Differential pulse wave form

We will consider five subtopics tast polarography and staircase voltammetry, normal pulse voltammetry, reverse pulse voltammetry, differential pulse voltammetry, and square wave voltammetry. Tast polarography, normal pulse voltammetry, and differential pulse voltammetry form a sequence of development rooted historically in polarography at the DME. To illustrate the motivating concepts, we will introduce each of these methods within the polarographic context, but in a general way, applicable to both the DME and SMDE. Then we will turn to the broader uses of pulse methods at other electrodes. Reverse pulse voltammetry and square wave voltammetry were later innovations and will be discussed principally outside the polarographic context. 

Fig. 7.13. Determination of copper obtained from the purification plant of a copper refinery using liquid chromatography, (a) UV detection (A = 420 nm, solvent and other chromatographic conditions as in Fig. 7.11, injection volume = 10 /il), (b) electrochemical detection applying a pulse wave form (initial potential = 400 mV vs Ag/AgCl, final potential = 480 mV vs Ag/AgCl, delay between pulses = 1 s, pulse duration = 0.40 s, (i) DC component, (ii) pulse component, (Hi) differential pulse component, injection volume = 10 /il), (c) UV detection (A = 420 nm, same conditions as in Fig. 7.11 except that acetonitrile has been replaced by methanol and the ii jection volume is 3 d). Reproduced by courtesy Anal. Chem. 55 (1983) 718.

Potentiodynamictechniques— are all those techniques in which a time-dependent -> potential is applied to an - electrode and the current response is measured. They form the largest and most important group of techniques used for fundamental electrochemical studies (see -> electrochemistry), -> corrosion studies, and in -> electroanalysis, -+ battery research, etc. See also the following special potentiodynamic techniques - AC voltammetry, - DC voltammetry, -> cyclic voltammetry, - linear scan voltammetry, -> polarography, -> pulse voltammetry, - reverse pulse voltammetry, -> differential pulse voltammetry, -> potentiodynamic electrochemical impedance spectroscopy, Jaradaic rectification voltammetry, - square-wave voltammetry. 

The differential pulse and square wave techniques are among the most sensitive means for the direct evaluation of concentrations, and they find wide use for trace analysis. When they can be applied, they are often far more sensitive than molecular or atomic absorption spectroscopy or most chromatographic approaches. In addition, they can provide information about the chemical form in which an analyte appears. Oxidation states can be defined, complexation can often be detected, and acid-base chemistry can be characterized. This information is frequently overlooked in competing methods. The chief weakness of pulse analysis, common to most electroanalytical techniques, is a limited ability to resolve complex systems. Moreover, analysis time can be fairly long, particularly if deaeration is required. 

If this is too difficult to understand, do not worry about it. The important facts to remember are (f) if a electrode reaction is reported to be irreversible the polarographic wave or peak will often be less well formed than for a reversible process (n) irreversibility causes classical dc and normal pulse polarographic waves to be less steep but has little effect on their height Hi) on the otherhand irreversibility broadens and lowers the height of the differential pulse polarographic peak but the area under the peak is little affected. 

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]... 

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... 

Table 12-4. Analytical conditions for the direct determination of trace elements (metals) in seawater using CSV employing the reduction of the element in the adsorbed complex. Hie wave-form used for the voltammetric scan is indicated by DP (differential-pulse), SW (square-wave) and LS (linear-sweep). LD is the limit of detection standardized to an adsorption period of 60s.

The potential wave-form for differential pulse voltammetry (DPV) is shown in Figure 2.8. The perturbation consists of a series of pulses having constant amplitude, superimposed at the same time upon a staircase wave-form. In contrast with NPV, the current is sampled twice at each pulse period (at the beginning and at the end of the pulse). The difference between these two current values is recorded and displayed as function of the applied potential, E, as shown in Figure 2.9, where it is observed that the current function response is characterized by symmetric peaks [4,8]. 

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