Cell without transference

We distinguish between concentration cells without and with transference. In the first type the solutions surrounding both electrodes arc not brought into direct contact, while in concentration cells with transference two solutions arc in direct contact. The name cell with transference originates from the fact that during flow of the current a simultaneous transfer of the electrolyte takes place owing to the different ionic mobility. In the case of cells without transference the direct transfer of the electrolyte from one solution to the other is prevented in this instance the transport of the electromotive active substance proceeds exclusively as a result of reactions taking place at the electrodes. 

The EMF of a cell with transference is increased or decreased, as compared with the EMF of a cell without transference, by the value of the liquid junction potential existing at the interface of both solutions according to the sign of this and to the sign of the electrode potentials. 

The difference in concentration which causes the potential difference within the cell is a result of either the difference in concentration of the electromotively active substance in the electrodes (with gas and amalgam concentration cells), or of the different concentration of solutions surrouding the electrodes (with electrolyte concentration cells). As will be seen later electrolyte concentration cells must be adjusted in special way in order to exclude liquid junction or diffusion potential. 

An example of a gas concentration cell is a system of two hydrogen electrodes saturated with hydrogen at different pressure (p1 p2), both dipping into the same solution with the hydrogen ions activity aH+  

By adding both equations the following result will be obtained  

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Were the FlCl in its standard state, AC would equal where is the standard emf for the reaction. In general, for any reversible chemical cell without transference, i.e. one with a single electrolyte solution, not one with any kind of junction between two solutions. 

A nonzero OCV of a galvanic cell implies that the potential of one of the electrodes is more positive than that of the other (there is a positive and a negative electrode). For the galvanic cell without transference, the OCV can be written as... 

We can readily show that for cells without transference, the OCV value is equal to the difference in electrode potentials of the two electrodes (i.e., it can be written in terms of two parameters that are measurable, and each of them refers to just one of the electrodes). In the expression for OCV, the (P(j values for the reference electrode cancel, so that the reference electrode itself has no effect on the results (provided that all potentials refer to the same reference electrode). In the case of cells with transference, the difference in electrode potentials is equal not to the total but to the corrected OCV value, %. ... 

Electrochemical Cell Without Transference Assume that we want to determine the activities of HCl solutions of various concentrations. We assemble a galvanic cell with hydrogen and calomel electrode ... 

In the previous chapters the condition of electroneutrality was applied to all systems that contained charged species. In this chapter we study the results when this condition is relaxed. This leads to studies of electrochemical systems, especially those involving galvanic cells. Cells without transference are emphasized, although simple cells with transference are discussed. At the end of the chapter the conditions of equilibrium across membranes in electrochemical systems are outlined. 

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. 

If these two solutions are the same, there is no liquid junction, and we have a cell without transference. 

Note that there is no direct transference of the electrolyte (HC1) from one side to the other. HCI is removed from the left-hand side by the left-hand electrode reaction and it is added to the right-hand side by the right-hand electrode reaction. This cell is an example of a electrolyte concentration cell without transference. 

Tho general formula for calculating the EMF of an electrolyte concentration cell without transference is ... 

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+. 

If reversible electrodes for both the cations and anions are available concentration cells without transference... 

Concentration Cells Cells without Transference.—In the operation of the cell... 

A cell of the type described above is called a concentiation cell without transference, for the e.m.f. depend.s on the relative concentrations, or molalities, of the two solutions concerned, and the operation of the cell is not accompanied by the direct transfer of electrolyte from one solution to the other. The transfer occurs indirectly, as shown above, as the result of chemical reactions. In general, a concentration cell without transference results whenever two simple cells whose electrodes are reversible with respect to each of the ions constituting the electrolyte are combined in opposition in the case considered above, the electrolyte is hydrochloric acid, and one electrode is reversible with respect to hydrogen ions and the other with respect to chloride ions. 

Amalgam Cells.—If the electrolyte in the concentration cell without transference is a salt of an alkali metal, e.g., potassium chloride, it is necessary to set up some form of reversible alkali metal electrode. This is achieved by dissolving the metal in mercury, thus forming a dilute alkali metal amalgam which is attacked much less vigorously by water than is the metal in the pure state." The amalgam nevertheless reactia with water to some extent, and also with traces of oxygen that may be... 

Determination of Activity Coefficients.—The e.m.f. of a concentration cell without transference is equal to Ei — Ez, where Ei and Ez are determined by the concentrations Ci and C2, respectively, of the electrolyte then for a cell to which equation (8) is applicable,... 

