Polarography, mercury electrodes

Drop time in polarography, 597, 608 Dropping mercury electrode 608, 628 Dry ashing 114 Dry box lOl Drying reagents 99 comparative efficiencies of, (T) 99 Drying of precipitates 119 Duboscq colorimeter 656 Duplication method 701... 

The term polarography basically refers to a method, where the current flowing across the electrochemical interface is recorded as a function of the applied electrode potential, historically in most cases a mercury electrode is involved. Thus polarography might be called also voltammetry. This sometimes results in confusing terms like e.g. AC voltammetry, which is obviously equivalent to AC polarography (see following entry). (Data obtained with this method are labelled DCP.)... 

Vittori O (1980) Polarographie study of adsorbed tellurium at the hanging and dropping mercury electrodes in 1 M hydrochloric or perchloric acid solutions. Anal Chim Acta 121 315-319... 

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. 

The gamma isomer of benzene hexachloride can also be determined by polarography (24, 0). The method is based on the fact that, under the conditions used, the gamma isomer is the only one of the five isomers that is reduced at the dropping mercury electrode. 

A modification of faradaic impedance measurement is a.c. polarography, where a small a.c. voltage is superimposed on the voltage polarizing the dropping mercury electrode (Fig. 5.181). 

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. 

In the course of the synthesis and the study of several group 14 metalloles (silacyclopentadienes and germacyclopentadienes) their reduction potentials were recorded (vs SCE), with 0.1 M TBAP in dimethoxyethane on DME by polarography and by hanging mercury electrode voltammetry176. In (r 4-gcrmacyclopcnladicnc)C.o(C.O)212... 

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. 

Polarography is the measurement of the current flowing at a dropping mercury electrode as the potential applied to this electrode is changed. Voltammetry is the measurement of the current flowing at a stationary electrode as the potential applied to this electrode is changed. 

One can see immediately that the easy oxidation of mercury renders it of little use for anodic scans. Note that to construct a solid mercury electrode one can simply immerse a gold electrode in mercury for a few seconds. The amalgam that forms produces a mercury electrode much more manageable than the dropping electrode used in polarography. 

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. 

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.

Dropping mercury electrode (DME) Historically, a popular choice of working electrode in polarography. 

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. 

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