Junction Potentials between Different Solvents

Liquid junction Potentials between Different Solvents 

As described above, establishing a reliable method to compare the potentials in different solvents is very important. One approach to this problem is to develop a 

The LJP between different solvents consists of three components (i) a component caused by the differences in electrolyte concentrations on the two sides of the junction and the differences between cationic and anionic mobilities (ii) a component due to the differences between ion solvation on the two sides of the junction (iii) a component due to the solvent-solvent interactions at the junction. 

The characteristics of the three components are schematically shown in Fig. 6.10 for a junction with the same electrolyte on the two sides (ci MX(S )/c2 MX(S2)). Component (i) is somewhat similar to the LJP between solutions in the same solvent (Section 6.1.4). Components (ii) and (iii), however, are specific to the junction between different solvents. Fortunately, under appropriate conditions, we can measure the variation in each of the three components separately. Thus, we can study the characteristics of each component. 

Here t is the ionic transport number, a is the electrolyte activity, and the subscripts 1 and 2 refer to the left and right sides of the junction. For Omxi = mx2. Ej(i)=0. If the experimental (actual) variations in component (i), obtained by changing the ratio C /c2, are plotted against the values calculated by Eq. (6.16), lin- 

---

Liquid junction Potentials between Different Solvents... 

Liquid Junction Potentials between Different Solvents I 197... 

Figure 5.19 summarizes the positive and negative voltage limits for some commonly used electrode materials in several solvents. Wherever possible, the data for a particular solvent has been referred to a single reference electrode. Absolute values of the electrode potential for different solvent systems cannot be directly compared, however, because they are often referred to different reference electrodes and because of the uncertainty in our knowledge of junction potentials between different solvent systems. 

As described in Section 4.3.S.2, the AglAg RE can be used with a variety of organic solvents. Using the same solvent in the RE filling solution as in the electrochemical cell will minimize the junction potential of the RE. The limitations of this electrode are that only solvents in which a silver salt is soluble and not oxidized by the Ag+ can be used. Similar to the difficulty in comparing potentials between different solvents using aqueous... 

Some Practical Considerations in the Use of Salt Bridges. Salt bridges are most commonly used to diminish or stabilize the junction potential between solutions of different composition and to minimize cross-contamination between solutions. For example, in working with nonaqueous solvents an aqueous reference electrode often is used that is isolated from the test solution by a salt bridge that contains the organic solvent. However, this practice cannot be recommended, except on the grounds of convenience, because there is no way at present to relate thermodynamically potentials in different solvents to the same aqueous reference-electrode potential furthermore, there is a risk of contamination of the nonaqueous solvent by water. 

Silver-silver ion electrodes have been employed to study liquid-junction potentials between electrolyte solutions in different solvents by use of the cell... 

Liquid Junction Potential Between Two Different Solvents. 226... 

The most satisfactory way of estimating solvent activity coefficients is by electrochemical measurements of the EMF of appropriate cells, or by polarographic methods (Kolthoff, 1964). Some measurements have been made (Kolthoff and Thomas, 1965 Nelson and Iwamoto, 1961 Koepp, Wendt and Strehlow, 1960 Coetzee et al., 1963 Alexander and Parker, 1966). The electrochemical methods are aimed at measuring liquid junction potentials between two half-cells in different solvents (Kolthoff and Thomas, 1965) and rely heavily on assumptions such as (i) and (ii). 

This paper reviews efforts to establish single ion activities for aqueous electrolytes. Nevertheless, a closely related problem, that of the energies of transfer of single ionic species from one solvent to another, has received much attention. Among the chief approaches on which these efforts are based are the following choice of a reference electrode the potential of which may be independent of the solvent, such as Rb /Rb or the ferrocinium/ferrocene couple assumption of the equality of the transfer energies of certain large ions such as tetraphenylarsonium and tetraphenylborate and efforts to nullify the liquid-junction potential between ionic solutions in different solvents. 

Such an assumption was proposed, namely that a bridge consisting of a 0.1 mol dm tetraethylammonium picrate in acetonitrile suppresses the liquid junction potential between two different nonaqueous electrolytes [6]. The argument in favor of such a salt bridge for nonaqueous electrolytes is the similar electrical mobility of the tetraethylammonium cation and the picrate anion in acetonitrile. This assumption was later expanded to allow for other nonaqueous solvents [28]. Agreement for the electrochemical data was found if the nonaqueous solvents did not have acidic hydrogen atom(s) in the solvent molecule (aprotic solvents) [29], 0.1 mol dm solutions of either tetrabutylammonium picrate or pyridinium trifluorosulftMiate [30] were also used. 

In this book, there are other chapters related to nonaqueous systems. Chapter 1 by Inzelt is on the electrode potentials and includes a section on the problem to relate the electrode potentials between different media. Chapter 2 by Gritzner is on the reference redox systems in nonaqueous systems and their relation to water. Chapter 3 by Tsirlina is on the liquid junction potential and somewhat deals with the problem between different solvents. Chapter 7 by Bhatt and Snook is on the reference electrodes for room temperature ionic liquids. See these chapters as well. 

With the reference electrodes of this group, there is a liquid junction between different solvents, i.e., between the solvent of the solution under smdy and that of the solution of the reference electrode. Of course, the reference electrodes themselves should fulfill the requirements that the electrode potentials are stable and reproducible. However, in this case, the LJP between different solvents should also be stable and reproducible. The reference electrodes of this group can be divided into two subgroups the case using aqueous solutions and the case using nonaqueous solutions. 

The influence of interfaeial potentials (diffusion or liquid junction potentials) established at the boundary between two different electrolyte solutions (based on e.g. aqueous and nonaqueous solvents) has been investigated frequently, for a thorough overview see Jakuszewski and Woszezak [68Jak2]. Concerning the usage of absolute and international Volt see preceding chapter. 

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) ... 

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. 

Fig. 1. Schematic representation of the liquid junction between two solvents S, and S2 containing electrolyte MA. Between lines 1 and 2 there is the intermediate layer, thickness AA", where the liquid junction potential arises due to the difference in activities and mobilities of ions and A in the two solvents and due to the interaction of molecules of both solvents. Line A represents the situation when there is no liquid junction, while line 2 represents the change in the potential in the layer. Its linear change with distance was assumed arbitrarily.

Potentials o<,+/coc(corr) given versus NHE are also reported in Table 2. Assuming that a liquid junction is formed between water and another solvent for which coc+/coc(corr) is known, the potential difference between coc+zcocCcorr) in water and this solvent gives directly the liquid junction potential. For instance, for H2O and acetone (AC) it amounts to -0.28 V (Table 2). 

Uqmd-Junction Potentials. At the boundary between two dissimilar solutions, a junction potential is always set up. The solvents, the nature of the electrolytes, and the concentration of a given electrolyte can all differ, and therefore the mobilities of positive and negative ions diffusing across the boundary will not be equal. Thus a... 

---