Analytical solution square wave voltammetry

The charging current must decay to zero, or be otherwise accounted for, before the analytically useful Faradaic current that results from electron exchange with electroactive species in the sample can be measured and used for calibration. Because the double layer is chaiged by ions that must move through the solution, the time constant for charging is the product of its capacitance (a few JJ.F) and the resistance of the solution (typically 100 Q). Charging can therefore be very fast. This is an important consideration because there is a relationship between the speed with which a current can be measured and the precision and detection limit of the assay. Various pulse and square-wave techniques, eg, pulse voltammetry and square-wave voltammetry (10), are used to increase the rate of charging and therefore the precision, accuracy, and/or speed of the assay. 

Fig. 7.25 Analytical solution in Cyclic Square Wave Voltammetry (SWV) for different situations with respect to the bulk concentrations of the ion (Eq. (7.50)). (a) net currents (b) forward (solid lines), and reverse (dashed lines) components. 7iim,ingress, ss = AzFD ac out, cj ln = 0,

Polarography (discovered by Jaroslav Heyrovsky in 1922) is a technique in which the potential between a dropping mercury electrode and a reference electrode is slowly increased at a rate of about 50 200 mV min while the resultant current (carried through an auxihary electrode) is monitored the reduction of metal ions at the mercury cathode gives a diffusion current proportional to the concentration of the metal ions. The method is especially valuable for the determination of transition metals such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, W, V, and Pt, and less than 1 cm of analyte solution may be used. The detection hmit is usually about 5 X 10 M, but with certain modifications in the basic technique, such as pulse polarography, differential pulse polarography, and square-wave voltammetry, lower limits down to 10 M can be achieved. 

The tracer is quantified by square wave voltammetry and the intensity of the peak current is proportional to the concentration of free tracer that accumulates at the surface of the electrode, and thus inversely related to the quantity of analyte present in the solution. The amplification of the signal (intensity of the current) obtained by use of Nafion is clearly shown in Fig. 8.14. The cobaltocenium tracers of amphetamine 35 and diphenylhydantoin 37 (Scheme 8.16a and b) were used for the development of the immunoassay of these substances [49,83]. This method allows analytes to be assayed in the range of 2-4 pM. 

The most common techniques that apply a constant and/or varying potential at an electrode surface, within a three-electrode system, measuring the resulting current intensity in an electrolytic solution are amperometry, cyclic voltammetry (CV), square wave voltammetry (SWV), and differential pulse voltammetry (DPV). These electro-analytical techniques evaluate the redox properties of a single compound or a mixture of compounds. The three-electrode system (Fig. 13.2) comprises an RE, a counter electrode (CE or auxiliary electrode) and a woiking electrode (WE). The RE contributes with a stable and known potential. 

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

The analytical use of GECE modified in situ by using bismuth solution for square wave anodic stripping voltammetry (SWASV) of heavy metals is also studied [36]. The use of this novel format is a simpler alternative to the use of mercury for analysis of trace levels of heavy metals. The applicability of these new surface-modified GECE to real samples (tap water and soil samples) is presented. 

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