Concentration cells with liquid junctions

Leckey JH, Horne FH (1981) Time-dependent cell potential tmd single-ion activity coeflScients for a concentration cell with liquid junction. J Phys Chem 85 2504-2511... 

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. 

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

Zi) A cell with liquid junction is employed, containing two identical ISEs selective for the determinand one ISE is immersed in a sample solution with concentration c and the other in a standard solution with concentration Cj. Assuming that the liquid-junction potential is constant, the potential difference measured is... 

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. 

The addition of organic solvents to water should modify acid-base phenomena, but assessment of such effects poses many problems, as only the measured pH of aqueous solutions can be interpreted in terms of hydrogen ion concentrations. The quantitative comparison of the acidities of partially aqueous solutions is therefore a problem of far greater complexity than the measurements of pH values in aqueous media. As mentioned earlier, a proton activity (paH) is defined in such a way that — log paH is equal to pH when the medium is water, and its value can be measured both by the electromotive force of a cell with liquid junction and by the spectrophotometry of colored indicators. 

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 the electrochemical shorthand the contact zone of two different electrolyte solutions (also, for example, for the same electrolyte at different concentrations) is represented by a double line. This is in contrast to the electrode/solution interface which is symbolized by a single line. Thus for a cell with liquid junction made up of an ion-selective measuring electrode M in contact with a solution containing the corresponding ions complemented with a standard hydrogen electrode we would write ... 

The next focus is on the electrolyte-concentration cells with a liquid junction. The dilution of HC1, which was the subject of the discussion above, can also be realized in a cell with a liquid junction, as shown in Figure 6.13. It is presupposed that the two HC1 solutions of different concentration can be brought together and averted from mixing. The flowing of two streams of solution synchronously sometimes attains this. One then can establish the cell ... 

The emf of the cell, contrary to that in the absence of a liquid junction, depends on the transference numbers. Such cells are usually identified as concentration cells with presence of transference, the second one in the electrolyte concentration cell classification list. This system, as has been seen, contains a liquid junction across which it is possible for direct transport of ions to occur. 

If two electrolytes of different concentrations are directly in contact with each other, this give rise to a junction potential. An example of a concentration cell with a liquid junction is... 

It has already been mentioned that the electromotive force of concentration cells with transference is the sum of the both electrode potentials and the liquid junction potential which arises, when two solutions of the same substance but of different concentrations are brought into contact the value of the mentioned potential is finally given by the equations (VI-28) and (VI-29). 

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... 

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). 

Liquid junctions are found in almost all electrochemical cells used in electroanalysis. In general, there is a potential drop across the liquid junction and it is important to be able to evaluate it. Because of the different electrolyte compositions and concentrations involved, the liquid junction is associated with an irreversible mass transfer process. In this section, methods of estimating the potential drop due to the liquid junction are outlined. 

A simple type of cell with which measurements of the potentials of concentration cells with transference may be carried out is shown in Fig. 3. The vessel A holds the three electrodes, e, e3 e and contains the more concentrated solution. The vessel B is filled with the dilute solution into which dip the electrodes er, ef, e A liquid junction between the two solutions is formed at the point j. Due to the slowness of the diffusion process the solution in the region of the electrodes is not disturbed until quite a long period has elapsed and the potential of such a cell remains constant. The potentials of cells containing most types... 

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

The transport number comes directly from a comparison of a concentration cell with a narrow tube as the liquid junction and the same cell but with a salt bridge as the liquid junction (see Sections 9.8.4 and 9.8.6). 

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. 

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