Polarograms derivative

In this method, the peak current was found to be directly proportional to the drug concentration over the range of 1 -20 mg/dL at a half-wave potential of-0.8 V vj. S.C.E. A derivative polarogram was reported for chlorpromazine. 

Derivative polarograms yield peaks that are convenient for qualitative identification of analytes based on the peak potential, EpeaR. 

There was available in the 1950 s electronic circuitry that was capable of differentiating the observed current and presenting d//d as a function of E. Fig. 1.6g illustrates this facility. E i emerges as the peak maximum and the peak height at the maximum is proportional to 7,1. This is easily seen in Eq. 1.6a. A useful secondary advantage of these derivative polarograms is that closely spaced waves are more easily distinguished as peaks, ie the resolution is improved. This... 

A typical conventional polarogram for 0.003 M-cadmium sulphate in 1M potassium chloride in the presence of 0.001 per cent gelatin, and the corresponding derivative curve, are shown in Fig. 16.7 (/max is the maximum current recorded in the derivative mode). 

Fig. 3.54. Normal and derivative oscillographic AC polarograms, (a) without and with acid, (b) with depolarisator. 

This favorable situation may not be encountered in every case. With radical reductions endowed with high intrinsic barriers, the half-wave potential reflects a combination between radical dimerization and forward electron transfer kinetics, from which the half-wave potential cannot be extracted. One may, however, have recourse to the same strategy as with the direct electrochemical approach (Section 2.6.1), deriving the standard potential from the half-wave potential location and the value of the transfer coefficient (itself obtained from the shape of the polarogram) under the assumption that Marcus-Hush quadratic law is applicable. 

The mathematical description of the normal pulse polarogram is easily derived from either eqn. (33) or eqn. (38). For sufficiently large lt 2 values, eqn. (35b) holds and reversible behaviour is observed corresponding to... 

The partial derivatives in eqns. (153) will be functions of the potential-dependent rate constants, kj, etc. and the mean concentrations c 0,c z, and cR. So, for application to experimental data, the results of the present section have to be combined with those of Sects. 5.1.1 or 5.1.2. The interested reader can find extensive theoretical predictions of the a.c. polarogram in ref. 131. 

In fact, the earlier treatments from the Czechoslovakian school [11] did not involve simplifying assumptions as described above. However, in most cases, only expressions for the d.c. limiting current were derived. Extensions to this work, describing the complete d.c. polarogram, are known from several workers [168—170]. Usually, the mathematics are kept simple by adopting the Nemst equation instead of some rate equation and by assuming equal diffusion coefficients, e.g. DA — DQ — Dqa > etc ... 

A comparison of the reactions of specially selected substrates at the electrode with those in the liquid phase reveals differences associated with the influence of the electrode. These differences cannot be established by direct analysis of the polarograms if there are no adsorption waves of the test substances. Let us consider the aromatic derivatives of divalent sulfur. 

Dumanovic with colleagues carried out polarographic investigation of various derivatives 3-, 5-nitropyrazoles and 2-, 4-, 5-nitroimidazoles in aqueous buffer solutions (pH 1.8-9.3) and 0.1 M solution NaOH [903-912], On polarograms all the investigated nitroazoles have one or two waves of reduction both in acid and neutral medium. The first four-electron wave corresponds to reduction of nitro group to hydroxylamine one. The second two-electron wave corresponds to further reduction of hydroxylamine derivative to aminoazole (Scheme 3.34). 

Polarograms of the alkoxyboron-capped iron(II) tris-dioximates [328] exhibited well-defined reduction waves with the Em from -1 330 to -1 390 mV. The reduction is accompanied by one-electron Fe2+/i+ process. The apical alkoxy groups do not significantly affect the reduction potentials of the various complexes in the same series. Two series of complexes derived from structurally different systems (dimethylglyoximates and acetylacetonedioximates) have almost similar Em values [328]. 

One advantage of the derivative-type polarogram is that individual peak maxima can be observed for substances with half-wave potentials differing by as little as 0.04 to 0.05 V in contrast, classical polarography requires a potential difference of about 0.2 V for resolution of waves. 

The current output has d.c. as well as a.c. components at the input frequency, to (the fundamental harmonic), and at higher harmonics. The shape of an a.c. polarogram (fundamental harmonic) for a reversible system resembles the first derivative of a d.c. polarogram (Fig. 1). The equations describing the peak current (I )-potential relationship (wave shape) and current-frequency relationship are complex but reduce to simpler expressions in limiting cases. Table 1 summarizes the results for the reversible case. 

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