Compensation potential difference

Farrell J. R. and McTigue P. (1982), Precise compensating potential difference measurements with a voltaic cell—the surface potential of water , J. Electroanal. Chem. 139, 37-56. 

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 similar equation can be written for the calomel system on the right-hand side, which involves Hg and the HCl solution, which has the same concentration as the solution on the left-hand side. It follows from equation (8.7.11) that the compensation potential difference also measures the difference between the electrochemical potentials of the Cff ions in the two HCl solutions ... 

The Donnan equilibrium allows the evaluation of the distribution of M and X" over both sides of the semipermeable membrane. If electrodes responding to either M+ or X" were inserted at either side of the membrane there would be no potential difference between them. This is a consequence of the system being in equilibrium which implies that no work can be performed. Nevertheless, because of the different ion concentrations, the potentials at the respective electrodes i/o,i and /o n are not equal. Consequently, there must be a compensating potential difference across the membrane, Axi/. If the electrodes respond reversibly to the ion concentrations so that Nernst s law (Equation 9.14) applies, it follows for the membrane potential... 

Information on defects can be obtained with good approximation from Eq. (3-5 la). The value of is all that is necessary for an overview. should be as high as possible to increase the sensitivity. In addition, to eliminate foreign voltages in the soil, it is necessary to switch the polarization current on and off with the help of a current interrupter periods of about 2 s off and 18 s on are convenient. Potential differences independent of the polarization current that are the result of foreign currents or electrode faults (see Section 3.2) are totally excluded by this method. On the other hand, the IR component of a compensation current can also be... 

FIG. 30 Top Square voltage waveform applied to the tip. Bottom. Corresponding changes in tip deflection (converted to force after multiplying by the lever spring constant). There is a net attractive force for both the positive and the negative cycles, but it takes time to reach the final force value. Note that the square-wave voltage is not symmetrical around zero. An offset is applied to compensate for the contact potential difference between the tip and the surface. This offset is dependent on humidity and is equal to the potential difference between the tip and the sample. (From Ref. 78.)... 

Adsorption of a dipolar substance at the w/a and w/o interfaces changes surface tension and modifies the surface potential of water (Fig. 11). As seen in Fig. 11, the change in compensation voltage due to adsorption is the surface potential difference, usually called the surface potential or better the adsorption potential and often indicated unnecessarily by AV. ... 

The electricity-producing system of electric fishes is built as follows. A large number of flat cells (about 0.1 mm thick) are stacked like the flat unit cells connected in series in a battery. Each cell has two membranes facing each other. The membrane potentials of the two membranes compensate for each other. In a state of rest, no electrostatic potential difference can be noticed between the two sides of any cell or, consequently, between the ends of the stack. The ends of nerve cells come up to one of the membranes of each cell. When a nervous impulse is applied from outside, this membrane is excited, its membrane potential changes, and its permeability for ions also changes. Thus, the electrical symmetry of the cell is perturbed and a potential difference of about 0.1 V develops between the two sides. Since nervous impulses are applied simultaneously to one of the membranes in each cell, these small potential differences add up, and an appreciable voltage arises between the ends of the stack. 

As seen in this sketch, the change of compensation voltage A.E, due to adsorption is the surface potential difference, usually called shortly surface potential, or better the adsorption potential... 

If this EMF is compensated by an external electrical potential difference of the same magnitude and of opposite polarity, then no current flows and the system is at equilibrium. The phase equilibria of communicating species are... 

The EMF of a galvanic cell is a thermodynamic equilibrium quatity. Thus, the potential of a cell must be measured under equilibrium conditions, i.e. without current flow. The measured EMF must be compensated by a known external potential difference. The measurement of the EMF of a cell is thus based on determination of a potential difference that exactly compensates the measured potential difference so that no current passes. This is easily achieved by the Poggendorf compensation method (see Fig. 3.13). 

FIGURE 2.45. Equivalent circuit for the cell and instrument. WE, RE, and CE, working, reference, and counter electrodes, respectively iph, photocurrent ij/, double-layer charging current Q, double-layer differential capacitance Rc, Ru, cell compensated (by the potentiostat) and uncompensated resistances, respectively Rs, sampling resistance RP, potentiostat resistance E, potential difference imposed by the potentiostat between the reference and working electrodes Vpu, photo-potential as measured across the sampling resistor. Adapted from Figure 1 of reference 51, with permission from Elsevier. 

Fortunately not, but to measure the absolute potential at an interface, another reference state would have to be used, as well as the nature of the metal-electron interactions. Later, in Chapter 8, it will be shown that relevant calculations can be made of this difference of inner potentials (sometimes called the Gcdvani potential difference), but their accuracy is on the order of 0.1 V, which is not yet enough to compensate for our lack of ability to measure the quantity. In the next sections, some useful concepts will be described and in Section 6.7.2 we will return to the concept of absolute electrode potential and the possibility of creating a scale of practical absolute-electrode potentials. 

Equilibrium is reached when the driving force for the diffusion (the concentration gradient) is compensated for by the electric field (the potential gradient). Under these equilibrium conditions, there is an equilibrium net charge on each side of the junction and an equilibrium potential difference d< >e. This process is analogous to the way charge transfer across a nonpolarizable electrode/solution interface results in the establishment of an equilibrium potential difference across the interface. 

IR drop compensation — The -> IR drop (or Voltage drop ) of a conducting phase denotes the electrical potential difference between the two ends, for example of a metal wire, during a current flow, equaling the product of the current I and the electrical resistance R of the conductor. In electrochemistry, it mostly refers to the solution IR drop, or to the ohmic loss in an electrochemical cell. Even for a three-electrode cell (- three electrode system), the IR drop in the electrolyte solution (between the... 

The tip and the adjacent surface form a capacitor. When the distance between tip and surface is changed by vibrating the probe an AC current flows its magnitude depends on the existing potential difference. By adjusting the external bias voltage Ucomp this potential difference can be compensated, as a consequence the AC current vanishes. In most cases relative changes of the local surface potential are of interest. In order to remove any unwanted influence of the probe surface potential a material with constant surface potential is used (typically an... 

Charge transport in the bulk has to obey electroneutrality. Figure 41 shows three simple experiments that comply with this restriction. For brevity let us call them the electrical (a), the tracer (b) and the chemical experiment (c) and, to be specific, let us consider an oxide. In the electrical experiment an electrical potential difference is applied, the electrons flowing in the outer circuit compensate the charge flow within the sample (Figure 41a shows this for the case of a pure ion conductor). If we apply reversible electrodes, in the steady state there is no compositional change involved. (At this point we are not interested in (electro-)chemical effects caused by non-reversible electrodes. This is considered in detail in Part II.1) The tracer transport (b) caused by the application of a chemical potential difference of the isotopes consists of a counter motion of the two isotope ions. Finally, experiment (c) presupposes a mixed conduction her the outer wire is, as it were,... 

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