Concentration cells without liquid junctions

Concentration cells are made up of two half-cells which are similar chemically but which differ in the activity of some (often common) component the difference giving rise to an e.m.f. because of the difference in potential of the two half-cells. This activity difference may be either between the solutions or between the electrode materials. 

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Present attention is first focused on the electrolyte concentration cells without liquid junctions. Such cells can be explained with two cells of the type... 

As indicated they are concentration cells, without liquid junctions, containing mixtures of electrolytes. Since the electrodes of the two halfcells are reversible to the chloride and hydrogen ions, the operation of such a cell involves per faraday of current the removal of one mol of hydrogen chloride from one of the two solutions and the reappearance of the same amount in the other, the mechanism being exactly that described for cell (22) of Chapter 6. The potential of the cell is therefore a measure of the difference of the chemical potential of hydrogen chloride in the two solutions, according to the equation ... 

In recent years much work has been carried out, particularly by H. S. Harned and his associates, on concentration cells without liquid junction for the purpose of obtaining ionization constants of weak electrolytes. The principle involved in these investigations is as follows. Galvanic cells are set up of the form ... 

As already stated the limiting value of K is the thermodynamic ionization constant, K, which in this case is 1.753 X 10 c. Another method for obtaining thermodynamic ionization constants is given in Chapter 11, depending on measurements of the electromotive force of concentration cells without liquid junction. Using that method Harned and Ehlers found 1.754 X 10"R for the ionization constant of acetic acid at 25°. However, that constant is based on molalities, m, rather than concentrations, C. The relation between the ionization constants may be readily shown to be... 

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. 

OCp]3 can be calculated directly if the concentrations of all ligands and of all competing cations are known (21). Where this information is not available, ISE s can in principle enable conventional single-ion activites to be measured directly. The limited sensitivity of present-day ISE s precludes their use in natural waters,/ although they Ccin be. used in experimental systems involving elevated concentrations of trace metals. Provided that the salinity remains constant, a cell without liquid junction, composed of perfectly selective chloride and lead ISE s could be used. The difference between the emfs measured in the sample (E ) and in a standard solution with the same tanperature and major ion composition (Eg) would be given by... 

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. 

Note that this cell does not require two compartments (nor a salt because molecular H2 has little tendency to react directly with the low concentration of Ag in the electrolyte solution. This is an example of a cell without liquid Junction (Figure 19-2). 

Workers at NIST and elsewhere have used cells without liquid junctions to study primary-standard buffers extensively. Some of the properties of these buffers are discussed in detail elsewhere. Note that the NIST buffers are described by their molal concentrations (mol solute/kg solvent) for accuracy and precision of preparation. For general use, the buffers can be prepared from relatively inexpensive laboratory reagents for careful work, however, certified buffers can be purchased from the NIST. 

The Determination of Transference Numbers from the Potentials of Concentration Cells. Another use of concentration cells, which involves the principles already discussed in this chapter, is that of the determination of transference numbers. Since a cell without liquid junctions of the type... 

In Eqs. (122) and (123), M(Hg) is an alkali metal amalgam electrode, MX the solvated halide of the alkali metal M at concentration c in a solvent S, and AgX(s)/Ag(s) a silver halide-silver electrode. Equation (124) is the general expression for the electromotive force " of a galvanic cell without liquid junction in which an arbitrary cell reaction 0)1 Yi + 0)2Y2 + coiYi + , takes place between k components in v phases. In Eq. (124) n is the number of moles of electrons transported during this process from the anode to the cathode through the outer circuit, F the Faraday number, and the chemical potential of component Yi in phase p. Cells with liquid junctions require the electromotive force E in Eq. (124) to be replaced by the quantity E — Ej), where Ey> is the diffusion potential due to the liquid junction. The standard potential E° for the cell investigated by Eq. (122) is given by the relationship... 

In electrochemical cells without liquid junctions, the two electrodes are in contact with the same electrolyte of uniform concentration. For example, the cell shown in Figure 2.23 is made of a lead electrode and a lead amalgam electrode (lead dissolved in mercury), in contact with an aqueous solution of PbCl2. This cell corresponds to the schematic representation (2.123), where M and M" refer to the metal of the two conductors attached to the voltmeter. 

When eliminating the liquid junction potential by one of the methods described in Section 2.5.3, we obtain a concentration cell without transport. The value of its EMF is given simply by the difference between the two electrode potentials. More exactly than by the described elimination of the liquid junction potential, a concentration cell without transport can be obtained by using amalgam electrodes or electrodes of the second kind. 

In the half-cell of Eq. (5.24), the concentration of AgClj" must be small compared to that of Cl-, or a liquid-junction potential will result because the mobilities of AgClJ and Cl- are not the same. Thus, for a reference electrode of the second kind to be elfective in cells without appreciable junction potentials, the equilibrium constant for the reaction of Eq. (5.25) must be smaller than unity (preferably <0.1). In water, methanol, formamide, and V-methyl-formamide, this criterion is met, but in most organic solvents the equilibrium constant for the reaction of Eq. (5.25) ranges from 30 to 100. The silver chloride electrode is not recommended for general use in organic solvents.27... 

The influence of the relative values of the transference numbers, affecting the resultant value of the EMF of the concentration cell without transference, is clearly to be seen from the equation (VI-29) should t.. > <+ then eK is positive and in a concentration cell reversible with respect to cations the liquid junction potential is added to the sum of the electrode potentials should, however, < t+, then the liquid junction potential will lower the resultant EMF. In a concentration cell reversible with respect to anions (e. g. in a cell with chlorine electrodes) the EMF is decreased when ( >(+, and increased when t. < t+. 

Potential Measurements of Cell Pt,H2(1 atm] HCI(/n),AgCI(sat d) Ag without Liquid Junction as a Function of Concentration (molality) and Temperature (°C) ... 

Use of concentration cells with and without liquid junctions in the determination of transport numbers... 

A concentration cell without transference (that is, without a liquid junction) is shown in Fig. 17.8. The cell consists of two cells in series, which can be symbolized by... 

As a liquid junction potential is avoided, the cell potential consists merely of the electrode potentials of the hydrogen and the silver/silver chloride reference electrode. Chloride at known concentrations, mcl, must be added to the (chloride-free) buffer solution to use the silver-silver chloride electrode in cells without transference as a reference. This is different from silver/silver chloride reference systems with fixed potentials used for example as standard references in single-rod glass electrodes. 

Acidity scales are used commonly to assess the chemical and biological state of seawater. The international operational scale of pH fulfills the primary, requirement of repro ducibility and leads to useful equilbrium data. Nevertheless, these pH numbers do not have a well defined meaning in respect to all marine processes. Seawater of 35%o salinity behaves as a constant ionic medium, effectively stabilizing both the activity coefficients and the liquid junction potential. It may be possible, therefore, to determine hydrogen ion concentrations in seawater experimentally. One method is based on cells without a liquid junction and is used to establish standard values of hydrogen ion concentration (expressed as moles of H /kg of seawater) for reference buffer solutions. 

In a cell without a liquid junction, the reactants of the cell reaction can react directly without the passage of an electtic current For instance, in the cell of Fig. 14.1 the electrolyte solution is saturated with respect to gaseous H2 and soUd AgCl, and therefore contains very small concentrations of dissolved H2 molecules and Ag" " ions. 

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