The dropping-mercury electrode

Although it was the first hydrodynamic electrode to be invented, the mathematical solution of mass transport at the DME is complex owing to the fact that no genuine steady-state can be attained such that dc/dt =h 0 

Detailed expositions of limiting current deviations may be found in standard polarography texts, e.g. refs. 5 and 6. Here, we try to pick out the more salient points. 

In the calculation, our time scale is the lifetime, r, of a mercury drop. Its radius at time t is given by 

Firstly, owing to the importance of the Ilkovic equation, we follow Ilkovic s argument [52] in the calculation of the limiting current, which although not completely rigorous, was later shown by MacGillavry and Rideal [53] to be correct under Ilkovic s assumptions. There are three steps to the argument. 

These are both forms of the Ilkovid equation. The first has been shown experimentally to be only reasonably correct in terms of time dependence however, the expression for the average current is substantially correct. The principal defect of this derivation is that it takes no account of electrode curvature. 

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Boujlel and Simonet used an electrochemical method to prepare a group of similar compounds, including compound ]5, shown in Eq. (3.41). In a typical case, benzil was reduced in DMF solution at the dropping mercury electrode in the presence of tetrabutylammonium iodide, used in this case as a supporting electrolyte rather than phase transfer catalyst. In the presence of diethylene glycol ditosylate, compound 15 (mp 77— 78°) was isolated in 10% yield. Using the same approach, acenaphthenedione was reduc-tively cyclized with triethylene glycol ditosylate to afford the product (mp 84—85°, 42% yield) shown in Eq. (3.42). 

Controlled-potential separation of many metals can be effected with the aid of the mercury cathode. This is because the optimum control potential and the most favourable solution conditions for a given separation can be deduced from polarograms recorded with the dropping mercury electrode see Chapter 16. 

The basic apparatus for polarographic analysis is depicted in Fig. 16.1. The dropping mercury electrode is here shown as the cathode (its most common function) it is sometimes referred to as the working or micro-electrode. The... 

The polarographic determination of metal ions such as Al3 + which are readily hydrolysed can present problems in aqueous solution, but these can often be overcome by the use of non-aqueous solvents. Typical non-aqueous solvents, with appropriate supporting electrolytes shown in parentheses, include acetic acid (CH3C02Na), acetonitrile (LiC104), dimethylformamide (tetrabutyl-ammonium perchlorate), methanol (KCN or KOH), and pyridine (tetraethyl-ammonium perchlorate), In these media a platinum micro-electrode is employed in place of the dropping mercury electrode. 

TECHNIQUE OF AMPEROMETRIG TITRATIONS WITH THE DROPPING MERCURY ELECTRODE... 

TABLE 3-1 Functional Groups Reducible at the Dropping Mercury Electrode... 

There are several types of mercury electrodes. Of these, the dropping mercury electrode (DME), the hanging mercury drop electrode (HMDE), and mercury film electrode (MFE) are the most frequently used. 

Copper(II) ions in the presence of chloride ions are reduced at the dropping mercury electrode (dme) in two steps, Cu(II) -> Cu(I) and Cu(I) -> Cu(0) producing a double wave at -1-0.04 and 0.22 V versus sce half-wave potentials. In the presence of peroxydisulphate , when the chloride concentration is large enough, two waves are also observed the first limiting current corresponds to the reduction of the Cu(II) to Cu(I) plus reduction of a fraction of peroxydisulphate and the total diffusion current at a more negative potential is equal to the sum of the diffusion currents of reduction of Cu(II) to Cu(0) and of the peroxydisulphate. There is evidence that peroxydisulphate is not reduced at the potential of the first wave because of the adsorption of the copper(I) chloride complex at... 

Studies in the field of electrochemical kinetics were enhanced considerably with the development of the dropping mercury electrode introduced in 1923 by Jaroslav Heyrovsky (1890-1967 Nobel prize, 1959). This electrode not only had an ideally renewable and reproducible surface but also allowed for the first time a quantitative assessment of diffusion processes near the electrode s surface and so an unambiguous distinction between the influence of diffusion and kinetic factors on the reaction rate. At this period a great number of efectrochemical investigations were performed at the dropping mercury efectrode or at stationary mercury electrodes, often at the expense of other types of electrodes (the mercury boom in electrochemistry). 

The electrolyte dropping electrode [63] method, introduced in 1976, and subsequently used in conjunction with the four-electrode potentiostat [64], is a hydrodynamic technique, offering controlled convective transport. In essence, this approach is identical to the dropping mercury electrode [65] however, the drop consists of a flowing electrolyte liquid phase which forms a polarized ITIES with an immiscible continuous (receptor) phase. In... 

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. 

Where a working electrode other than the dropping mercury electrode is used, the electrode material is indicated by brackets. 

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. 

This equation is analogous to Eq. (5.4.18) or (5.4.19) for the steady-state current density, although the instantaneous current depends on time. Thus, the results for a stationary polarization curve (Eqs (5.4.18) to (5.4.32)) can also be used as a satisfactory approximation even for electrolysis with the dropping mercury electrode, where the mean current must be considered... 

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). 

Mercury electrodes require far less maintenance than solid metal electrodes. Especially for the dropping mercury electrode, a noticeable amount of impurities present in the solution at low concentrations (<10-5mol dm-3) cannot appreciably reach the surface of the electrode through diffusion during the drop-time (see Section 5.7.2). 

Mishra and Gode developed a direct current polarographic method for the quantitative determination of niclosamide in tablets using individually three different buffer systems, namely Mcllraine s buffers (pH 2.20 8.00), borate buffers (pH 7.80—10.00), and Britton Robinson s buffers (pH 2.00—12.00) as well as 0.2 M sodium hydroxide. The drug was extracted from the sample with methanol, appropriate buffer was added to an aliquot and the solution then polarographed at the dropping-mercury electrode versus saturated calomel electrode at 25°C [36], The resultant two-step reduction waves observed were irreversible and diffusion-controlled. For the quantitative determination, the method of standard addition was used. Niclosamide can be determined up to a level of 5—10 pg/mL. 

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