Activity Coefficients from Cells With Transference.—In order to set up a cell without transference it is necessary to have electrodes reversible with respect to each of the ions of the electrolyte this is not always possible or convenient, and hence the use of cells with transference, which require electrodes reversible with respect to one ion only, has obvious advantages. In order that such cells may be employed for the purpose of determining activity coefficients, however, it is necessary to have accurate transference number data for the electrolyte being studied. Such data have become available in recent years, and in the method described below it will be assumed that the transference numbers are known over a range of concentrations. ... 

Determination of Transference Numbers.—Since activity coefficients can be derived from e.m.f. measurements if transference numbers are known, it is apparent that the procedure could be reversed so as to make it possible to calculate transference numbers from e.m.f. data. The method employed is based on measurements of cells containing the same electrolyte, with and without transference. The e.m.f. of a concentration cell without transference E) is given by equation (11), and if the intermediate electrodes are removed so as to form a concentration cell with transference, the e.m.f., represented by Et, is now determined by equation (25), provided the transference numbers may be taken as constant within the range of concentrations in the cells. It follows, therefore, on dividing equation (25) by (11), that... 

Elimination of Liquid Junction Potentials.—Electromotive force measurements are frequently used to determine thermodynamic quantities of various kinds in this connection the tendency in recent years has been to employ, as far as possible, cells without transference, so as to avoid liquid junctions, or, in certain ca.ses, cells in w hich a junction is formed between two solutions of the same electrolyte. As explained above, the potential of the latter type of junction is, within reasonable limits, independent of the method of forming the boundary. 

If the cell metal concentrated solution dilute solution metal is capable of yielding a current, the direction of the current must be such that the concentrated solution becomes more dilute and the dilute solution more concentrated. The positive current must therefore fiow from the dilute to the concentrated solution inside the cell, so that the electrode dipping into the concentrated solution becomes the cathode. As 1 —i/ equivalents of the electrolyte = 2(l —j/) equivalents of the ions are transferred by unit quantity of electricity from the more concentrated (cathode) solution to the more dilute (anode) solution, the E.M.F. of this concentration cell can be calculated by the same two methods (p. 354) which Helmholtz and Nemst employed in the calculation of the e.m.f. of concentration cells without transference. Thus, for dilute solutions of an w-valent metallic salt, we have the equation... 

This chapter is concerned with the determination of activity coefficients with the aid of various types of concentration cells, and with the comparison of such activity coefficients with the predictions of the Debye-Hiickel theory, developed in the previous chapter. The types of cells discussed are (a) cells without transference, including those containing amalgam electrodes, (b) cells with transference, and (c) cells without transference containing mixtures of electrolytes. 

Amalgam Cells. F6r the determination of activity coefficients of electrolytes the method depending upon the measurement of the potentials of concentration cells without transference has already been discussed in Chapter 6. Two examples of such cells were mentioned. These were the following ... 

Table I. The Computation of the Activity Coefficients of Sodium Hydroxide From Cells Without Transference at 25°...

The computation of activity coefficients from A log j values will be illustrated for hydrochloric acid since in that case direct comparison can be made with the results of measurements on concentration cells without transference of the type described in Chapter 6. The relevant data are given in Table II and are from the work of Shedlovsky and Maclnnes.18 The emf data in the second column were obtained from a cell of the type illustrated in Fig. 4. The transference numbers in the third column were interpolated from the measurements of Longs-worth given in Table IV of Chapter 4. The A log f values in the fourth column were computed as described in the last paragraph. 

In Chapter 10 standard potentials were obtained from measurements on galvanic cells involving only one electrolyte. These cells without transference thus do not involve surfaces between solutions of electrolytes, more commonly called liquid junctions. Although the measurements on cells without liquid junctions can be much more readily interpreted than the results from cells containing such boundaries most of the earlier work was carried out with the latter type of cell. Thus for instance instead of measuring the potential of the cell ... 

Cells using a salt bridge are called cells without transference since the migration process is largely independent of the ions of the electrode compartments. 

But the derivation now deviates from that for the cell without transference. 

Cells without liquid junction are always used for the most accurate measurements because there are no uncertain potentials to account for and were used for measuring the pH of NIST standard buffers (see below). However, there are few examples of cells without liquid junction (sometimes called cells without transference), and these are inconvenient to use. Therefore, the more convenient but less accurate cells with liquid junction are most commonly used. 

The total potential of the cell with transference is that of the cell without transference plus... 

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... 

Write the cell reaction and calculate the potential of the following cells without transference. 

Tknmodymtmc Measurements The cell (I), is a cell without transference... 

Other cells without transference such as redox cells follow the same general principles as discussed above and further details are given else-where. Some description of concentration cells with and without transference is necessary, however, since measurements of this type are often made in organic solvents. A concentration cell without transference is illustrated by cell (II) which is a combination of two cells of type I... 

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