Differential pulse techniques

Sensitivity In many voltammetric experiments, sensitivity can be improved by adjusting the experimental conditions. For example, in stripping voltammetry, sensitivity is improved by increasing the deposition time, by increasing the rate of the linear potential scan, or by using a differential-pulse technique. One reason for the popularity of potential pulse techniques is an increase in current relative to that obtained with a linear potential scan. 

Since 1978, the microprocessor has been increasingly used in electrochemical apparatus34. For instance, the above-mentioned microprocessor-controlled pH meters appeared on the market in about 1979 and the first application of a microprocessor in the differential pulse technique was even reported in 197735, in stripping voltammetry in 197836 and in combinations of various techniques37 41 from 1980 to 1982. 

It is necessary to mention that there is no universal and unambiguous nomenclature for the different differential pulse techniques, something which can lead to inaccurate analyses and misinterpretations. The DDPV technique is usually called Differential Pulse Voltammetry (DPV) referring to the double pulse... 

According to the above, the electrochemical response in the different differential pulse techniques can be very different, and it is worth analyzing the advantages and disadvantages of each method. Regarding the double pulse methods, in normal mode, DNDPV, this has the inconvenience of presenting asymmetrical peaks that can hinder the experimental determination of the peak current. In addition, the peak... 

The electrochemical characterization of multi-electron electrochemical reactions involves the determination of the formal potentials of the different steps, as these indicate the thermodynamic stability of the different oxidation states. For this purpose, subtractive multipulse techniques are very valuable since they combine the advantages of differential pulse techniques and scanning voltammetric ones [6, 19, 45-52]. All these techniques lead to peak-shaped voltammograms, even under steady-state conditions. 

Anodic stripping voltammetry (ASV) was applied to the determination of copper traces present as Cu(dik)2. The differential pulse technique was used to strip the amalgamated copper from a hanging mercury drop electrode. The experimental variables such as scan rate of electrode potential, deposition potential, deposition time and stirring speed of the solution could be optimized. The linear range of the calibration plot was 0.05-1 (xM and the LOD was 0.014 fiM Cu(II). A method was used for the determination of copper in breast milk and beer as typical examples of application, consisting of minerahzation of the sample, extraction of Cu(II) from the aqueous solution with a 1 M solution of acacH in chloroform and ASV end analysis . 

Most of the other voltammetric procedures described in the previous section have also been applied to the stripping step. The most widely used of these appears to be an anodic differential pulse technique. Often, narrower peaks are produced by this procedure, which is desirable when mixtures are to be analyzed. Use of the mercury... 

Signals obtained with linear-sweep voltammetry are less sensitive than those obtained with the differential-pulse technique. Therefore, the time needed for accumulating a sufficient amount of the preelectrolysis product is typically 10-60 min for the linear-sweep technique as opposed to 30 sec to 5 min for the differential-pulse mode. Since it is more difficult to control the conditions of the electrolysis over a prolonged period of time, the results obtained with differential-pulse voltammetry are usually more reproducible than those obtained with linear-sweep stripping voltammetry. 

The greater sensitivity of the differential pulse technique can sometimes even allow the use of a pre-electrolysis deposition step in an unstirred solution, thus avoiding the problems of the reproducibility of the stirring. For concentrations below 50 ppb stirring is essential but at higher concentrations it can sometimes be avoided. 

Y. Wang, E. O. Barnes, E. Laborda, A. Molina, and R. G. Compton. Differential pulse techniques in weakly supported media. Changes in the kinetics and thermodynamics of electrode processes resulting from the supporting electrolyte concentration, J. Electroanal. Chem. 673, 13 23 (2012). 

The readout in the semidifferential electroanalysis resembles a differential pulse polarogram (cf. section 4). Both techniques have much in common for enough small pulse amplitudes AE the peak potentials and peak widths coincide in both techniques. That is the reason why the differential pulse technique is preferred to that presented above. 

A recent innovation is the static mercury drop electrode [55]. The mercury flow to the capillary is controlled by an electromagnetic valve opened periodically for a short time only so that a mercury drop of a constant area is formed. The current is registered some time after the drop formation when the charging current is reduced practically to zero and does not interfere. The measurement is performed by d.c. or differential pulse technique and the sensitivity of the measurement is highly increased. A chromatographic detector with SMDE is manufactured commercially [ 54 ] ... 

The polarographic cell assembly comprising cell base and cover with a dropping mercury electrode as the cathode, a platinum wire auxiliary electrode as the anode and a saturated calomel reference electrode making solution contact via a liquid junction bridge. The mercury column is kept at a constant height, to have a natural drop time of about 4 s. The instrumental conditions for the differential pulse technique were 0.5 s drop time, -10 mV/S scan rate, 50 mV modulation amplitude. 

Fig. 2.36 Potential-time programs of the differential pulse techniques considered. Reproduced from Ref. [20] with permission from Elsevier...

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