Analytical dropping mercury electrode

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

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. 

In contrast, the coupling of electrochemical and spectroscopic techniques, e.g., electrodeposition of a metal followed by detection by atomic absorption spectrometry, has received limited attention. Wire filaments, graphite rods, pyrolytic graphite tubes, and hanging drop mercury electrodes have been tested [383-394] for electrochemical preconcentration of the analyte to be determined by atomic absorption spectroscopy. However, these ex situ preconcentration methods are often characterised by unavoidable irreproducibility, contaminations arising from handling of the support, and detection limits unsuitable for lead detection at sub-ppb levels. 

Although the carbon-paste electrode is quite effective for readily oxidizable analytes, a full range of electrochemical response is only available with the mercury electrode, particularly the versatile version called dropping mercury electrode. Witli this electrode, analyses are conducted at a frequently renewed mercury... 

The following types of mercury electrodes have been widely used for voltammetry dropping mercury electrode (DME), hanging mercury drop electrode (HMDE), static mercury drop electrode (SMDE), streaming mercury electrode (SME), and mercury film electrode (MFE). We begin our discussion with a description of the construction and properties of the DME because this electrode has a long history and continues to be used for both analytical and fundamental studies. 

The dropping-mercury electrode. The usefulness of the dropping-mercury electrode (DME) for analytical voltammetry was discovered by Heyrovsky, and the history of this discovery has been recounted.102 The DME usually is prepared from a 10-20-cm length of glass-capillary tubing with an approxi-... 

Polarography (discovered by Jaroslav Heyrovsky in 1922) is a technique in which the potential between a dropping mercury electrode and a reference electrode is slowly increased at a rate of about 50 200 mV min while the resultant current (carried through an auxihary electrode) is monitored the reduction of metal ions at the mercury cathode gives a diffusion current proportional to the concentration of the metal ions. The method is especially valuable for the determination of transition metals such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, W, V, and Pt, and less than 1 cm of analyte solution may be used. The detection hmit is usually about 5 X 10 M, but with certain modifications in the basic technique, such as pulse polarography, differential pulse polarography, and square-wave voltammetry, lower limits down to 10 M can be achieved. 

The last major development is not of a purely analytical nature it is based on the idea of applying polarographic results for working out electrochemical routes of organic synthesis. The experience obtained from d.c. polarographic experiments can be utilized. By preparative electrolysis in extremely small volumes (about 1 ml) with a dropping mercury electrode in connection with the identification of products thus... 

Diffusion Current at the Dropping Mercury Electrode To derive an equation for polarographic diffusion currents, we must take into account the rate of growth of the spherical electrode, which is related to the drop time in seconds, t the rate of flow of mercury through the capillary m in mg/s and the diffusion coefficient of the analyte D in cm-/s. These variables are taken into account in the Ilkovic equation ... 

This electrode is constituted of a glass tube, maintained in a vertical position, of which the central passageway (10-70 pm in diameter), allows the transfer of mercury between a reservoir and the capillary tip, where very small droplets are formed. This dropping-mercury electrode is immersed in an unstirred solution containing the target analyte mixed with a support electrolyte. The surface of the droplet of mercury increases until it falls (around 4-5 s). The fall is most often provoked by a device producing a small impact on the electrode. Instantaneously, a new droplet, identical to the previous one, is formed presenting a fresh uncontaminated surface. 

Meites L (1963) Voltammetry at the dropping mercury electrode (Polarography). In Kolthoff IM, Elving PJ and Sandell EB, eds. Treatise on analytical chemistry. Part I (Theory and practice), Vol 4, section D-2, Electrical methods of analysis (Reilley CN, section advisor), pp. 2303-2380. John Wiley Sons, New York. 

DIN direct injection nebulizer, see also above German DIN norm diuresis excretion of urine DMBA dimethylbenzanthracene DME dropping mercury electrode DMG dimethylglyoxime (analytical reagent)... 

Polarography is then an example of voltammetry at controlled potential in which the working electrode consists of a dropping mercury electrode and the potential of the working electrode is changed in a linear mode. We shall not be concerned in this unit with any other parts of the above family of methods. Students who wish to obtain a broad overview of the whole family of methods should study ACOL Principles of Electro-analytical Methods. 

You will be introduced to the role of solvent, pH, and complexa-tion in polarography, to permit an intelligent choice of a suitable supporting electrolyte for a given analytical problem. The use and advantages of electrodes other than the dropping mercury electrode will be considered. 

It is important to realise that an analytical technique should not be given the name polarography unless the working electrode is specifically a dropping mercury electrode (DME). Polarography is voltammetry at controlled potential using a DME as the working electrode. 

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