Potential peak, differential pulse polarography

The concentration of As(III) in water can be determined by differential pulse polarography in 1 M HCl. The initial potential is set to -0.1 V versus the SCE, and is scanned toward more negative potentials at a rate of 5 mV/s. Reduction of As(III) to As(0) occurs at a potential of approximately —0.44 V versus the SCE. The peak currents, corrected for the residual current, for a set of standard solutions are shown in the following table. 

The polymers can be oxidized by differential pulse polarography. Their oxidation is metal-centered and leads to Ru(III) compounds. The potential is located aroimd -1-1.26 V (SCE). It can be stated that the polymers which contain the triphenylamine structure imits in the main chain show, as expected, an additional peak caused by the amine nitrogen. Substitution at the triphenylamine by electron-donating substituents lowers these potentials to 1.05 V (25), whereas acceptor substituents cause an increase of the oxidation potential (23). 

Cyclic voltammetric methods, or other related techniques such as differential pulse polarography and AC voltammetry,3 provided a convenient method for the estimation of equilibrium constants for disproportionation or its converse, comproportionation. In this respect, the experimentally measured quantity of interest in a cyclic voltammetric experiment is E>A, the potential mid-way between the cathodic and anodic peak potentials. For a one-electron process, E,A is related to the thermodynamic standard potential Ea by equation (4).13 In practice, ,/2 = E° is usually a good approximation. 

With the differential pulse polarography [245], the antibiotics can be determined at low concentration, if necessary, at the ppm or even sub-ppm level. Tetracycline hydrochloride is determined in aqueous acetate buffer pH 4 (detection limit 0.1 ppm), but for the analysis of chlortetracycline hydrochloride, oxytetracycline hydrochloride and free tetracycline, a non-aqueous medium must be used. Streptomycin sulphate is analysed in alkaline solution, trace quantities of zinc being masked by Na2EDTA, and the detection limit is 1 ppm. A determination in blood serum or urine is also possible but the peak potentials are shifted here to more negative values. The polarographic determination is preceded by ultrafiltration. Penicillin G potassium and ampicillin must be first functionalised by nitrosation. The authors also recommend an analysis of mixtures which is however demonstrated only with chloramphenicol and tetracycline, at 2.4 and 4.2 ppm, respectively. 

Figure 23-19 Voltammogram for a differential pulse polarography experiment. Here At = ig, - is, (see Figure 23-18). The peak potential, Epeak is closely related to the polarographic half-wave potential.

Higuera etal. [141] have studied reduction of 4-chloro-2,6-diisopropyloamino-5 -triazine in acidic media up to pH 5, applying dc differential pulse polarography. In the recorded voltammograms, two main reduction peaks were observed, with a prepeak at less negative potentials, and a postpeak at more negative potentials, what points to adsorption of the compound at the electrode. Two main peaks corresponded to two-electron reduction process. 

Differential pulse polarography also produces an ambiguous record for this kind of situation, as shown in Figure 7.3.18c. A peak is seen only for the Cd " reduction, because the trace covers potentials only on the negative side of the Fe wave. We note again that the differential pulse polarogram approximates the derivative of the normal pulse record hence distinct peaks will not be seen in DPV (or in SWV) unless distinct waves appear in NPV. 

Separation of adjacent peaks in differential-pulse polarography, where the symmetry of the peak can also be made use of, is usually easier than that of consecutive peaks in linear-sweep voltammetry. As is the case in conventional polarography, more positive peaks that interfere can sometimes be shifted to more negative potentials and the sequence of peaks inverted by change in supporting electrolyte. 

In linear-sweep voltammetry and differential-pulse polarography, the problem of an excess of a species reduced at more positive potentials is of considerably smaller consequence than in conventional polarography. When the current peaks of the species present in excess and that of the components to be determined are separated by more than about 0.3 V in the former and about 0.2 V in the latter technique, the presence of the more positive peak has almost no influence. 

The potential of the peak Ep is indicative of which species is involved. If the reduction (or oxidation) mechanism is diffusion-controlled the concentration of the species controls the Faradaic current. Since differential pulse polarography effectively displays the derivative of this current, theoretically it is the area under the peak which is proportional to the concentration. However, provided the shape of the peak does not change, the height of the peak is also proportional to concentration. The choice between the two modes of measurement will be discussed later. 

Each electroanalytical technique has certain characteristic potentials, which can be derived from the measured curves. These are the half-wave potential in direct current polarography (DCP), the peak potentials in cyclic voltammetry (CV), the mid-peak potential in cyclic voltammetry, and the peak potential in differential pulse voltammetry (DPV) and square-wave voltammetry. In the case of electrochemical reversibility (see Chap. 1.3) all these characteristic potentials are interrelated and it is important to know their relationship to the standard and formal potential of the redox system. Here follows a brief summary of the most important characteristic potentials. 

As in differential pulse polarography, the characteristic parameters of the AC wave are the peak height, Ip,Ac the peak potential, Ep ac and the half-peak width, Wi/2-... 

DPP = differential pulse polarography EC = electrode and chemical reactions Ei/2 = half-wave potential, V Ep — peak potential, V... 

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