Electrodes for polarography

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. 

Fig. 5.30 Cells for ITIES measurements (a) cell with a stationary interface for voltammetry and (b) cell with a dropping electrolyte electrode for polarography. 1 aqueous electrolyte phase 1 aqueous electrolyte drop 2 organic phase 3 Teflon capillary 4 reservoir of aqueous electrolyte. CE and RE denote the counter and reference electrodes.

Figure 5.7 Controlled growth mercury electrode for polarography and C-3 cell stand for voltammetry (Reproduced by permission of BAS Inc.).

Tatsumi, H. and Tanaka, S. (2014) Development of dropping carbon paste electrodes for polarography. Electrochim. Acta, 135, 255-259. 

Mark Borisovich Bardin-Shteyn (1919, Odessa-1987, Kinshinev) (Fig. 5.7.6) was doctor of chemical science and fought in WW II. In 1947 he started to teach at KSU and rose from assistant to professor. From 1964 to 1975, he was dean of the chemical faculty. He was a specialist in polarography of platinum metals. His studies on the determination of ft and Pd on solid electrodes and on the application of a rotating Pt microdisc electrode for polarography are well known. 

In hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. The same potential profiles used for polarography, such as a linear scan or a differential pulse, are used in hydrodynamic voltammetry. The resulting voltammograms are identical to those for polarography, except for the lack of current oscillations resulting from the growth of the mercury drops. Because hydrodynamic voltammetry is not limited to Hg electrodes, it is useful for the analysis of analytes that are reduced or oxidized at more positive potentials. 

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 Schematic representation of a typical dropping-mercury electrode (DME) for polarography, where the DME acts as a working electrode in a cell such as that shown in Figure 6.1. The platinum electrode at the top right of the diagram is needed to give an electrical connection. The rate of mercury flow is altered by adjusting by changing the height h.

Hanging mercury-drop electrode (HMDE) A commonly employed working electrode in polarography. The HMDE is often preferred to the experimentally simpler dropping mercury electrode (DME) because resultant polarograms do not have a sawtoothed appearance and because accumulation (for stripping purposes) is readily achieved at its static surface. 

Indicator Electrode and Working Electrode In polarography and voltammetry, both terms are used for the microelectrode at which the process under study occurs. 

Three-Electrode Instruments for Polarography and Voltammetry In Fig. 5.45, if E connected to point a is a DC voltage source that generates a triangular voltage cycle, we can use the circuit of Fig. 5.45 for measurements in DC polarography as well as in linear sweep or cyclic voltammetry. An integrating circuit as in... 

Dropping Mercury Electrode and Zinc Reference Electrode for Continuous Polarography. Chem. Ind. 1957, 223. 

Fig. 5 Electrical circuitry for polarography/voltammetry. (A) A simple two-electrode system. (B) Illustrates a modern three-electrode system incorporating a potentiostat circuit.

Mechanistic studies can employ CPE if the coupled chemical reactions are slow. Conventional bulk electrolyses require typically 10-30 min for completion, longer than the typical longest time for voltammetric techniques (ca. 20 s maximum for cyclic voltammetry, CV, ca. 8 s for polarography, etc.). This is important to recall when comparing CPE with voltammetry data. An electrode reaction that is chemically reversible in a slow CV experiment may be irreversible in bulk electrolysis if the electrode product has a half-life of, e.g., a minute or two. Conversely, an electron transfer that is quasi- or irreversible in a relatively fast voltammetric experiment may be electrochemically reversible in the long timescale of bulk electrolysis. 

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