Liquid junction between same electrolytes

The case of a liquid junction between electrolyte solutions of the same composition was examined earlier for electrochemical cells with transport (section 9.5). This situation is now re-examined using Onsager s method for dealing with mass transfer. The system considered is... 

These cells have an electrolyte-electrolyte boundary, more commonly referred to as a liquid junction between two solutions of the same salt, which differ only in their concentrations.11... 

Liquid Junctions between Two Solutions of the Same Electrolyte. A type of cell involving liquid junctions has already been considered, in Chapter 8, and the measurements have been shown to yield potentials which can be interpreted thermodynamically. A cell of the type is... 

The foregoing text highlights the fact that at the interface between electrolytic solutions of different concentrations (or between two different electrolytes at the same concentration) there originates a liquid junction potential (also known as diffusion potential). The reason for this potential lies in the fact that the rates of diffusion of ions are a function of their type and of their concentration. For example, in the case of a junction between two concentrations of a binary electrolyte (e.g., NaOH, HC1), the two different types of ion diffuse at different rates from the stronger to the weaker solution. Hence, there arises an excess of ions of one type, and a deficit of ions of the other type on opposite sides of the liquid junction. The resultant uneven distribution of electric charges constitutes a potential difference between the two solutions, and this acts in such a way as to retard the faster ion and to accelerate the slower. In this way an equilibrium is soon reached, and a steady potential difference is set up across the boundary between the solutions. Once the steady potential difference is attained, no further net charge transfer occurs across the liquid junction and the different types of ion diffuse at the same rate. 

When the electrolytes on either side of a liquid junction are different, the mathematical analysis of the interfacial potential becomes complex. In nearly all these cases the potential is a function of the geometrical characteristics of the boundary itself. In one general case, however, i.e., for the junction between two uni-univalent electrolytes at the same concentration and having a common ion (e.g., the pair KC1, NaCl), the liquid junction potential is independent of the structure of the boundary and is provided by following equation ... 

Another less precise but frequently used method employs a liquid bridge between the analysed solution and the reference electrode solution. This bridge is usually filled with a saturated or 3.5 m KCl solution. If the reference electrode is a saturated calomel electrode, no further liquid bridge is necessary. Use of this bridge is based on the fact that the mobilities of potassium and chloride ions are about the same so that, as follows from the Henderson equation, the liquid-junction potential with a dilute solution on the other side has a very low value. Only when the saturated KCl solution is in contact with a very concentrated electrolyte solution with very different cation and anion mobilities does the liquid junction potential attain larger values [2] for the liquid junction 3.5 M KCl II1 M NaOH, A0z, = 10.5 mV. 

In aqueous solutions, the method of measuring electrode potentials has been well established. The standard hydrogen electrode (SHE) is the primary reference electrode and its potential is defined as zero at all temperatures. Practical measurements employ reference electrodes that are easy to use, the most popular ones being a silver-silver chloride electrode and a saturated calomel electrode (Table 5.4). The magnitude of the liquid junction potential (LJP) between two aqueous electrolyte solutions can be estimated by the Henderson equation. However, it is usual to keep the LJP small either by adding the same indifferent electrolyte in the two solutions or by inserting an appropriate salt bridge between the two solutions. 

Liquid Junction Potential Between Electrolyte Solutions in the Same Solvent... 

If two electrolyte solutions that are of different concentrations but in the same solvent contact each other at a junction, ion transfers occur across the junction (Fig. 6.3). If the rate of transfer of the cation differs from that of the anion, a charge separation occurs at the junction and a potential difference is generated. The potential difference tends to retard the ion of higher rate and accelerate the ion of lower rate. Eventually, the rates of both ions are balanced and the potential difference reaches a constant value. This potential difference is called the liquid junction potential (LJP) [10]. As for the LJP between aqueous solutions, the LJP between non-aqueous solutions can be estimated using the Henderson equation. Generally the LJP, Lj-, at the junction Ci MX(s) c2 NY(s) can be expressed by Eq. (6.1) ... 

