Cells with liquid junction

These are cells where the two electrode solutions must be kept physically separate. Basically there are two types  

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Electrolyte-concentration cells are based on electrolyte dilution, and have two identical electrodes that are immersed in two solutions of the same electrolyte containing ions of the electrode material at two different activities. Electrolyte concentration cells are classified as (i) cells without liquid junctions and (ii) cells with liquid junctions. 

Dunsmore, H. S. Midgley, D., The calibration of glass electrodes in cells with liquid junction, Anal. Chim. Acta 61, 115-122 (1972). 

Two types of methods are used to measure activity coefficients. Potentiometric methods that measure the mean activity coefficient of the dissolved electrolyte directly will be described in Section 3.3.3. However, in galvanic cells with liquid junctions the electrodes respond to individual ion activities (Section 3.2). This is particularly true for pH measurement (Sections 3.3.2 and 6.3). In these cases, extrathermodynamical procedures defining individual ion activities must be employed. 

The dissociation constants of acids and bases are determined either exactly, by means of a suitable cell without liquid junction and without measuring the pH directly, or approximately on the basis of a pH measurement in a cell with liquid junction, the potential of which is reduced to a minimum with the help of a salt bridge. In the former case we shall use, for example, the cell... 

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... 

Cells in which only one electrolytic solution is used are called cells without liquid junction. Cells in which it is necessary to use two solutions with a boundary between them are called cells with liquid junction. Such cells are discussed in Section 12.12. 

Cells in which at least two electrolytic solutions are in contact are known as cells with liquid junction or with transference. Such cells are inherently irreversible and a complete thermodynamic development of them is beyond the scope of this book. However, cells with liquid junction are of sufficient importance that we discuss here the type that approximates a reversible cell most closely. 

Cells with Liquid Junctions and Elimination of Junction Potentials. When electrochemical cells are employed to obtain thermodynamic data, high accuracy ( 0.05 mV) requires the use of cells that are free from liquid junction (in the sense that the construction of the cell does not involve bringing into contact two or more distinctly different electrolyte solutions). Otherwise, the previously discussed uncertainties in the calculation of liquid-junction potentials will limit the accuracy of the data. 

The extrapolation procedures used for cells with liquid junction are time-consuming, and the method is not entirely free of theoretical pitfalls.9 Because salt bridges usually involve double junctions, an important distinction needs to be made betwen the behavior of single-junction and double-junction salt... 

The silver-silver chloride electrode. The silver chloride reference electrode is not generally suitable as an electrode of the second kind because of the large solubility of AgCl in many aptotic solvents from formation of anionic complexes with chloride ion. In many cases the silver chloride solubility will essentially be that of the added chloride. This contributes significantly to the junction potential in cells with liquid junction and makes the electrode unsuitable for precise potentiometric work. 

For the cell with liquid junction Hg(i) Hg2S04(s) IKzSO Caq.mi) K2S04(aq,m2) Hg2S04(s) Hg(i) show that under reversible operating conditions... 

Cells with Liquid Junction.—In the cases described above it has been possible to utilize cells without liquid junctions, but this is not always feasible the suitable salts may be sparingly soluble, they may hydrolyze in solution, their dissociation may be uncertain, or there may be other reasons which make it impossible, at least for the present, to avoid the use of cells with liquid junctions. In such circumstances it is desirable to choose, as far as possible, relatively simple junctions, e.g., between two electrolytes at the same concentration containing a common ion or between two solutions of the same electrolyte at different concentrations, so that their potentials can be calculated with fair accuracy, as shown in Chap. VI. 

Electromotive Force Methods.—The earliest e.m.p. methods for evaluating the ionic product of water employed cells with liquid junction the e.m.f. of the cell... 

