Polarity mercury

Opinions differ on the nature of the metal-adsorbed anion bond for specific adsorption. In all probability, a covalent bond similar to that formed in salts of the given ion with the cation of the electrode metal is not formed. The behaviour of sulphide ions on an ideal polarized mercury electrode provides evidence for this conclusion. Sulphide ions are adsorbed far more strongly than halide ions. The electrocapillary quantities (interfacial tension, differential capacity) change discontinuously at the potential at which HgS is formed. Thus, the bond of specifically adsorbed sulphide to mercury is different in nature from that in the HgS salt. Some authors have suggested that specific adsorption is a result of partial charge transfer between the adsorbed ions and the electrode. 

Mar. 22,1874 Semily, then Austro-Hungarian Empire -Apr. 16, 1921, Prague, Czechoslovakia) Since 1912, Professor of experimental physics at Charles University, Prague. Kucera introduced the measurement of surface tension of polarized mercury by applying the dropping mercury electrode [i] rather than the Lippmann capillary electrometer, and he inspired thereby -> Heyrovsky, J. to introduce - polarography. 

In connection with the foregoing, double layers of course also play an important role in electroanalysis. Transfer of. say, electroactive ions through the polarized mercury-solution interface is preceded by passage through the double layer. Therefore current-potential plots depend in principle on double layer properties. Historically, it was his interest In charge-transfer and corrosion problems that induced Grahame to start his seminal double layer Investigations. 

In 1940, Frumkin explored the relationships among the double-layer structure on mercury electrodes, the capacitance measured by use of a Wheatstone bridge, and the surface tension, following the theoretical underpinnings of the Lippmann equation. Grahame ° expanded this treatment of the mercury electrode, providing a fundamental understanding of the structure of the electrical double layer. Dolin and Ershler applied the concept of an equivalent circuit to electrochemical kinetics for which the circuit elements were independent of frequency. Randles developed an equivalent circuit for an ideally polarized mercury electrode that accounted for the kinetics of adsorption reactions. ... 

Figure 1.2.5 Left Two-electrode cell with an ideal polarized mercury drop electrode and an SCE. Right Representation of the cell in terms of linear circuit elements.

Capacitance, C, provides direct information on the structure of the adsorbed layer (10, 11). The change in the differential capacity of the electrical double layer between a polarized mercury surface and a 0.15 M NaCl solution containing various concentrations of protein... 

When maxima are observed on polarographic curves, gelatin or other surface active substances are sometimes added to the solution. The concentration of surface active substance should be as low as possible, because a higher concentration of gelatin, deforms the polarographic curve. The surface active agent damps the streaming of electrolyte around the polarized mercury drop. 

The primary element of polarography, the dropping mercury electrode, was introduced in science by the Czech physicist Bohumil Kucera (1874—1921) (Fig. 3.2.1) [344] in order to improve the Lippmann s method of surface tension measurement of polarized mercury [345]. 

In the case of the completely polarized (mercury) electrode the charges occurring in the thermodynamical calculations and in the model are identical ... 

Using Langmuir s principle of independent surface action, make qualitative calculations and decide whether the polar or the nonpolar end of ethanol should be oriented toward the mercury phase at the ethanol-mercury interface. 

It is necessary that the mercury or other metallic surface be polarized, that is, that there be essentially no current flow across the interface. In this way no chemical changes occur, and the electrocapillary effect is entirely associated with potential changes at the interface and corresponding changes in the adsorbed layer and diffuse layer. 

In contrast to spectrophotometry, hght-scattering experiments are generally conducted at constant wavelength. Mercury vapor lamps are the most widely used light sources, since the strong lines at 436 and 546 nm are readily isolated by filters to allow monochromatic illumination. Polarizing filters are also included for both the incident and scattered beams so that depolarization can... 

Stripping voltammetry procedure has been developed for determination of thallium(I) traces in aqueous medium on a mercury film electrode with application of thallium preconcentration by coprecipitation with manganese (IV) hydroxide. More than 90% of thallium present in water sample is uptaken by a deposit depending on conditions of prepai ation of precipitant. Direct determination of thallium was carried out by stripping voltammetry in AC mode with anodic polarization of potential in 0,06 M ascorbic acid in presence of 5T0 M of mercury(II) on PU-1 polarograph. 

Consider two examples. In the first example, hydrogen is evolved cathodicaUy by ion discharge at metals where this reaction occnrs with high polarization (e.g., at mercury, z, = 1, = 0.5). hi this case the reaction occurs at potentials mnch more... 

Adsorption of surface-active substances is attended by changes in EDL structure and in the value of the / -potential. Hence, the effects described in Section 14.2 will arise in addition. When surface-active cations [NR] are added to an acidic solution, the / -potential of the mercury electrode will move in the positive direction and cathodic hydrogen evolution at the mercury, according to Eq. (14.16), will slow down (Fig. 14.6, curve 2). When I ions are added, the reaction rate, to the contrary, will increase (curve 3), owing to the negative shift of / -potential. These effects disappear at potentiafs where the ions above become desorbed (at vafues of pofarization of less than 0.6 V in the case of [NR]4 and at values of polarization of over 0.9 V in the case of I ). 

FIGURE 14.6 Influence of surface-active ions [N(C4H9)4]+ (curve 2) and I (curve 3) on the polarization curve for hydrogen evolution at a mercury electrode in acidic solutions (curve 1 is for the base electrolyte). 

Experience shows that in the deposition of a number of metals (mercury, silver, lead, cadmium, and others), the rate of the initial reaction is high, and the associated polarization is low (not over 20 mV). For other metals (particularly of the iron group), high values of polarization are found. The strong inhibition of cathodic metal deposition that is found in the presence of a number of organic substances (and which was described in Section 14.3) is also observed at mercury electrodes (i.e., it can be also associated with the initial step of the process). 

FIGURE 15.3 pH dependence of potential (1) and polarization (2) in cathodic hydrogen evolution at a mercury electrode (lOmA/cm ), and the pH dependence of equilibrium potential of the hydrogen electrode (3). 

At mercury and graphite electrodes the kinetics of reactions (15.21) and (15.22) can be studied separately (in different regions of potential). It follows from the experimental data (Fig. 15.6) that in acidic solutions the slope b 0.12 V. The reaction rate is proportional to the oxygen partial pressure (its solution concentration). At a given current density the electrode potential is independent of solution pH because of the shift of equilibrium potential, the electrode s polarization decreases by 0.06 V when the pH is raised by a unit. These data indicate that the rate-determining step is addition of the first electron to the oxygen molecule ... 

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