DIFFUSION POTENTIAL. When liquid junctions exist where two electrolytic solutions are in contact, as in the case of two solutions of different concentrations of the same electrolyte, diffusion of ions occurs between the solutions, and the differences in rales of diffusion of different ions set up an electrical double layer, having a difference of potential, known as Ihe diffusion potential nr liquid junction potential. 

Therefore the fraction of the total cell potential due to the junction potential cannot be unambiguously assigned. However, it is possible to estimate junction potentials indirectly or to make calculations based on assumptions about the geometry and distribution of ions in the region of the junction. For a junction between two dilute solutions of the same univalent electrolyte (concentrations C] and C2), the liquid-junction potential is described by... 

The total potential difference across the terminals of a cell is the sum of the potential differences, arising at the boundaries of two different phaseH. The most important boundary is that one between the electrode and the electrolyte. At the junction of two solutions of the same electrolyte but of different concentrations or solutions containing different electrolytes a potential difference will also arise, i. e. the so called liquid junction potential or diffusion potential which is, however, rather small (it will be dealt with in more detail later on). 

A salt bridge is a liquid junction filled with a normal solution, or better, with a saturated solution of potassium chloride where the formation of a precipitate would be the result of the contact between the solution and the electrolyte containing e. g. ions Ag+, T1+, Hg4+, a saturated solution of ammonium nitrate is applied. This junction is interposed between the electrolytes surrounding both electrodes and at the same time is an intermediary of their electric connection. In this way, instead of one interface between the electrolytes, two are formed, as is evident, e. g. from the schematic representation of this system ... 

Interface between two liquid solvents — Two liquid solvents can be miscible (e.g., water and ethanol) partially miscible (e.g., water and propylene carbonate), or immiscible (e.g., water and nitrobenzene). Mutual miscibility of the two solvents is connected with the energy of interaction between the solvent molecules, which also determines the width of the phase boundary where the composition varies (Figure) [i]. Molecular dynamic simulation [ii], neutron reflection [iii], vibrational sum frequency spectroscopy [iv], and synchrotron X-ray reflectivity [v] studies have demonstrated that the width of the boundary between two immiscible solvents comprises a contribution from thermally excited capillary waves and intrinsic interfacial structure. Computer calculations and experimental data support the view that the interface between two solvents of very low miscibility is molecularly sharp but with rough protrusions of one solvent into the other (capillary waves), while increasing solvent miscibility leads to the formation of a mixed solvent layer (Figure). In the presence of an electrolyte in both solvent phases, an electrical potential difference can be established at the interface. In the case of two electrolytes with different but constant composition and dissolved in the same solvent, a liquid junction potential is temporarily formed. Equilibrium partition of ions at the - interface between two immiscible electrolyte solutions gives rise to the ion transfer potential, or to the distribution potential, which can be described by the equivalent two-phase Nernst relationship. See also - ion transfer at liquid-liquid interfaces. 

If the right-hand side is constant, for cells with transference containing different chlorides at definite concentrations, it may be concluded that the approximate equation (36) gives a satisfactory measure of the liquid junction potential between two solutions of the same electrolyte. The results in Table XLV provide support for the reliability of this equation, within certain limits the transference numbers employed are the mean values for the two solutions, the individual figures not differing greatly in the range of concentrations involved. 

III. Free Diffusion Junction.—The free diffusion type of boundary is the simplest of all ir. practice, but it has not yet been possible to carry out an exact integration of equation (41) for such a junction. In setting up a free diffusion boundary, an initially sharp junction is formed between the two solutions in a narrow tube and unconstrained diffusion is allowed to take place. The thickness of the transition layer increases steadily, but it appears that the liquid junction potential should be independent of time, within limits, provided that the cylindrical symmetry at the junction is maintained. The so-called static junction, formed at the tip of a relatively narrow tube immersed in a wdder vessel (cf. p. 212), forms a free diffusion type of boundary, but it cannot retain its cylindrical symmetry for any appreciable time. Unless the two solutions contain the same electrolyte, therefore, the static type of junction gives a variable potential. If the free diffusion junction is formed carefully within a tube, however, it can be made to give reproducible results. ... 

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. 

