Electrometer, capillary

Usually one varies the head of mercury or applied gas pressure so as to bring the meniscus to a fixed reference point [118], Grahame and co-workers [119], Hansen and co-workers [120] (see also Ref. 121), and Hills and Payne [122] have given more or less elaborate descriptions of the capillary electrometer apparatus. Nowadays, the capillary electrometer is customarily used in conjunction with capacitance measurements (see below). Vos and Vos [111] describe the use of sessile drop profiles (Section II-7B) for interfacial tension measurements, thus avoiding an assumption as to the solution-Hg-glass contact angle. 

The simplest device for measuring ECC at mercury is Gouy s capillary electrometer (Eig. 10.5). Under the effect of a mercury column of height h, mercury is forced into the slightly conical capillary K. In the capillary, the mercury meniscus is in contact with electrolyte solution E. The radius of the mercury meniscus is practically equal to the capillary radius at that point. The meniscus exerts a capillary pressure Pk = directed upward which is balanced by the pressure = ftpegg of... 

In the case of a solution with a previously known aH+ (see below), we could determine 2°H+-.H2(iatm)> provided that a reference electrode of zero potential is available however, experiments, especially with the capillary electrometer of Lippmann, did not yield the required confirmation about the realization of such a zero reference electrode16. Later attempts to determine a single electrode potential on the basis of a thermodynamic treatment also were not successful17. For this reason, the original and most practical proposal by Nernst of assigning to the standard 1 atm hydrogen potential a value of zero at any temperature has been adopted. Thus, for F2H+ H2(iatm) we can write... 

Fig. 4.10 Capillary electrometer. The basic component is the cell consisting of an ideally polarized electrode (formed by the mercury meniscus M in a conical capillary) and the reference electrode R. This system is connected to a voltage source S. The change of interfacial tension is compensated by shifting the mercury reservoir H so that the meniscus always has a constant position. The distance between the upper level in the tube and the meniscus h is measured by means of a cathetometer C. (By courtesy of L. Novotny)...

G. Lippmann introduced the capillary electrometer to measure the surface tension of mercury (Fig. 4.10). A slightly conical capillary filled with mercury under pressure from a mercury column (or from a pressurized gas) is immersed in a vessel containing the test solution. The weight of the mercury column of height h is compensated by the surface tension according to the Laplace equation... 

If solutions of two electrolytes are brought into contact there is, generally speaking, a potential difference between them, just as there is one at the interface mercury-electrolyte in the capillary electrometer. This potential difference has been shown by Nemst to depend on the differences in the concentrations and the migration velocities of the ions. Smith uses dilute solutions containing equivalent amounts of KI and KC1 the kation is thus the same in both solutions, and the migration velocities of the I and Cl ions are nearly equal, so that, according to Nemst s theory, there should be no potential difference or double layer at the interface. These... 

Surface tension-potential difference curves for each electrolyte against mercury are plotted in the capillary electrometer, the result being shown in Fig. 16. At P there is, according to Helmholtz, no potential difference between Hg — KC1, and at R none between Hg — KI. If the effects at the interface were purely electrostatic, i.e., dependent only on the lines of force, and if the anions had no specific influence, then QS should be zero. Actually, however, it represents a potential difference of 0 2 volt. 

In order to produce the maximum interfacial tension in a capillary electrometer, it is generally necessary to apply a polarising E.M.F. Palmaer... 

Lippmann Ann. v. 494, 1875), independently made the same observation as a result of his study of the capillary electrometer. 

Gabriel Lippmann, 1845-1921. Professor of mathematical physics at the University of Paris. Inventor of the capillary electrometer and of a process of direct color photography. The phenomenon of piezo-electricity in crystals predicted by Professor Lippmann was first demonstrated experimentally by Pierre and Jacques Curie. 

The thermodynamic equations applicable to a polarizable interface, which can be studied by means of a capillary electrometer, can now be summarized. The general equation is... 

The potential of an electrode is determined in combination with a second constant electrode which does not belong to the actual electrolytic system. This subsidiary or standard electrode, whose potential is either arbitrarily taken, as zero or has a certain absolute value, is connected by a siphon with the liquid surrounding the experimental electrode. The electromotive force of this galvanic combination is then measured by one of the well-known methods, with a galvanometer or capillary electrometer. If the potential difference of the standard electrode is correctly subtracted from the obtained value, the difference in potential of the reaction electrode, based on the agreed-upon zero) value of the potential, is... 

In general, the repulsion between similar electric charges present at a surface lowers the surface tension in Chap. II, 21, we have already seen cases where the development of similar charges, by dissociation, on the end groups of a surface film, increases the surface pressure. In the well-known capillary electrometer, in which a potential difference can be applied across a mercury-water interface, simultaneously with measurement of the surface tension, any changes in the potential difference will alter the density of electrification at the interface, and consequently alter the surface tension. 

Lippmann was the first to record an accurate study of the relation between electrical potential and surface tension, in the classical memoir1 in which the capillary electrometer is described. 

If the relation between the e.m.f. applied to the terminals of the capillary electrometer, and the interfacial tension between mercury and the electrolyte, is plotted with tension as ordinate and the (negative) potential applied to the small mercury meniscus increasing as abscissae, the curve is called the electrocapillary curve.5... 

In cases when the concentration of mercury ions in the solution is high, the mercury may become negatively charged and only the descending part of the curve be realizable. The capillary electrometer works poorly under these conditions, as the current is carried by the Hg ions across the interface, and the polarization is very imperfect. 1 Phil. Trans., 161, 129 (1871). 

See also -> adhesion, -> Dupre equation, -> Lippmann equation, -> Lippmann capillary electrometer, -> point of zero charge, -> Young equation. 

See also - electrocapillarity, - electrocapillary curve, -r Gibbs-Lippmann equation, - Wilhelmy plate (slide) method, - ring method, - Lippmann capillary electrometer. 

Electrocapillarity — (a) as a branch of science, this term covers all phenomena related to the thermodynamics of charged - interfaces, esp. of metal-solution interfaces. The term is practically synonymous with -> capillarity, but emphasizes the electric aspects, (b) The term electrocapillarity is often used in a restricted sense to mean the study of the equilibrium properties of metal solution interfaces, such as the - interfacial tension of mercury solution interfaces, the height of a mercury column (in the case of the - Lippmann capillary electrometer), or the -> drop time (in the case of the - dropping mercury electrode). More generally, however, the equilibrium properties of many other interfaces fall... 

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. 

Lippmann capillary electrometer — Figure 1. Lippmann capillary electrometer [ii]. For explanation see text... 

Lippmann capillary electrometer — Figure 2. (a), (b), (c) from left to right Lippmann capillary electrometers. (Reproduced (a) from [iii], and (b) and (c) from [iv]). For explanation see text and sources... 

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