The Volta potential difference

In contrast to differences in the inner potentials (the Galvani potential difference), the difference in the outer potentials A pip (the Volta potential difference or the contact potential) and the real potential or, can be measured. The real potential <, is defined as... 

Figure 3. Comparison of the Volta potential differences at the Hg/solvent interface vs. (a) donor number and (b) donor and acceptor numbers. Jt=0.007 AN-0.011DN-0.485 ...

The inner potential difference between two contacting phases is cafied in electrochemistry the Galvani potential difference, and the outer potential difference is called the Volta potential difference. The outer potential difference corresponds to what is called the contact potential between the two phases. We call, in this test, the inner potential difference across an interface the interfacial potential. 

For correct function of the ISFET, a sufficiently large gate voltage, Vq, must be applied between the leads to the reference electrode and to the substrate, so that a sufficiently large potential difference is formed between the surface and the interior of the substrate for formation of the n-type conductive channel at the insulator/substrate interface. This channel conductively connects drain 1 and source 2, which are connected with the substrate by a p-n transition. On application of voltage Vj between the drain and the source, drain current /p begins to pass. Under certain conditions the drain current is a linear function of the difference between Vq and the Volta potential difference between the substrate and the membrane. 

At this stage, let the two conceptually separated pails of the double layer be brought together again. The interface has been reassembled. One can now refer to the outer potential difference, sometimes called the Volta potential difference, between the metal and solution. This outer potential difference is written... 

The Volta potential difference is a quantity that can be measured experimentally (see Appendix 6.1). Further, if one chooses a plausible model of the arrangement of charges at the interface, the Volta potential difference can be calculated by simple electrostatic reasoning [see Eq. (6.118)]. It is for these reasons that the outer potential is an important aspect of the study of the electrified interface. 

The value of the Volta potential difference between a metal a and a solution can be obtained by performing two measurements, in one of which the electrode a is in contact with the solution (Fig. A6.1), whereas, in the second, it is separated from the latter by a space filled with ionized unreactive gas at a low pressure (Fig. A6.2). Electrode a and another electrode of a metal y, reversible to ions Mz+ in solution, are connected to a potentiometer P by wires P and P made of the same metal. 

Fig. 1. The definition of the Volta-potential difference in the system electrode-electrolyte solution.

We can use the relations derived in (30) and (31) unchanged, if Sif/ is to represent the Volta potential difference between a free surface and a similar one occupied by a concentration of chemisorbed gas equal to... 

In the second step the charge arrives at the internal phase passing through the interface. The associated potential is known as the surface potential jump (also called surface potential, surface electrical potential, etc.). It is determined by dipoles aligned at the interface and by surface charges. It is not identical with the Volta potential difference (also sometimes called the surface potential) that has so far been used for the description of the electrical double layer. For the treatment of the electrical double layer, dipoles did not play a role. In particular in water, however, the aligned water molecules contribute substantially to the surface potential jump x- The Galvani potential, Volta potential, and surface potential jump are related by... 

Concerning the potential differences, that corresponding to the outer potentials between two phases a and /> is known as the Volta potential difference defined as... 

The electrostatic potential of a phase is defined as the potential of the space just outside it, for a phase a let this be Fa. The Volta potential difference between two phases is the difference in electrostatic potential between two points just outside the phases. The electrostatic potentials, and therefore the Volta potential, are determined by the amount of charge on the phase and its electrostatic capacity. 

The Volta potential difference between two phases a and j3 is Fa— F it is sometimes called the external contact potential, more commonly simply the contact potential. 

If Ma is copper and M zinc, the equations giving the Volta potential differences at the three phase boundaries are... 

Kelvin probe — The surface (or contact) potential of a solid or a liquid-film covered solid can be measured with a Kelvin probe [i]. Essentially the -> Volta potential difference AT between the two employed surfaces as described below is measured. In common abbreviation this is also called measurement of a Volta potential T. As depicted below a probe tip is brought close to the surface under investigation. 

