Interfacial potential differences measurement

The dependence of the interfacial tension at the W/NB interface on the interfacial potential difference [29,30] was investigated by using an aqueous solution dropping electrode [26,31]. In this investigation, the aqueous solution was forced upward dropwise in NB and the drop time of W was measured as a function of potential difference applied at the W/ NB interface. When W contained 1 MMgS04 and NB contained 4 x 10 " M Cs" TPhB ... 

Direct measurement of the change in interfacial potential difference at the oxide-electrolyte interface with change in pH of solution can be measured with semiconductor or semiconductor-oxide electrodes. These measurements have shown d V g/d log a + approaching 59 mV for TiC (36, 37). These values are inconsistent with the highly sub-Nernstian values predicted from the models with small values of K. (Similar studies 138.391 have been performed with other oxides of geochemical interest. Oxides of aluminum have yielded a value of d t)>q/A log aH+ greater than 50 mV, while some oxides of silicon have yielded lower values.)... 

One can have more complicated cells (Fig. 6.30), and in all of them it can be seen that the attempted measurement of a metal-solution potential difference will conclude with the measurement of the sum of at least two interfacial potential differences, i.e., the desired PDM /s and as many extra potential differences as there are new phase boundaries created in the measurement. In symbolic form, therefore, the potential difference V indicated by the measuring instrument can be expressed as... 

Thus, suppose that when one wishes to measure an interfacial potential difference PDm /s, one connects the electrode with another electrode and measures the potential of this cell. Suppose one always keeps this second electrode constant in nature (i.e, the algebraic sum of the potentials associated with it is kept constant). Then, measurements of the potential of a cell in which the one (same) electrode and its associated solution were always present and the other [i.e., the first electrode mentioned here (Mt)] and its solution were changed would clearly reflect the changing interfacial potential difference FDM /s. This is in fact what is done to measure the relative values of PDM /sas Mjor S (or both) are varied. 

Use of a reference electrode to measure the electrode—electrolyte potential difference also introduces a new reference—solution interface, but this is designed so that the potential gradient at the new interface is constant regardless of whatever electrode process occurs isothermally at the working electrode. Changes in electrode potential are thus proportional to changes in the interfacial potential difference... 

Interfacial potential differences are not directly observable. The usual way of measuring a potential difference between two points is to bring the two leads of a voltmeter into contact with them. It s simple enough to touch one lead of the meter to a metallic electrode, but there is no way you can connect the other lead to the solution side of the interfacial region without introducing a second second electrode with its own interfacial potential, so you would be measuring the sum of two potential differences. Thus single electrode potentials, as they are commonly known, are not directly observable. 

The concept of interfacial potential difference can be a major stumbling block for those new to electrochemistry. The measurement of voltage and its analogy to pressure in fluid flow is reasonable, but the reference point of zero volts and the sign conventions can be confusing. Moran and Gileadi wrote an excellent article on these topics to which the interested reader is referred (1). The key issues are summarized here. 

When the current does not flow through battery the measurable diflerence in electric potential between the terminals of the two electrodes is the result of all the equilibrium potential differences at the interphase between the conducting phases in contact. In the example of the Daniell cell, with both electrodes having copper terminals, there are three interfacial potential differences (apart from the small liquid junction potential difference at the contact between the two electrolyte phases) one potential difference at the contact between the zinc rod and the copper terminal (Zn/Cu) and two potential differences at the metal-solution interphases (Zn/Zn + and Cu/Cu +), which are mainly due to the charge transfer processes. 

Traditionally, electrochemical equilibria are explained in terms of thermodynamic cell potentials. However, in electro analytical applications, such a description is of little use, because one almost always uses a non-thermodynamic measurement, with a reference electrode that includes a liquid junction. It is then more useful to go back to the basic physics of electrochemistry, i.e., to the individual interfacial potential differences that make up the total cell potential. This is the approach we will use here. 

There are two types of interfacial potential differences equilibrium and non-equilibrium potentials. (From now on we will use potential as shorthand for potential difference . Potentials of individual phases cannot be measured, but some potential differences can be.) The equilibrium potentials can again be subdivided into two categories electron transfer and ion transfer potentials. The metal/metal ion potentials can be considered as... 