A procedure for the elimination of liquid junction potentials, suggested by Nemst (1897), is the addition of an indifferent electrol3rte at the same concentration to both sides of the cell. If the concentration of this added substance is greater than that of any other electrolyte, the former will carry almost the whole of the current across the junction between the two solutions. Since its concentration is the same on both sides of the boundary, the liquid junction potential will be very small. This method of eliminating the potential between two solutions fell into disrepute when it was realized that the excess of the indifferent electrolyte has a marked effect on the activities of the substances involved in the cell reaction. It has been revived, however, in recent years in a modified form a series of cells are set up, each containing the indifferent electrolyte at a different concentration, and the resulting e.m.p. s are extrapolated to zero concentration of the added substance. 

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. 

Reviewing all the results, we may say that there was reasonable agreement between theory and experiment except when the electrolyte concentration in the oil phase was very low. It was concluded that the variations in liquid junction potential had been made negligible, and when the same oil phase was used throughout the cell, the liquid-junction potentials themselves were probably very small. 

The results obtained for the liquid junction potential show that its magnitude for a system with the same electrolyte on either side depends on the relative values of the transport numbers for the cation and anion. For an electrolyte like KCl for which and are approximately equal, the value of As2< ) is very small. On the other hand, if the liquid junction is formed between two HCl solutions, the value of sj Asj4> is considerably larger because the transport number for the FT cation is larger than that for the CP anion. 

Values of the liquid junction potential between 4.2 M KCl and more dilute solutions of the same electrolyte estimated by equation (9.7.32) are shown in table... 

To this point, we have examined only systems at equilibrium, and we have learned that the potential differences in equilibrium electrochemical systems can be treated exactly by thermodynamics. However, many real cells are never at equilibrium, because they feature different electrolytes around the two electrodes. There is somewhere an interface between the two solutions, and at that point, mass transport processes work to mix the solutes. Unless the solutions are the same initially, the liquid junction will not be at equilibrium, because net flows of mass occur continuously across it. 

If the two electrode systems that compose a cell involve electrolytic solutions of different composition, there will be a potential difference across the boundary between the two solutions. This potential difference is called the liquid junction potential, or the diffusion potential. To illustrate how such a potential difference arises, consider two silver-silver chloride electrodes, one in contact with a concentrated HCl solution, activity = the other in contact with a dilute HCl solution, activity = Fig. 17.7(a). If the boundary between the two solutions is open, the and Cl ions in the more concentrated solution diffuse into the more dilute solution. The ion diffuses much more rapidly than does the Cl ion (Fig. 17.7b). As the ion begins to outdistance the Cl ion, an electrical double layer develops at the interface between the two solutions (Fig. 17.7c). The potential difference across the double layer produces an electrical field that slows the faster moving ion and speeds the slower moving ion. A steady state is established in which the two ions migrate at the same speed the ion that moved faster initially leads the march. 

Every measurement of the potential of a cell whose two electrodes require different electrolytes raises the problem of the liquid junction potential between the electrolytes. The problem can be solved in two ways Either measure the junction potential or eliminate it. The junction potential can be eliminated by designing the experiment, as above, so that no liquid junction appears. Or, rather than using two cells, choose a reference electrode that uses the same electrolyte as the electrode being investigated. This is often the best way to eliminate the liquid junction however, it is not always feasible. 

Consider the simple diffusion of an electrolyte in the absence of an external electric field. The diffusion occurs because of a concentration gradient. The situation shown in Fig. 31.12(a) illustrates the initial condition of an electrolytic solution over which there is a layer of pure water. We assume that initially the boundary between the two layers is sharp. Suppose that the ion moves more rapidly than the ion. Then we soon have the situation illustrated in Fig. 31.12(b). In the first few moments of the process the positive ions outdistance the negative ions. An electrical double layer forms, with an associated electric field. The effect of this electric field is to speed up the slower ion and to slow down the faster ion. The system quickly adjusts so that both ions move, in the same direction with the same velocity. If this adjustment did not occur, large departures from electrical neutrality would occur because of the difference in velocity between the positive and negative ions. Correspondingly enormous electric potential differences would develop in the direction of diffusion. In fact, the potential difference that develops and that equalizes the velocities of the ions is rather small (< 100 mV) it is the diffusion potential and is responsible for the liquid junction potential that was described in Section 17.18. 

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