Null electrodes

Schwan, H. P. and Ferris, C. D., 1968, Four-electrode null techniques for impedance measurement with high resolution. Rev. Sci. Instr. 39(4) 481-485. 

There are two procedures for doing this. The first makes use of a metal probe coated with an emitter such as polonium or Am (around 1 mCi) and placed above the surface. The resulting air ionization makes the gap between the probe and the liquid sufficiently conducting that the potential difference can be measured by means of a high-impedance dc voltmeter that serves as a null indicator in a standard potentiometer circuit. A submerged reference electrode may be a silver-silver chloride electrode. One generally compares the potential of the film-covered surface with that of the film-free one [83, 84]. 

There are five major methods for the determination of fluonde ions m aqueous solution (1) volumetric, (2) gravimetnc, (3) null pointpotentiometry, (4) fluonde lon-selecti ve electrode (also known as specific ion electrode), and (5) ion chromatography... 

Principles and Characteristics Contrary to poten-tiometric methods that operate under null conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise occur spontaneously. It is thus possible to analyse ions and organic compounds that can either be reduced or oxidised electrochemi-cally. Polarography, which is a division of voltammetry, involves partial electrolysis of the analyte at the working electrode. 

Apart from the necessity of excluding interferences from any diffusion potential, normal potentiometry requires accurate determination of the emf, i.e., without any perceptible drawing off of current from the cell therefore, usually one uses the so-called Poggendorff method for exact compensation measurement the later application of high-resistance glass and other membrane electrodes has led to the modern commercial high-impedance pH and PI meters with high amplification in order to detect the emf null point in the balanced system. 

The cell voltage measurement in itself represents a point of decisive significance, where factors such as temperature of the measurement, and Nemstian behaviour and asymmetry of the electrode play a role together with the reliability and flexibility of the pH/mV meter. Such a meter consists of a null-point or a direct-reading meter. 

Electrode Systems. Direct Potentiometric Measurements. Potentiometric Titrations. Null -point Potentiometry. Applications of Potentiometry. 

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

Potentiometric measurements are based on the determination of a voltage difference between two electrodes plunged into a sample solution under null current conditions. Each of these electrodes constitutes a half-cell. The external reference electrode (ERE) is the electrochemical reference half-cell, which has a constant potential relative to that of the solution. The other electrode is the ion selective electrode (ISE) which is used for measurement (Fig. 18.1). The ISE is composed of an internal reference electrode (IRE) bathed in a reference solution that is physically separated from the sample by a membrane. The ion selective electrode can be represented in the following way ... 

D Dj Do, DR E Ea E° Ea Ec Em, Eoui m-3) diffusion coefficient (m2 s 1) diffusion coefficient of species (m2 s-1) diffusion coefficient of species O and R (m2 s-1) electrode potential (vs. some reference electrode) (V) null or equilibrium potential (V) standard potential (V) interelectrode potential (V) voltage input and output of a circuit (V)... 

The null potential , En, is the potential exhibited by an electrode when equilibrium reigns. The surface concentrations of O and R are then identical to their bulk values, Cq = Cq and, and the current is... 

There Is one null entry in this table, and that is the one for the dropping mercury electrode. This reflects the same fact that led to the omission of polarography from Table VII. There are some compounds in Table I for which no information obtained with a dropping mercury electrode is given in... 

Note that the finite electrode volume has not been considered to deduce Eq. (2.137), i.e., we have used the so-called Koutecky approximation (see Eq. (2.134) and [52]). Therefore, when amalgamation takes place, these equations with the lower sign cannot be used for very small spherical electrodes for which numerical treatments considering null flux at the center of the electrode are needed. 

In the first case, when only species O is initially present in the electrolytic solution (Fig. 2.13a), it is observed that the amalgamation of species R leads to a shift of the wave to more negative potential values, and this shift is greater the more spherical the electrode, i.e., when the duration of the experiment increases or the electrode radius decreases. In the second case (Fig. 2.13b), both species are initially present in the system so we can study the anodic-cathodic wave. In the anodic branch of the wave, the amalgamation produces a decrease in the absolute value of the current. As is to be expected, the null current potential, crossing potential, or equilibrium potential ( Eq) is not affected by the diffusion rates (D0 and Z)R), by the electrolysis time, by the electrode geometry (rs), nor by the behavior of species R... 

In Fig. 3.14a, the dimensionless limiting current 7j ne(t)/7j ne(tp) (where lp is the total duration of the potential step) at a planar electrode is plotted versus 1 / ft under the Butler-Volmer (solid line) and Marcus-Hush (dashed lines) treatments for a fully irreversible process with k° = 10 4 cm s 1, where the differences between both models are more apparent according to the above discussion. Regarding the BV model, a unique curve is predicted independently of the electrode kinetics with a slope unity and a null intercept. With respect to the MH model, for typical values of the reorganization energy (X = 0.5 — 1 eV, A 20 — 40 [4]), the variation of the limiting current with time compares well with that predicted by Butler-Volmer kinetics. On the other hand, for small X values (A < 20) and short times, differences between the BV and MH results are observed such that the current expected with the MH model is smaller. In addition, a nonlinear dependence of 7 1 e(fp) with 1 / /l i s predicted, and any attempt at linearization would result in poor correlation coefficient and a slope smaller than unity and non-null intercept. 

In order to establish the most appropriate conditions for the determination of the diffusion coefficients of both electroactive species by using Eqs. (4.38) and (4.39), it has been reported that when the reaction product is absent (i.e., cR = 0) neither planar electrodes nor ultramicroelectrodes can be used in DPC for determining diffusion coefficients (see Eqs. (4.48) and (4.50)) because in these situations the anodic limiting current is either independent of DR or null, respectively. 

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