Controlled-current techniques chronopotentiometry

Chronopotentiometry — is a controlled-current technique (- dynamic technique) in which the - potential variation with time is measured following a current step (also cyclic, or current reversals, or linearly increasing currents are used). For a - nernstian electrode process,... 

Current step— The excitation signal used in controlled current techniques in which the potential is measured at a designated time [i]. See also - chronopotentiometry, -> cyclic chronopotentiometry, - staircase voltammetry. Ref. [i] Heineman WR, Kissinger PT (1984) In Kissinger PT, Heine-man WR (eds) Laboratory techniques in electroanalytical chemistry. Marcel Dekker, New York, pp 129-142... 

Chronopotentiometry is a controlled-current technique in which the potential variation with time (0 is measiued following a current step. Other ciurent perturbations such as linear, cyclic, or current reversals are also used [1,3,4,12]. For a reversible electrode reaction following a ciurent step, chronopotentiograms shown in Fig. 7 can be obtained. 

For controlled-current DC polarography, especially its current density mode, see under Chronopotentiometry at a dme (p. 172). For charge-step polarography, i.e., a controlled charge of coulostatic technique, see ref. 9, pp. 424-429, and ref. 3, pp. 270-276. 

Controlled-current chronoabsorptometry involves the simultaneous optical monitoring of the product or other redox component in the electrode mechanism during a chronopotentiometry experiment [14]. Although this technique has been demonstrated with Sn02 optically transparent electrodes, it has generally received little use, since the resistance effects in thin-film electrodes can give unequal current densities across the electrode face. This results in distorted potential-time and absorbance-time responses. Consequently, the more prevalent spectro-electrochemical methods utilize potential rather than current as the excitation signal. 

In this section, we will show that the stationary responses obtained at microelectrodes are independent of whether the electrochemical technique employed was under controlled potential conditions or under controlled current conditions, and therefore, they show a universal behavior. In other words, the time independence of the I/E curves yields unique responses independently of whether they were obtained from a voltammetric experiment (by applying any variable on time potential), or from chronopotentiometry (by applying any variable on time current). Hence, the equations presented in this section are applicable to any multipotential step or sweep technique such as Staircase Voltammetry or Cyclic Voltammetry. 

A cell (Fig. 54) that allowed the precise control of potential and current was designed by Goldberg and Bard, who also demonstrated the advantage of combining ESR spectroscopy with electrochemical techniques such as CA, CV, and chronopotentiometry [366]. The latter approach was taken in a study of the reduction of cyclooctatetraene (COT) in which it was demonstrated that the COT radical anion is stable in the presence of tetra-butylammonium ion, which had been a matter of dispute in previous work [378] (Fig. 55). 

Studies of the oxidation of formaldehyde have been carried out primarily with nonstationary techniques, i.e., cyclic voltam-etry and chronopotentiometry. The polarograms obtained with formaldehyde show evidence of two oxidation peaks, which are shifted depending on the pH of the electrolyte. The peak currents which are not diffusion controlled, were found to be larger for formaldehyde than for methanol or formic acid. Plots of log i vs, V from the cyclic voltametric data for the oxidation... 

Nowadays, most chemists know the name "Sand" only because it appears attached to a partic ilar equation in texts that describe chronopotentiometry. This technique can be useful in the diagnosis of electrode reactions (l). A current step i is Impressed across an electrochemical cell containing iinstirred solution, the potential of the working electrode is measured with respect to time and the transition time x is noted. According to the Sand equation, the product ix /2 should be constant for an uncomplicated linear diffusion-controlled electrode reaction at a planar electrode. 

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