Vibrating dropping mercury electrode

Narayan, R. Studies on the Vibrated Dropping Mercury Electrode. Effect... 

Convection terms commonly crop up with the dropping mercury electrode, rotating disk electrodes and in what has become known as hydrodynamic voltammetry, where the electrolyte is made to flow past an electrode in some reproducible way (e.g. the impinging jet, channel and tubular flows, vibrating electrodes, etc). This is discussed in Chap. 13. 

Originally, polarography was defined as a method making use of current-voltage curves obtained in the electrolysis at a dropping mercury electrode of the solutions to be analyzed. Later, the application of solid electrodes [8] which may be stationary, rotated or vibrated was introduced. Polarography with solid electrodes is often called voltammetry. 

The compound is an air-stable, white, crystalline solid, soluble in water. Its aqueous solution is stable between pH 7 and 8.S. In O.SM tris(hydroxy-methyl)aminomethane -l-O.SM NaCI buffer of pH 8.1, the polarogram, obtained with a dropping mercury electrode, ofa6xlO M solution of the polyanion shows two waves with half-wave potentials —1.15 and — 1.26 V versus SCE. The 250 MHz WNMR spectrum in HjO-DjO (90 10) solution at 40°C, shows four resonance lines with relative intensities 1 2 2 2 at — 16.8, — 68.5, — 129.4, and — 224.6 ppm (reference external 2 M NajWO in alkaline D2O). The IR spectrum (KBr pellet) shows numerous bands characteristic of (a) crystallization water molecules 3400-3420 and t630cm" , (b) ammonium counterions 3160, 3000, 2820, and 1405cm , and (c) vibration modes of the polyanion 925, 865 (sh), 825, 770, 730, 680, 560, 480, 410, 350, and 320cm ... 

A further method relies on the fact that the natural drop time of a dropping mercury electrode is proportional to the interfacial tension [18]. Again, drop birth can be detected electrically by the sudden change in impedance, so the method is easily automated. Unlike the other methods there is no adequate theory describing the mechanism of drop detachment, so the proportionality constant is again obtained by calibration with a solution of known properties. The method is extremely sensitive to vibrations and impurities, and consequently it is difficult to obtain results better than 1%. Also as a dropping mercury electrode is used, the system is dynamic and may not be in equilibrium if the rate of adsorption is slow. Similarly, capacitance measurements will be frequency dependent if adsorption is slow compared with the period of a.c. perturbation, and this provides a useful check of whether equilibrium is obtained. 

That effect does not occur in measurements with dropping mercury electrodes (DME) and a great effort has been employed to develop an electrode with similar features but without the charging current, sensitivity to vibration and to impurities and, of course, without the need to clean the mercury. Such an universally applicable electrode does not yet exist. For some applications, however, electrodes... 

Forced convection is often applied to enhance the rate of the mass transport process, ft can be achieved by stirring the solution with the help of a separate stirrer, or the electrode itself can rotate, vibrate, or even simply expand its volume (which is a movement of its surface against the solution) as the dropping mercury electrode does. In the case of forced convection, the fluid flow can be laminar flow or turbulent flow. The flux of species i driven by one-dimensional motion along the a -axis can be given as follows ... 

The dropping mercury electrode with adjustable vibrating frequency of 5 Hz up to a record-high value of 1 kHz was then made and studied in detail ([172, 173], p. 76) (cf. Fig. 9.17). This was achieved using wider glass capillaries which were drawn out at the end. With conventional flow rates of mercury (m 0.002 g/s), this electrode made it possible to lower the time of contact U of the mercxury with the solution to so low values like in case of the Heyrovsky s streaming mercury... 

So far, five types of mercury electrode have been used in specular reflection measurements (i) hanging mercury drop electrode (HMDE) [38, 39], (ii) mercury film deposited on a platinum or gold substrate [40-42], (iii) mercury pool electrode with or without an amalgamated platinum ring guide [43, 44], (iv) mercury drop bottom electrode placed on the optical window [45—47], and (v) mercury drop bottom electrode placed on an underlying ion-conductive optically transparent polymer film [48]. To avoid the difficulty due to mechanical vibration and shape change, the mercury drop bottom electrode would be useful. 

This can be achieved in two different ways. In the first, a slow scan, comparable to that used in DC polarography, is carried out using an electrode with a constant, unchanging surface such as a mercury pool, a hanging mercury drop, or a solid electrode. The electrode can be stationary and the solution stationary or stirred or the electrode can be periodically displaced in the solution by rotation, vibration, etc. 

The most popular of the electrodes used in reductive electrochemical detection utilise an electrode consisting of mercury deposited as a thin film on a gold substrate. This design overcomes the problems of vibration and mechanical instability observed with the older design of mercury drop electrodes. 

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