The experimentally measured reversible electrode potential, E q, includes not only the above emf but also the potential difference at the metal-platinum contact. The electrons are the electromotively active particles at this junction, and it may be assumed that at equilibrium an electrical potential difference exists between the two metals which equalizes the electrochemical potential of the electrons in the two phases. As is well known, it is equivalent to the Volta potential difference and is given by the following ... 

Since each outer potential may be determined as described above, the Volta potential difference may also be found experimentally. A method for measuring o Ap / is discussed later in this chapter. 

The relationship between the Galvani potential difference, the Volta potential difference, and the surface potentials of two condensed phases in contact is illustrated in fig. 8.5. Both of the condensed phases a and p are in contact with an inert gas or vacuum. The outer potential / is the potential difference between charge-free infinity and a point just outside of but close to phase a is the corresponding quantity for phase p. The Volta potential difference is simply the difference between these quantities. Xa and xp give the surface potentials of phases ot and p, respectively. They are measured from vacuum into the condensed phase. The Galvani potential difference aApcj) is defined within the condensed phases. Since the sum of the potential drops around a loop must be zero, one obtains... 

Experimental Measurement of the Volta Potential Difference at Interfaces... 

A Volta potential difference usually arises between two condensed phases when they come into contact. Because these phases have different chemical properties, charge may be redistributed at their interface with air, even though this interface carried no net charge before contact. Measurement of the Volta potential difference is possible in a high-impedance electrochemical cell which contains an air gap. The potential drop across this cell is determined under conditions for which the potential drop across the air gap is zero, and is known as the compensation potential difference. 

A schematic diagram illustrating the experimental determination of the Volta potential difference between two metals, a and p, is given in fig. 8.11. For this system, electrons are the species common to each phase. The experiment involves an arrangement in which there is an air gap between a and p. On the other side of P, there is a second portion of phase a which is designated ot, so that the system can be described as... 

Fig. 8.11 Experimental setup for determining the Volta potential difference between two metals a and p. The compensation potential is determined with potentiometer P by moving a with respect to P until there is zero current in galvanometer G.

Fig. 8.12 Schematic diagram of the Kenrick apparatus used to determine the Volta potential difference between two liquids a and p. Liquid a (Hg in this example) flows down the center of tube T in a stream of droplets, whereas p (O.IM HCl) flows down the walls.

Another important experiment used to measure the Volta potential difference between two liquids was described by Kenrick [9] and is illustrated here for the mercury electrolyte solution interface (see fig. 8.12). An air gap between the two liquids is established in a cylindrical tube T. The mercury emerges from a central reservoir in a stream of small droplets which flow down the center of the tube. On the other hand, the solution, namely 0.1 M HCl, flows down its walls. The experiment is designed so that the two liquids flow sufficiently rapidly that no charge can be built up on their surfaces. As a result the Volta potential difference across the air gap is zero. The HCl solution is part of an electrochemical half-cell connected to a calomel electrode. The total cell may be described as... 

The object of the experiment is to measure the Volta potential difference between Hg" and the HCl solution with which it is in contact. As described above, any Volta potential difference between the surfaces of Hg" and the HCl solution on the other side of the air gap is eliminated by streaming these liquids in the central cylinder of the apparatus. In addition, nitrogen gas is passed up the gap in the cylinder to prevent condensation of water vapor on the mercury droplets. Since the resistance of this system is very high it is important that the device used to measure the potential difference have an even higher resistance, so that charge does not flow from the experimental cell to the measuring device. 

In a slightly different configuration, the Kenrick apparatus can be used to measure the Volta potential difference between two miscible liquids. Such a cell is... 

When the compensation potential is determined using the Kenrick apparatus, the mercury streams down the center of the vertical tube and the HCl solution down its walls so that the Volta potential difference across the air gap is eliminated. In this system both the concentration of HCl and the pressure of hydrogen gas can be varied. It differs from cells (8.7.8) and (8.7.16) in that no attempt is made to balance the charge carriers on opposite sides of the cell. The Galvani potential difference measured between the two copper leads can be related to the electrochemical potentials of electrons in the mercury and platinum as follows ... 

Since the Volta potential difference across the air gap between the mercury and the aqueous solution is maintained at zero, this result may also be written as... 

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