Pulse and square wave voltammetry

Pulse and square wave voltammetry are much better candidates than linear sweep or cyclic voltammetry for incorporation into a chemical sensor. The primary reason for this is the ability of the former techniques to discriminate between faradaic and capacitative currents. When a potential pulse is applied to an electrode, the capacitative current that flows is proportional to the 

The technique of square wave voltammetry (22) (see Table 8.1) has even more to offer as a voltammetric method of probing selective chemistry, because of the speed with which a scan can be carried out. The analytical signal in this technique is the difference between the current for the forward pulse and the current for the reverse pulse. Because of the large amplitude of the square wave, for a reversible reduction, the reduced electroactive species formed at the electrode during the forward pulse is re-oxidized by the reverse pulse. Consequently, the sensitivity of this method is enhanced when compared to differential pulse voltammetry. For identical conditions, an approximately 30% improvement in signal is obtained, but when the higher scan rates that are 

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The detection limit for TLV has been improved substantially by using differential pulse and square-wave voltammetry (Chap. 5). For example, detection limits in the 10 8 M range and below have been demonstrated in thin-layer cells requiring less than 100 /xL of sample [61,62]. One practical application of twin-electrode thin-layer cells is in the automatic electrochromic rearview mirror for automobiles. A cell with optically transparent electrodes is placed in front of a mirrored surface. At night, electrolysis in the cell to generate colored material can rapidly reduce glare from following vehicles. 

Bond, A.M., Czerwinski, W.A. and Llorente, M. (1998) Comparison of direct current, derivative direct current, pulse and square wave voltammetry at single disc, assembly and composite carbon electrodes stripping voltammetry at thin film mercury microelectrodes with field-based instrumentation. Analyst, 123, 1333-1337. 

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]... 

Notes Only a small fraction of a complete applied potential waveform scan is shown for differential pulse and square wave voltammetry. The entire measured signal for a single analyte is shown. 

Proper selection of the measuring time permits a signal-to-noise ratio to be improved dramatically. This characteristic is exploited in normal-pulse, differential-pulse, and square-wave voltammetry. In the first of these techniques it represents the only mechanism for decreasing the effect of capacitive current. Further elimination of the capacitive current can be achieved in differential-pulse voltammetry by limiting the duration of the applied pulse and by subtracting the current observed immediately prior to the imposition of the pulse. Increased rejection of the charging cur-... 

In pulse techniques such as differential pulse and square wave voltammetry, the capacitative contribution is eliminated via subtraction. Differential pulse voltammetry (DPV) measures the difference between two currents just before the end of the pulse and just before its application. Figure 2.32 shows the waveform of pulse utilised which is superimposed on a staircase. 

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