ipc for the cathodic wave or ipa for the anodic wave. Again, this technique will yield redox potentials only if the couple is reversible in the electrochemical sense, but this is now very readily established through the above relationship that pa — -Epc = A p = 57/n mV and by the requirement that ipjip3 = 1. In addition it should be established that Ep is independent of the scan rate, v, and that the process is diffusion controlled by showing ip/v h to be constant. 

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