Liquid junction computed potentials

TABLE 5.1 Computed Potentials of the Liquid Junction in Cells of the Type Ag AgCl MCl(Ci) MCl(C2) AgCl Ag... 

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. 

Concentration Cells Involving Mixtures of Electrolytes. There is a type of concentration cell, in addition to those already discussed, the study of which yields data of value. The data are useful, for instance, in the computation of liquid junction potentials as described in Chapter 13. These can be represented by the example ... 

The standard potential of am electrode is defined as the standard potential of a cell in which the other (reference) electrode is the arbitrary zero of potential (equation (2)) as described above. In this chapter the methods for obtaining standard potentials from emf measurements of cells without liquid junctions will be discussed, and the available data will be used for computing such potentials. The order adopted will be, more or less, that of the increasing complexity of the methods employed, Later chapters will deal with liquid junctions and the less accurate standard potentials that can be obtained from emf values of cells containing such junctions. 

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

It is however possible to evaluate the liquid junction potential using all the data available for the transference numbers and the ionic activities and any assumed distribution of electrolytes in the boundary with the aid of graphical methods. In the discussion below the procedure followed so far in this chapter will be reversed. The potential for a complete cell will be computed, after which the liquid junction will be obtained by subtracting a computed value of the electrode potentials. 

The two examples given above indicate that the graphical method may be used in computing the potentials of cells including liquid junctions if the necessary thermodynamic data are available. Any distribution of the electrolytes in the boundary may be used. Such distributions may be experimentally determined, or based on assumptions, and may be of any complexity. 

Having computed values for the total potential of cell (35) it is of interest to discuss the matter of the distribution of this potential between the differences of the electrode potentials Ei — 2 and the liquid junction potential EL. To do this wc must again resort to non-thermodynamic assumptions. The assumption made by Maclnnes 25 and frequently used in computations of this sort is that at any concentration... 

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 ... 

The uncertainty of computations involving liquid junctions is however indicated by the fact that if use is made of equation (26b) instead of (26a), Chapter 13, a potential is obtained that is almost one millivolt lower, numerically, than the one just given.22... 

This equation is not thermodynamic, for reasons made dear in foregoing chapters. In the first place the liquid junction potential EL and the potential of the reference electrode Er cannot be computed without non-thermodynamic assumptions. In general also, some of the activities, a, a, etc. are those of individual ions, which cannot be... 

This establishes the value of S from which the liquid junction potential may be computed with the aid of equation (27) in terms of the other variables contained in the equation. 

In order to illustrate the use of this formula, we apply it to the computation of the liquid-junction potential between phases containing two electrolytes with a common ion, for example,... 

The accuracy of the approximations inherent in the Planck and Henderson methods for the computation of liquid-junction potentials can be judged from the results tabulated in Table 13-1. The experimental values tabulated in Table 13-1 were measured in the cell... 

In the words of Harned (1924), Ve are confronted with the interesting perplexity that it is not possible to compute liquid-junction potentials without a knowledge of individual ion activities, and it is not possible to determine individual ion activities without an exact knowledge of liquid-junction potentials. ( ) The statement This is a dilemma from which there is apparently no escape is pertinent. ... 

Concerns with liquid junctions—that is, electrolytes with different ionic concentrations or different ionic species meeting at a junction, such as a membrane or simply a small hole in a Luggin capillary, go back at least to the works of Nernst [4, 5], Planck [6] in the 1880s and 1890, and that of Henderson [7] in 1907. It is Henderson who is credited with the derivation of the equation named after him, for the potential difference across such a junction, see below, although we find essentially the same equation in the 1890 work of Planck [6]. These works were concerned with steady state solutions. Helfferich (in 1958) [8] and Cohen and Cooley [9] computed, by finite differences, time-dependent behaviour at liquid junctions. Many subsequent works were of course published since then, including the recent work of Strutwolf et al. [10, 11], Dickinson et al. [12] and Britz and Strutwolf [13],... 

Among others, this approach has been used to calculate concentration profiles and liquid junction potentials at controlled electrolyte flow, current-induced concentration changes of mobile ions in liquid-membrane electrodes, as well as for the computer simulation of membranes exposed to different sample ions. It was also applied to calculate non-steady-state responses of membranes under current polarization. Also, the response behavior of ISMs operated in a thin-layer coulometric detection mode has been simulated with this model. - ... 

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