Potential difference, measurement

This method involves very simple and inexpensive equipment that could be set up m any laboratory [9, 10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed m the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluondes and has been applied to unbuffered solutions from Willard-Winter distillation, to lon-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10 M This method is based on complexing by fluonde ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(tll) ions... 

It is relevant to follow up the description given above on reversible electrodes with attention focused on the potential difference of a cell. The potential difference measured under reversible conditions, is called the electromotive force, or emf, of the cell, E. It is clear that if a cell is reversible then it is implied that the half-cells of which it is composed are also reversible. 

Contact potential difference measurements, os-cillatoiy reactions, 39 85 Contact synergy model, 40 183 Contaminant distribution... 

When a constant ionic strength of the test solution is maintained and the reference electrode liquid bridge is filled with a solution of a salt whose cation and anion have similar mobilities (for example solutions of KCl, KNO3 and NH4NO3), the liquid-junction potential is reasonably constant (cf. p. 24-5). However, problems may be encountered in measurements on suspensions (for example in blood or in soil extracts). The potential difference measured in the suspension may be very different from that obtained in the supernatant or in the filtrate. This phenomenon is called the suspension (Pallmann) effect [110] The appearance of the Pallmann effect depends on the position of the reference electrode, but not on that of ISE [65] (i.e. there is a difference between the potentials obtained with the reference electrode in the suspension and in the supernatant). This effect has not been satisfactorily explained it may be caused by the formation of an anomalous liquid-junction or Donnan potential. It... 

Zi) A cell with liquid junction is employed, containing two identical ISEs selective for the determinand one ISE is immersed in a sample solution with concentration c and the other in a standard solution with concentration Cj. Assuming that the liquid-junction potential is constant, the potential difference measured is... 

The terms /qRp and E ere are independent of the concentration q of analyte i and, as described previously, the use of saturated KC1 minimises the value of the E term. Therefore, the potential difference measured is almost solely due to the membrane and is related to the activity of the ionic species under analysis in the sample solution by the following Nernst-type equation ... 

E is electric potential difference measured in volts (V). E is the work (J) needed to move a coulomb of positive charge from one point to the other. 

The mitochondrial membrane potential Em (or Ay) is the potential difference measured across a membrane relative to a reference electrode present in the surrounding solution.176 For both mitochondria and bacteria Em normally has a negative value. The Gibbs energy change AyH+ for transfer of one mole of H+ from the inside of the mitochondrion to the outside, against... 

Practically, AE is calculated by measuring the potential over two reference electrodes (e.g. Agl AgCl electrodes), each electrode being positioned at one side of the glass membrane (Fig. 3.4a). The potential of both reference electrodes is the same because they are identical, so if a difference in potential is measured over these electrodes, this difference should be related to the presence of the glass membrane. As explained above, the contribution of Ei is constant, so the potential difference measured over the entire cell system is related to E2, or to the hydrogen-ion activity of the solution of unknown pH. 

Returning to the three-electrode setup, it could seem that no ohmic drop would affect the measurement of the potential difference between the working and reference electrodes, since there is practically no current flow between both electrodes. However, this is not totally true. The reference electrode is located at a given distance from the working electrode surface, and, as a result of this separation, the potential difference measured contains a part of the ohmic drop in the solution which is called residual ohmic drop, IRU (with I being the current and Ru the uncompensated resistance). For more details concerning the minimization of the ohmic distortion of the current-potential response, see Sects. 1.8 and 5.4. 

For endosmose a potential difference is applied between the two ends of a capillary tube, or the two sides of a porous plug or membrane, and the rate of motion, or the pressure required to prevent motion, of the liquid, is noted. For streaming potentials the liquid is forced through the tube and the potential difference measured. In every case it is the relative velocity of motion of the two phases in the electric field that is measured, or conversely, the intensity of the field set up when the particles are caused to move. 

The value of the electrical potential difference measured without any current flowing (obtained by extrapolation to zero current) will be called -> electromotive force A E = U. 