Sometimes it is useful to break the inner potential into two components called the outer (or Volta) potential, if/, and the surface potential, x- Thus, (f) = if/ + x- There is a large, detailed literature on the establishment, the meaning, and the measurement of interfacial potential differences and their components. See references 23-26. Although silver chloride is a separate phase, it does not contribute to the cell potential, because it does not physically separate silver from the electrolyte. In fact, it need not even be present one merely requires a solution saturated in silver chloride to measure the same cell potential. 

Obviously, the first two components of E are the expected interfacial potential differences at the copper and zinc electrodes. The third term shows that the measured cell potential depends also on the potential difference between the electrolytes, that is, on the liquid junction potential. This discovery is a real threat to our system of electrode potentials, because it is based on the idea that all contributions to E can be assigned unambiguously to one electrode or to the other. How could the junction potential possibly be assigned properly We must evaluate the importance of these phenomena. 

The interface between two immiscible liquids is used as a characteristic boundary for study of charge equilibrium, adsorption, and transport. Interfacial potential differences across the liquid-liquid boundary are explained theoretically and documented in experimental studies with fluorescent, potential-sensitive dyes. The results show that the presence of an inert salt or a physiological electrolyte is essential for the function of the dyes. Impedance measurements are used for studies of bovine serum albumin (BSA) adsorption on the interface. Methods for determination of liquid-liquid capacitance influenced by the presence of BSA are shown. The potential of zero charge of the interface was obtained for 0-200 ppm of BSA. The impedance behavior is also discussed as a function of pH. A recent new approach, using a microinterface for interfacial ion transport, is outlined. 

The potential difference measured in the cell (I) consists of the sum of two interfacial potential drops ... 

Two reference electrodes can then be immersed in the two solutions and the electromotive force (e.m.f.) measured in this electrochemical cell. The measured value consists of liquid junction potential(s), which can be maintained constant within a certain experimental error by maintaining a constant, relatively high ionic strength and a suitable composition of the solutions, and the interfacial potential differences at the sides of the membrane adjacent to the sample and standard solutions. As the potential difference between the membrane and the standard solution is constant, the overall change in the e.m.f. corresponds to the change in the composition of the sample solution. 

The two perturbation terms are specific to the given interface and are experimentally inseparable. They measure the contact potential difference at the M/S contact. However, since no cpd is measured in this case <5/M + S%s are grouped into a single quantity denoted by X, called the interfacial... 

In initial ET rate measurements, both the NB and aqueous phases contained 0.1 M TEAP, enabling measurements to be made with a constant Galvani potential difference across the liquid junction. In these early studies, the concentration of Fc used in the organic phase (phase 2) was at least 50 times the concentration of the electroactive mediator in the aqueous phase which contained the probe UME (phase 1). This ensured that the interfacial process was not limited by mass transport in the organic phase, and that the simple constant-composition model, described briefly in Section IV, could be used. 

The automatic measurement of the extracellular and intracellular electrical potential difference can be effectively used in plant electrophysiology for studying the molecular interfacial mechanisms of ion transport, the influence of external stimuli on plants, and for investigating the bioelectrochemical aspects of the interaction between insects and plants. 

Figure 9 (C) and (B) show interfacial potential of the two-phase system in the presence of SDS and inorganic and organic electrolytes in the aqueous phase and tetra-butylammonium chloride in the octanol phase, iiD,sDS> and interfacial potential in the absence of SDS, iJc.sDS respectively. The potential of the octanol phase was measured in reference to the aqueous phase. The effects of electrolytes on c,sds and i D,sDS were the same for the most part as those on a,sds and iJs.sDS respectively, and differences between Eqsds and Tsdsds were essentially as much as those between a,sds and...

Neither electric fields nor absolute potentials can be directly measured in the interfacial region. Instead, potential differences are measured against a reference electrode. Although it cannot be directly measured, the absolute electrode potential may be defined as... 

The outer potential difference between two contacting phases can be measured because it is a potential difference between two points in the same vacuum or gas phase outside the free surfaces of the two phases. On the other hand, the inner potential difference can not be measured, because the potential measuring probe introduces its interfacial potential that differs with the two phases and thus can not be canceled out this gives rise to an unknown potential in the potential measurement. 

---