Electrometer interface

Fig. 20.4 Lippmann electrometer for studying the variation of the excess charge on mercury with variation in potential difference at the mercury solution interface...

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. 

Consider the operations necessary to measure the potential difference across a metal/solution interface. Various potential-measuring instruments can be used potentiometers, electrometers, etc. All these instruments have two metallic terminals that must he connected to the two points between which the potential difference is to he measured. 

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... 

Let us now return to the nonpolarized interface within the context of the working of the entire electrochemical cell, which we have to use in order to obtain useful information about concentration of fluoride ion, using (6.20). It is connected to a high-input impedance electrometer (e.gR > 10 2), so that current cannot pass through it. This ensures that the condition of zero current is satisfied. The general schematic of ISE is shown in Fig. 6.10a. In the usual cell notation we can write for the complete cell... 

The potential of a single electrode or half-cell cannot be directly measured in a simple way. Any potential measuring device such as a voltmeter or electrometer has two metallic terminals that must be connected across the two points between which the potential difference is measured. One terminal can be connected to the metallic electrode, but the other terminal must make connection to the solution through a wire as illustrated in Figure 5.1. The immersion of the connecting wire in the solution creates a second metal-solution interface whose potential difference will be included in the measurement. A fuller discussion of this point is available.1... 

A test system, controlled by personal computer (PC), was developed to evaluate the performance of the sensors. A schematic of this system is shown in Figure 3. The signals from the sensors were amplified by a multi-channel electrometer and acquired by a 16 bit analog to digital data acquisition board at a resolution of 0.0145 mV/bit. The test fixture provided the electrical and fluid interface to the sensor substrate. It contained channels which directed the sample, reference and calibrator solutions over the sensors. These channels combined down stream of the sensors to form the liquid junction as shown in Figure 1. Contact probes were used to make electrical connection to the substrate. Fluids were drawn through the test fixture by a peristaltic pump driven by a stepper motor and flow of the different fluids was controlled by the pinch valves. 

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. 

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). 

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... 

Refs. [i] Bottomley JT (1877) Electrometers (Science lectures at South Kensington). Macmillan and Co [ii] Electrometer measurements. (1972) Keithley Instruments [iii] Anso MK, Roos ME, Saks OV, Shor VG, Khyammalov YA (1989) Instrum Exp Tech 32 1257 [iv] Volkov AG, Deamer DW (eds) (1996) Liquid-liquid interfaces. Theory and methods. CRC-Press, Boca Raton... 

See also - electrode surface area, -> Gibbs-Lippmann equation, - interfacial tension, -> interface between two liquid solvents, -> interface between two immiscible electrolyte solutions -> Lippmann capillary electrometer, -> Lippmann equation -> surface, -> surface analytical methods, - surface stress. 

Figure 3.47. Sketch of a Lippmann-type capillary electrometer. The mercury-solution interface resides in the slightly conical capillary A. in which a certain height h is chosen at which the measurements are carried out. The potential is externally applied R is the reference electrode (in Llppmann s experiments it was a calomel electrode, connected to the solution S via a salt bridge). The interfacial tension between mercury and solution is obtained from the height h, defined in the sketch...

For practical reasons the capillary rise technique is rarely used for the measurement of interfacial (rather than surface) tensions large amounts of the two liquids are needed and there are suitable and convenient alternatives. An exception to this is the measurement of the Interfacial tension between mercury and (mostly) aqueous solutions at various potential differences applied across the liquid-liquid interface. Such measurements are done in a so-called Lippmann capillary electrometer, already described in the chapter on electric double layers (fig. 11.3.47). 

On the basis of the GAI, it is clear that the interfacial tension y is the most important experimental quantity. Three methods are commonly used to determine y at liquid liquid interfaces, namely, the capillary electrometer method, the maximum bubble pressure method, and the drop weight or drop time method. 

The capillary electrometer method first used by Lippmann [4] is based on the capillary rise principle. The interface between the liquid Hg electrode and the solution is established in a fine capillary, the position of the interface being determined by the interfacial tension (see fig. 8.2). More specifically, y is given by... 

Thus, the E values (relative interface potential differences, or interface potentials) represent differences in potentials across the metal/solution interface minus the potential difference across the standard hydrogen reference electrode interface. The E values are physically measured by attaching one lead of an electrometer to the metal, the other lead to a reference electrode in the solution and very close to the metal surface (a point discussed further in Chapter 6). If the positive electrometer lead is... 

It is not obvious why (13.1.31) is called an electrocapillary equation. The name is a historic artifact derived from the early application of this equation to the interpretation of measurements of surface tension at mercury-electrolyte interfaces (1-4, 6-8). The earliest measurements of this sort were carried out by Lippmann, who invented a device called a capillary electrometer for the purpose (9). Its principle involves null balance. The downward pressure created by a mercury column is controlled so that the mercury-solution interface, which is confined to a capillary, does not move. In this balanced condition, the upward force exerted by the surface tension exactly equals the downward mechanical force. Because the method relies on null detection, it is capable of great precision. Elaborated approaches are still used. These instruments yield electrocapillary curves, which are simply plots of surface tension versus potential. 

Schufle et al. have proposed, from surface conductance results of dilute HCl in small diameter glass capillaries, that long-range ordering of the structure of water may exist near Interfaces (dlams. ranged 2 mm - 5 microns). This controversial issue is Inherently linked with models for electroosmosis, if dc methods are employed for the conductivity determination and a concurrent electroosmotic effect occurs (electrometer measurement methods have presumably been dc). 

Figure I. (a) Experimental arrangement for the measurement of freezing potentials (10 K, resistor not in circuit) and currents (10 K. resistor shunting the phases), V = electrometer C = recorder, (b) Electric analog of the system in the shunt case, Rb = interface barrier resistance = external shunt resistance Rj = ice resistance Ri = solution resistance Rm = ice metal interface resistance c = interface charge separation...

The key issue in these sensors is the interface between the ion selective membrane and the contact. The most convenient way to present this problem is in the form of the equivalent electrical circuit in which the resistances and capacitances have their usual electrochemical meaning (Fig.2). It is necessary to include the electrometer (or at least its input stage) in the analysis of these sensors. In most modern instruments the amplifier is an insulated gate field-effect transistor (IGFET) which has the input dc resistance of greater than 10 and the input capacitance on the order of picofarads. 

It is quite impossible to determine the absolute potential difference across a sin e met /solution interface, and the potential must be evaluated indirectly from the e.m.f. of a cell comprising the interface under consideration and another electrified interface. The e.m.f. of the cell can be determined readily by a suitable measuring device such as a potentiometer, vacuum-tube voltmeter or an electrometer, which are capable of measuring the e.m.f. with the minimum passage of electrical charge. This is essential since if a significant current is allowed to pass, the electrodes (electrified interfaces) become polarised and the e.m.f. will be less than the equilibrium value. Consider the determination of the interfacial potential at the surface of a zinc electrode in equilibrium with Zn ions in solution. In order to determine the potential it is necessary to couple it with another electrode, and for the purpose of this discussion the equilibrium between ions in solution and gas will be chosen, i.e. the reversible hydrogen electrode in which the equilibrium between and H takes place at a platinised-platinum surface. The spontaneous cell reaction will be... 

---