Polarography with dropping-mercury

The analytical method described here is based on the results obtained with dc polaro-graphy and differential pulse polarography at dropping mercury electrode. The detection and determination of the drug metronidazole (I) continues to be of interest, particularly because of its status as a drug of abuse. The nitro group in metronidazole is useful in electrochemical analysis. 

Investigation of chemical kinetics meet two limitations in direct current polarography (DCP) operating with dropping mercury electrodes the first is the narrow kinetic window given by the drop time 0.1 s < < 5 s... 

At Charles University in Prague, Jaroslav Heyrovsky (Fig. 3.1.4), first docent and from 1926 an ordinary professor of physical chemistiy, was dealing with a number of electrochemical problems. He was interested in the cmicept of electrode potential [22-25], in the theory of overpotential [26], in the electrokinetic potentials of galvanic elements [27], and in the electrolytic potential of iron amalgam [28]. After the discovery of electrolysis with dropping mercury electrode, he founded polarographic school and devoted himself to polarography (cf. Sect. 3.2). 

In voltammetry as an analytical method based on measurement of the voltage-current curve we can distinguish between techniques with non-stationary and with stationary electrodes. Within the first group the technique at the dropping mercury electrode (dme), the so-called polarography, is by far the most important within the second group it is of particular significance to state whether and when the analyte is stirred. 

Lopez-Fonseca et al. [11] discussed the theory of reverse pulse polarography and the technique was applied in the determination of penicillamine electrochemically coated on a dropping-mercury electrode. Using long drop times and short pulses, the drug can be determined at levels as low as 50 nM in the presence of Cu(II), and the technique compares well with normal-pulse and differential-pulse polarography. 

Voltammetry is a part of the repertoire of dynamic electrochemical techniques for the study of redox (reduction-oxidation) reactions through current-voltage relationships. Experimentally, the current response (i, the signal) is obtained by the applied voltage (.E, the excitation) in a suitable electrochemical cell. Polarography is a special form of voltammetry where redox reactions are studied with a dropping mercury electrode (DME). Polarography was the first dynamic electrochemical technique developed by J. Heyrovsky in 1922. He was awarded the Nobel Prize in Chemistry for this discovery. 

The electrode potential obtained with linear-sweep polarography, for example, at a dropping-mercury electrode (DME), is different again and is called the halfwave potential, 1/2, which is also discussed in Chapter 6. 

Figure 6.6 shows a schematic diagram of the apparatus required as a working electrode for polarography. Such a set-up is almost universally called a dropping mercury electrode (DME), with the mercury drop being immersed in a cell that is essentially the same as that shown in Figure 6.1. 

Powdered chlorpromazine hydrochloride (50 mg) was dissolved in 50 mL of water, a portion of the solution treated with 0.5N KCl and 0.2% gelatin solution, and diluted. Nitrogen was passed through the solution before polarography was performed out at 20 C. The polarographic scan initiated at -1.2V, and used an internal calomel compression electrode and a dropping mercury electrode. The method was used for the determination of chlorpromazine in injectable solutions and tablets [153]. 

Many of the experimental parameters for normal-pulse polarography are the same as with differential-pulse polarography. Differential-pulse polarography is a technique that uses a series of discrete potential steps rather than a linear potential ramp to optimize specific applications (130). Unlike normal-pulse polarography, each potential step has the same amplitude, whereas the return potential after each pulse is slightly negative of the potential prior to the step. In this manner, the total waveform applied to the dropping mercury electrode is very much like a combination of a linear ramp with a superimposed square wave. 

Equation (4.5) is also valid in this case. Reactions of this type are realized in polarography at a dropping mercury electrode, and the standard potentials can be obtained from the polarographic half-wave potentials ( 1/2)- Polarographic studies of metal ion solvation are dealt with in Section 8.2.1. Here, only the results obtained by Gritzner [3] are outlined. He was interested in the role of the HSAB concept in metal ion solvation (Section 2.2.2) and measured, in 22 different solvents, half-wave potentials for the reductions of alkali and alkaline earth metal ions, Tl+, Cu+, Ag+, Zn2+, Cd2, Cu2+ and Pb2+. He used the half-wave potential of the BCr+/BCr couple as a solvent-independent potential reference. As typical examples of the hard and soft acids, he chose K+ and Ag+, respectively, and plotted the half-wave potentials of metal ions against the half-wave potentials of K+ or against the potentials of the 0.01 M Ag+/Ag electrode. The results were as follows ... 

Voltammetry conducted with a dropping-mercury electrode, is called polarography (Figure 17-14). The dispenser suspends one drop of mercury from the bottom of the capillary. After current and voltage are measured, the drop is mechanically dislodged. Then a fresh drop is suspended and the next measurement is made. Freshly exposed Hg yields repro-... 

Voltammetry is a collection of methods in which the dependence of current on the applied potential of the working electrode is observed. Polarography is voltammetry with a dropping-mercury working electrode. This electrode gives reproducible results because fresh surface is always exposed. Hg is useful for reductions because the high overpotential for H+ reduction on Hg prevents interference by H+ reduction. Oxidations are usually studied with other electrodes because Hg is readily oxidized. For quantitative analysis, the diffusion current is proportional to analyte concentration if there is a sufficient concentration of supporting electrolyte. The half-wave potential is characteristic of a particular analyte in a particular medium. 

Equation (11) is also applicable as a good, or reasonably good, approximation to a number of techniques classified as d.c. voltammetry , in which the response to a perturbation is measured after a fixed time interval, tm. The diffusion layer thickness, 5/, will be a function of D, and tm and the nature of this function has to be deduced from the rigorous solution of the diffusion problem in combination with the appropriate initial and boundary conditions [21—23]. The best known example is d.c. polarography [11], where the d.c. current is measured at the dropping mercury electrode at a fixed time, tm, after the birth of a new drop as a function of the applied d.c. potential. The expressions for 5 pertaining to this and some other techniques are given in Table 1. 

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