Polarography cell with dropping mercury electrode

A potentially promising route to learn more about the complex redox chemistry of hydrogenase is to look at the direct electron transfer between the enzyme and an electrode. A study of this phenomenon may also be relevant to a possible coupling of solar cells with hydrogenasecatalyzed H2 production. Results from this line of research have thus far been very limited. A response was obtained in differential-pulse polarography on the dropping mercury electrode modified with polylysine (cf. van Dijk et al., 1985). Attempts to use... 

From the experimental point of view, for reductions, DC-polarography at the dropping mercury electrode (DME) or cyclic voltammetry (CV) at the hanging mercury drop electrode (HMDE) were used and for controlled-potential electrolyses at negative potentials, a mercury pool electrode was employed. For both oxidative and reductive experiments, voltammetry at the platinum rotating disk electrode (RDE) and CV at the stationary platinum electrode were applied. All experiments were performed in a three-electrode system with a platinum counter electrode. For measurements in analytical scale (a standard aminocarbene concentration was 3x 10-" mol/1), an undivided cell for 5-10 ml was used and for preparative electrolyses a two-compartment cell of the H-type was employed [14]. The potentials were referred to the saturated calomel electrode (SCE), which was separated from the investigated solution by a double-frit bridge. 

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. 

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. 

Figure 1.3.11 Typical two- and three-electrode cells used in electrochemical experiments, a) Two-electrode cell for polarography. The working electrode is a dropping mercury electrode (capillary) and the N2 inlet tube is for deaeration of the solution. [From L. Meites, Polarographic Techniques, 2nd ed., Wiley-Interscience, New York, 1965, with permission.] (Jb) Three-electrode cell designed for studies with nonaqueous solutions at a platinum-disk working electrode, with provision for attachment to a vacuum line. [Reprinted with permission from A. Demortier and A. J. Bard, /. Am. Chem. Soc., 95, 3495 (1973). Copyright 1973, American Chemical Society.] Three-electrode cells for bulk electrolysis are shown in Figure 11.2.2.

Polarography is voltammetry conducted with a dropping-mercury electrode. The cell in Figure 17-11 has a dropping-mercury working electrode, a Pt auxiliary electrode, and a calomel reference electrode. An electronically controlled dispenser suspends one drop of mercury from the tip of a glass capillary tube immersed in analyte solution. A measurement is made in 1 s, the drop is released, and a fresh drop is suspended for the next measurement. There is always fresh, reproducible metal surface for each measurement. 

Polarography, invented in 1922 by Czech electrochemist J. Heyrovsky, was in the period 1930 up to the end of the 1950s the worldwide commonly used electroanalytical method. In its classic form it represents a special case of linear sweep voltammetry characterized by the use of the dropping mercury electrode (DME). The linear voltage scan applied to the electrolytic cell is slow (typically 0.1 V up to 0.4 V min ). With regard to the usual mercury drop-life (1 -4 s) the fundamental assumption can be accepted that each single drop is polarized at nearby constant potential. 

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