Linear current—potential analysis

Aalstad and Parker, 1980, 1981 Linear current potential analysis df/df/Z/P) p Normalized potential sweep voltammetry Convolution potential sweep voltammetry... 

Parker and Bethell, 1981a. Measurements at an Hg electrode at 23°C with MejNBF (sat.) as electrolyte. The numbers in parentheses refer to the standard deviations in 5 measurements. Peak potentials are relative to a bias setting of — 1.680 V vs Ag/Ag+ in acetonitrile. It should be noted that the reduction process does not fulfill the requirements for purely kinetic waves, linear current potential analysis indicates a slope at 100 mV s" of about 80 mV rather than 69 mV for a purely kinetic wave... 

Most of the more advanced techniques have only rarely been used outside the laboratories where they have been developed, and for that reason it is not easy to give recommendations. Examples include normalized sweep voltammetry [34,35,157,158], linear current-potential analysis [33], and the so-called global analysis and related techniques [159-161]. However, one such technique, convolution potential sweep voltammetry, has gained some popularity, and is introduced briefly here. 

The film electrodeposition process was studied by means of linear sweep voltammetry. The rate of electrochemical reaction was determined from current density (current-potential curves). The film deposits were characterized by chemical analysis, IR - spectroscopy, XRD, TG, TGA and SEM methods. 

FIGURE 1.10. Rotating disk electrode voltammetry. A + e B, with a concentration of A equal to C° and no B in the solution a Linearized concentration profiles —, at the plateau (vertical arrow in b), , at a less negative potential (horizontal arrow in b). b Current potential curve, c Concentrations of A and B at the electrode surface, d Logarithmic analysis of the current potential curve. 

A more simple analysis of LSV waves can give essentially the same information as CPSV and NPSV. Analysis of theoretical current-potential data for Nemstian and purely kinetic waves revealed that a nearly linear region... 

Upon evaluating the convolution integral from the experimental current-potential (time) curve and its limiting values (Eq. 77), kinetic analysis can be performed with the help of Eq. (76). Conversely, Eq. (76) or similar equations can be used to calculate the theoretical current-potential curve, e.g., for the linear potential sweep voltammogram, provided that the values of all the parameters are known. Some illustrative examples were provided by Girault and coworkers [183]. 

Linear sweep voltammetry is based on the potential being ramped up between the working and auxiliary electrodes as current is measured. The working electrode is usually a SMDE nowadays, in which case this technique would be called linear sweep polarography. In this set-up, the auxiliary electrode is a mercury pool electrode and may also serve as the reference electrode. The resultant current-potential recording (the polarogram) can yield much information which can be used to qualitatively identify the species and the medium in which it is determined as well as calculate concentrations. Analysis of mixtures is also possible. The detection limit is of the order of 10 M. 

Fast linear sweep voltammetry can give a complete current potential curve in times of a second or much shorter and is thus suitable for following the kinetics of chemical reactions or where ever very rapid analysis is necessary. For more conventional polarographic methods scan times of 10 to 20 minutes are more typical. However the fast scan rates of fast linear sweep voltammetry result in large charging or capacitive currents necessary to charge up the electrode surface to the potential required. This results in a loss of sensitivity etc,in consequence fast linear sweep voltammetry is not a popular analytical technique. 

CV. A cyclic voltanunogram is obtained by applying a linear sweep potential (the potential increases or decreases linearly with time) at the WE. When a potential change occurs, the current flows through the WE, oxidizing or reducing the compound under analysis. The intensity of this current is proportional to the concentration of the compound in the solution, thus enabling the use of this technique in analytical... 

In hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the mercury drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 

The two dashed lines in the upper left hand corner of the Evans diagram represent the electrochemical potential vs electrochemical reaction rate (expressed as current density) for the oxidation and the reduction form of the hydrogen reaction. At point A the two are equal, ie, at equiUbrium, and the potential is therefore the equiUbrium potential, for the specific conditions involved. Note that the reaction kinetics are linear on these axes. The change in potential for each decade of log current density is referred to as the Tafel slope (12). Electrochemical reactions often exhibit this behavior and a common Tafel slope for the analysis of corrosion problems is 100 millivolts per decade of log current (1). A more detailed treatment of Tafel slopes can be found elsewhere (4,13,14). 

To improve the selectivity of chronoamperometric in vivo analysis, a differential measurement ta hnique has been employed Instead of a single potential pulse, the potential is alternately pulsed to two different potentials giving rise to the name double chronoamperometry. This waveform is shown in Fig. 15 B. Because the current contributions of individual electroactive components add linearly to produce the observed current output, the difference in current response at the two potentials is the current due to only those compounds which are oxidized at the higher potential and not oxidized at the lower potential. This system provides two responses, the current due to easily oxidized compounds and the current due to harder to oxidize compounds. This gives greater selectivity than the direct chronoamperometric method. 

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