Hanging mercury drop

Mercury electrodes (a) hanging mercury drop electrode (b) dropping mercury electrode (c) static mercury drop electrode. 

Anodic stripping voltammetry consists of two steps (Figure 11.37). The first is a controlled potential electrolysis in which the working electrode, usually a hanging mercury drop or mercury film, is held at a cathodic potential sufficient to deposit the metal ion on the electrode. For example, with Cu + the deposition reaction is... 

Potential-excitation signal and voltammogram for anodic stripping voltammetry at a hanging mercury drop electrode. 

Faraday s law (p. 496) galvanostat (p. 464) glass electrode (p. 477) hanging mercury drop electrode (p. 509) hydrodynamic voltammetry (p. 513) indicator electrode (p. 462) ionophore (p. 482) ion-selective electrode (p. 475) liquid-based ion-selective electrode (p. 482) liquid junction potential (p. 470) mass transport (p. 511) mediator (p. 500) membrane potential (p. 475) migration (p. 512) nonfaradaic current (p. 512)... 

Electrodes. The Hanging Mercury Drop Electrode is traditionally associated with the technique of stripping voltammetry and its capabilities were investigated by Kemula and Kublik.51 In view of the importance of drop size it is essential to be able to set up exactly reproducible drops, and this can be done as explained in Section 16.8 for the S.M.D.E. 

DME = dropping mercury electrode HMDE = hanging mercury drop electrode MFE = mercury film... 

Anodic shipping voltammetry (ASV) is the most widely used form of stripping analysis, hi this case, the metals are preconcenhated by elechodeposition into a small-volume mercury electrode (a tiiin mercury film or a hanging mercury drop). The preconcenhation is done by catiiodic deposition at a controlled tune and potential. The deposition potential is usually 0.3-0.5 V more negative than E° for the least easily reduced metal ion to be determined. The metal ions reach die mercury electrode by diffusion and convection, where diey are reduced and concentrated as amalgams ... 

FIGURE 3-14 Stripping voltammograms for 2 x 10 7 M Cu2+, Pb2+, In3+ and Cd2+ at the mercury film (A) and hanging mercury drop (B) electrodes. (Reproduced with permission from reference 21.)... 

The mercury film electrode has a higher surface-to-volume ratio than the hanging mercury drop electrode and consequently offers a more efficient preconcentration and higher sensitivity (equations 3-22 through 3-25). hi addition, the total exhaustion of thin mercury films results in sharper peaks and hence unproved peak resolution in multicomponent analysis (Figure 3-14). 

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. 

Irreversible potentials obtained with a hanging mercury drop working electrode a Ag/AgI/Me2SO reference electrode, and 0.1 M Et4N BF4 in Me2SO as a supporting electrolyte at a scan rate of 25 V/s. 

The properties of anodic layers of HgS formed on mercury in sulfide solutions have been investigated in comparison with anodic sulfide layers of cadmium and bismuth. Also, the electrochemistry of mercury electrodes in aqueous selenite solutions has been studied (see Sect. 3.2.1). The problem with the presence of several cathodic stripping peaks for HgSe in acidic Se(IV) solutions has been addressed using various voltammetric techniques at a hanging-mercury-drop electrode [119]. 

Fig, 16. Anodic stripping voltammogram for at the hanging mercury drop electrode. (Reprinted with permission from W. R. Heineman, in Water Quality Measurement The Modem Analytical Techniques , (H. B. Mark, Jr. and J. S. Mattson, eds.) Marcel Dekker New York, 1981)... 

HMDE Hanging mercury dropping electrode mass spectrometry... 

The above considerations also apply to the ion of an amalgamating metal with the reversible equilibrium M"+ + ne M(Hg) at a stationary mercury electrode such as an HMDE (hanging mercury drop) or an MTFE (mercury thin-film) with the restriction, however, that the solution can contain only ox, so that merely the cathodic wave (cf., eqn. 3.15) represents a direct dependence of the analyte concentration, whilst the reverse anodic wave concerns only the clean-back of amalgam formed by the previous cathodic amplitude. When one or both of the electrodic reactions is or becomes (in the case of a rapid potential sweep) irreversible, the cathodic wave shifts to a more negative potential and the anodic wave to a more positive potential (cf., Fig. 3.10) this may even result in a complete separation of the cathodic and anodic waves (cf., Fig. 3.11). 

The electrodes usually consist of mercury or deposited mercury or occasionally of inert solid material further, they are mainly of a stationary type (in the stripping step as the crucial analytical measurement, but not in the concentration step, where often the solution is stirred or the electrode is rotated). Considering the mercury, only exceptionally has a sessile mercury drop electrode (SMDE)91 or a slowly growing DME(drop time 18 min and phase-selective recording of stripping curve)92 been applied. Most popular are the hanging mercury drop electrode (HMDE) and the mercury film or thin-film electrode (MFE or MTFE). 

Figure 12. Current-potential curves for Ni(II)-cyclam (1 mM) in an aqueous 0.1 M KC104 solution (pH 4.5) under N2 (a) or C02 (b) at a hanging mercury drop electrode.135 Scan rate 0.1 V/s.

J.M. Sevilla, T. Pineda, A.J. Roman, R. Madueno, and M. Blazquez, The direct electrochemistry of cytochrome c at a hanging mercury drop electrode modified with 6-mercaptopurine. J. Electroanal. Chem. 451, 89-93 (1998). 

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