In certain cases, all of the quantities in Equation 3.20 —namely, the permeabilities and the internal and the external concentrations of K+, Na+, and Cl- —have been measured. The validity of the Goldman equation can then be checked by comparing the predicted diffusion potential with the actual electrical potential difference measured across the membrane. 

The colloid vibration potential (difference) E or CVP is the a.c. potential difference measured between two Identical relaxed electrodes, placed in the dispersion if the latter Is subjected to an (ultra)sonlc field. CVP Is a particular case of the more general phenomenon, ultrasonic vibration potential (UVP), applying to any system, whether or not colloids are present. This field sets the particles into a vibrating motion, as a result of which the centres of particle charge and countercharge are periodically displaced with respect to each other. This phenomenon is the a.c. equivalent of that observed in the Dorn effect. Counterpart to this is the electrokinetic sonic amplitude, ESA, the amplitude of the (ultra)sonlc field created by an a.c, electric field in a dispersion. 

The reaction free energy is hence directly related to the electrochemical-potential difference measured between the two metal electrodes with a voltmeter (see Eqs. 32-34). Furthermore, the electrochemical potential of a redox system in solution is probed by measurement of the potential of a metal phase, in equilibrium with the redox system, with respect to a reference electrode. 

Davis, G.R. Santa Ana, C.A. Morawski, S.G. Fordtran, J.S. Permeability characteristics of human jejunum, ileum, proximal colon and distal colon results of potential difference measurements and unidirectional fluxes. Gastroenterology 1982, 83 (4), 844-850. 

The flame ionization detector (FID) is one of the most commonly used detectors in beverage analysis by GC, as it is suitable to most groups of compounds investigated in alcoholic beverages [9]. This occurs because almost all compounds of interest in such samples are able to burn in the flame, forming ions and producing a potential difference measured by a collector electrode. 

What would be the approximate potential difference measured by a pH meter if the test solution had a pH of 2 and the reference solution had a pH of 4 ... 

The potential difference measured by a pH meter is directly proportional to ... 

If the reference solution of a pH meter were 1 MHC1, and the potential difference measured by the meter were 59 mV, what would be the pH of the test solution ... 

E5.7 The standard reduction potentials are equal to the potential difference measured against SHE ... 

Davies GR, Santa Ana GA, Morawski SG, and Fordtran JS. Permeability Characteristics of Human Jejunum, Proximal Colon and Distal Colon Results of Potential Difference Measurements and Unidirectional Fluxes. Gastroenterology 1982 83 844-850. 

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. 

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 ... 

From Volta potential difference measurements at the metal solution interface discussed in section 8.8, the value of sAHg r at the potential of zero charge (PZC) of mercury is —0.248 V. However, the PZC is 0.192 V negative of the standard potential of the SHE. Thus, the value of the Volta potential difference at the standard potential of the hydrogen electrode is —0.056 V. PFsing the work function for an electron in Hg (4.50 eV), the estimate of E °, is... 

This relationship states that the change in charge density on the electrode with electrolyte concentration measured at constant cell potential difference is equal to the change in ionic surface excess with cell potential difference measured at constant electrolyte concentration. Cross-differentiation of equation (10.2.33) gives... 

Selected values of standard reduction potentials are listed in Table 7.1 (see also Appendix 11). Most of these values have been obtained directly from potential difference measurements, but a few values have been calculated from data obtained by calorimetric methods. This latter technique is for systems that cannot be investigated in aqueous media because of solvent decomposition (e.g. F2/2F ) or for which equilibrium is established only very slowly, such that the electrode is non-reversible (e.g. O2, 4H /2H20). Table... 

The potential difference between the electrodes in a working electrochemical cell is called the cell potential. The cell potential is not a constant and changes with time as the cell reaction proceeds. Thus the cell potential is a potential difference measured under non-equilibrium conditions as electric current is drawn from the cell. Electromotive force is the zero-current cel potential and corresponds to the potential difference of the cell when the cell (not the cell reaction) is at equilibrium. Infinitesimally small changes from this equilibrium are reversible with constant concentration and, consequently, it is possible to relate emf to thermodynamic properties. 

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