Mercury gallium amalgam

Gallium makes a safe substitute for mercury amalgams in dental filhngs when it is combined with tin or silver. 

Working electrodes are normally solid. The mercury electrode is the only liquid electrode at room temperature (with the rare exceptions of gallium and amalgam electrodes) and has been most used as a dropping electrode. 

III.l [see also Eq. (17) and Fig. 2], and that in the presence of a faradaic reaction [Section III. 2, Fig. 4(a)] are found experimentally on liquid electrodes (e.g., mercury, amalgams, and indium-gallium). On solid electrodes, deviations from the ideal behavior are often observed. On ideally polarizable solid electrodes, the electrically equivalent model usually cannot be represented (with the exception of monocrystalline electrodes in the absence of adsorption) as a smies connection of the solution resistance and double-layer capacitance. However, on solid electrodes a frequency dispersion is observed that is, the observed impedances cannot be represented by the connection of simple R-C-L elements. The impedance of such systems may be approximated by an infinite series of parallel R-C circuits, that is, a transmission line [see Section VI, Fig. 41(b), ladder circuit]. The impedances may often be represented by an equation without simple electrical representation, through distributed elements. The Warburg impedance is an example of a distributed element. 

The impedance of ideally polarizable liquid electrodes (e.g., mercury, amalgams, indium-gallium) may be modeled by an R-C circuit (Fig. 4.1a). However, most impedance studies are now carried out at solid electrodes. At these electrodes the double-layer capacitance is not purely capacitive and often displays a certain frequency dispersion. Such behavior cannot be modeled by a simple circuit consisting of R, L, and C elements. To explain such behavior, a constant phase element (CPE) is usually used. 

Obviously this method is limited to liquid metals like mercury and gallium and their amalgams respectively alloys. Modifications of this method have been reported [86FIor]. At elevated temperatures with molten salt electrolytes alloys with an appropriately low melting point can be investigated, too. 

The concept of the potential of zero charge (PZC or E, has already been discussed in the context of electrocapillary thermodynamics, where we showed that, for an ideally polarizable interphase, the PZC coincides with the electrocapillary maximum. In view of the very high accuracy attainable with the electrocapillary electrometer, it is possible to measure E for liquid metals near room temperature to within about 1 mV. This accuracy is limited, however, to mercury, some dilute amalgams, and gallium. 

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