An operational definition endorsed by the International Union of Pure and Applied Chemistry (lUPAC) and based on the work of Bates determines pH relative to that of a standard buffer (where pH has been estimated in terms of p"H) from measurements on cells with liquid junctions the NBS (National Bureau of Standards) pH scale. This operational pH is not rigorously identical to p H defined in equation 30 because liquid junction potentials and single ion activities cannot be evaluated without nonthermodynamic assumptions. In dilute solutions of simple electrolytes (ionic strength, I < 0.1) the measured pH corresponds to within 0.02 to p H. Measurement of pH by emf methods is discussed in Chapter 8. 

Due to the disadvantage of working with cells without liquid junctions, in practice the operational scale uses buffer solutions with known conventional pH for the calibration of cells with liquid junction [e.g., the convenient glass electrode (with the calomel or silver electrode, respectively, as reference)]. After calibration of the measuring cell (with a buffer of known conventional pH), the acidities of unknown samples can be measured in the same solvent. It is clear that for the standard buffers used, the conventional and the operational pH are identical. However, we cannot assume such an identity for the unknown samples. This is because the activities and the mobilities of the different ionic species might change the potential on the boundary with all liquid junctions (even without taking effect of the nonelectrolytes into account). 

Figure 1. pH cell with liquid junction. Calomel reference electrode on the left, hydrogen electrode compartment on the right, A liquid junction is formed in the capillary tube below the hydrogen electrode. 

This conclusion was reinforced in another way. The observed emf for the glass electrode-calomel cell (with liquid junction) in eight solutions with pH 2-9 differed by a constant amount from that for the glass-AgCl Ag combination (without liquid junction) in the same cell. The range was from 9.0 mV in the acetate buffer to 9.6 mV in the tris buffer the mean was 9.43 mV, and the standard deviation was 0.19 mV. 

The addition of organic solvents to water should modify acid-base phenomena, but assessment of such effects poses many problems, as only the measured pH of aqueous solutions can be interpreted in terms of hydrogen ion concentrations. The quantitative comparison of the acidities of partially aqueous solutions is therefore a problem of far greater complexity than the measurements of pH values in aqueous media. As mentioned earlier, a proton activity (paH) is defined in such a way that — log paH is equal to pH when the medium is water, and its value can be measured both by the electromotive force of a cell with liquid junction and by the spectrophotometry of colored indicators. 

There is some confusion in the literature concerning the expression "cells with transference" and "cells with liquid junction. In this book the first of these will be reserved for cells of the type represented by (10) and (11) which contain only one solute, and the second expression will include these and also cells containing more complicated types of liquid junctions. 

These values are in fair agreement with the earlier work by Lewis and associates involving cells with liquid junctions, which have been used in all the available determinations of the standard potentials of the other alkali metals, and will be discussed in Chapter 14. 

Galvanic Cells with Liquid Junction Potentials... 

Graphical Methods for Computing Potentials of Cells with Liquid Junctions. The assumptions made by Planck and Henderson in obtain-... 

Standard Potential Determinations Involving Cells with Liquid Junctions, or Equilibrium... 

The Standard Potentials of the Alkali Metals from Cells with Liquid Junctions. The determination of the standard potentials of sodium and potassium, using cells without liquid junctions, has already been described in Chapter 10. It is of interest to compare the value obtained in that way for potassium with the result of measurements on cells with liquid junctions, especially as the available data for computing the standard potentials of lithium, rubidium and cesium are of the latter type. Lewis and Keyes2 have found the potentials of the cells ... 

E. Larson, Z. physik. Chem., A165, 53 (1933), emf measurements in salt solutions, cells with liquid junctions (at 18°),... 

Under these conditions the current is carried in solution by ions migrating in the solutions and also through the narrow tube, or the porous pot, or through the salt bridge. Such cells do not operate reversibly in a rigorous manner because the processes occurring at the junctions will contribute to the thermodynamic quantities. Such cells are called cells with liquid junctions. 

VOLTAIC CELLS WITH LIQUID JUNCTION—THE PRACTICAL KIND... 

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