Cell without liquid junction

In view of all the difficulties associated with liquid junction potentials, it is desirable to construct cells without liquid junctions. In practice this is possible more often than one might think. All that is needed is for the sample solution to contain an ion with constant activity which does not interfere with the ion-selective indicator electrode used, and which can itself be specifically sensed with a second ion-selective electrode. This second ion-selective electrode can then function as a reference electrode, since it fulfills the primary requirement of a good reference electrode, i.e. it has a constant equilibrium Galvani potential at the interface electrode/solution. With such a set-up problems of a technical nature arise only if the resistance of the ion selective electrode connection with the reference electrode jack is too large. In this case an ion-meter with a high-ohmic differential input must be used. Such devices are already sold commercially (see Fig. 43). 

If the test solution does not contain any detectable ion in a constant, stable amount, as is the case in most analytical work, then a suitable compound which does not interfere with the indicator electrode can often be added before the EMF measurement. The amount of such an ion-pair (an ion can only be added as a cation-anion ion-pair) needed to produce a constant equilibrium Galvani potential at the second electrode can lie between 10 and 10 M, if this ion is originally present in the test solution only in traces. For cases where the individual sample solution contains vari- 

Two electrodes can dip into one solution with each electrode being reversible with respect to one of the ions of the solution, e.g. a hydrogen electrode, Pt(s) H2(g) HCl(aq) could be coupled with a Ag(s) AgCl(s) Cl (aq). The hydrogen electrode is reversible with respect to 

Such a cell can operate rigorously as a reversible cell, and is termed a cell without a liquid junction. Such cells are ideal since, by operating reversibly, they furnish exact thermodynamic quantities. 

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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... 

The potentiometric measurement of physicochemical quantities such as dissociation constants, activity coefficients and thus also pH is accompanied by a basic problem, leading to complications that can be solved only if certain assumptions are accepted. Potentiometric measurements in cells without liquid junctions lead to mean activity or mean activity coefficient values (of an electrolyte), rather than the individual ionic values. 

The dissociation constants of acids and bases are determined either exactly, by means of a suitable cell without liquid junction and without measuring the pH directly, or approximately on the basis of a pH measurement in a cell with liquid junction, the potential of which is reduced to a minimum with the help of a salt bridge. In the former case we shall use, for example, the cell... 

If we consider a cell without liquid junction - which in fact is nonexistent. 

Cells in which only one electrolytic solution is used are called cells without liquid junction. Cells in which it is necessary to use two solutions with a boundary between them are called cells with liquid junction. Such cells are discussed in Section 12.12. 

The cell reaction for cells without liquid junction can be written as the sum of an oxidation reaction and a reduction reaction, the so-called half-cell reactions. If there are C oxidation reactions, and therefore C reduction reactions, there are C C — 1) possible cells. Not all such cells could be studied because of irreversible phenomena that would take place within the cell. Still, a large number of cells are possible. It is therefore convenient to consider half-cell reactions and to associate a potential with each such reaction or electrode. Because of Equation (12.88), there would be (C - 1) independent potentials. We can thus assign an arbitrary value to the potential associated with one half-cell reaction or electrode. By convention, and for aqueous solutions, the value of zero has been assigned to the hydrogen half-cell when the hydrogen gas and the hydrogen ion are in their standard states, independent both of the temperature and of the pressure on the solution. 

Some galvanic cells without liquid junction... 

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... 

For buffer solutions, pcH can be calculated from the equation pcH = pifc + log ([A ] /[HA]), provided pKc of the buffer acid HA is known for the particular ionic strength and neutral salt. Otherwise, pcH values must be measured with a glass electrode (and a silver- silver chloride electrode) in a cell without liquid junction [7]. 

When a metal forms a soluble, highly dissociated chloride, e.g., zinc, the standard potential is best obtained from measurements on cells without liquid junction, viz.,... 

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. 

Electromotive Force Method.—An alternative procedure for the evaluation of dissociation constants, which also leads to very accurate results, involves the study of cells without liquid junction. The chemical reaction occurring in the cell... 

Dissociation Constants of Dibasic Acids by E.M.F. Measurement.— If the ratio of the dissociation constants of a dibasic acid, or of any two successive stages of ionization of a polybasic acid, is greater than about 10 or 10 , it is possible to treat each stage as a separate acid and to determine its dissociation constant by means of cells without liquid junction in the manner already described. In a mixture of the free dibasic acid H2A with its salt NallA, the essential equilibria are... 

The values of these dissociation constants may be determined by means of cells without liquid junction in a manner similar to that described in Chap. IX. For the first stage the acid is the hydrochloride C1- NH8RC02H, i.e., RH Cl and the corresponding salt is the electrically neutral form NHsRCOi, i e., RH=, and so the appropriate cell without liquid junction is... 

In the determination of the second dissociation constant (Kt) the acid is the neutral form +NH3RCO2", i.e., RH= =, whereas the corresponding salt is the sodium salt NH2RCOi Na ", i.e., Na+R the cell without liquid junction will thus be... 

For cations and anions generally, the assumption that liquid-junction potentials are the same in the measurement of standards and unknowns is less likely to be valid than for pH measurements. It has been suggested that a quantity A ) expressed in pM or pA units be included in (13-26) and (13-27) to correct for changes injunction potential arising from differences in ionic strengths of standard and test solutions. Alternatively, these effects could be eliminated through the use of two reference half-cells composed of electrodes without liquid-junction potentials. For example, if the test solution contained chloride ion, both reference half-cells could be Ag/AgCl, and the liquid-junction potential would be eliminated. In practice, external reference half-cells without liquid junction are not always convenient. 

Separate each of the following reactions into its half-reactions and in each case write down the schematic representation of a galvanic cell in which the reaction would take place. Wherever possible devise a cell without liquid junction potentials. 

Measurements in the standard acidity scale are carried out in cells without liquid junctions (e.g., with the following setup Pt/H2/HCl in SH/AgCl/Ag). It is assumed, here, that the activities of the solvated proton and the counterion, chloride, are equal. In this case, the electromotive force (emf) of the cell can be expressed by... 

Due to the disadvantage of working with cells without liquid junctions, in practice the operational scale uses buffer solutions with known conventional pH for the calibration of cells with liquid junction [e.g., the convenient glass electrode (with the calomel or silver electrode, respectively, as reference)]. After calibration of the measuring cell (with a buffer of known conventional pH), the acidities of unknown samples can be measured in the same solvent. It is clear that for the standard buffers used, the conventional and the operational pH are identical. However, we cannot assume such an identity for the unknown samples. This is because the activities and the mobilities of the different ionic species might change the potential on the boundary with all liquid junctions (even without taking effect of the nonelectrolytes into account). 

Cell A is a cell without liquid junction while cell B, with a liquid junction, resembles the cell assembly of the common pH meter. 

Figure 3 is a plot of the pmn at 25°C calculated by Equation 10b as a function of buffer molality m (m = madd = sait) Data obtained in the sulfate-free seawater I are connected by dashed lines, and those for seawater II (with sulfate) are joined by a solid line. Two features of these results require fmther study—the separation of the two curves for tris buffers and the separation and curvature of the lines for bis-tris. It seems unlikely that the presence of sulfate can alter the ion-ion interactions sufficiently to cause departures from Equation 9 of the magnitude (0.02 in pmn at m = 0.02) found with tris buffers. Consequently, specific interactions, possibly complexation with cations, are suspected. In this case, the differences in calculated pmn would be real. As will be seen presently, measurements of cells without liquid junction suggest that this is the case see Table V). 

Table V. pH and ptn-g. in S3 tlietic Seawater at 25°C from Measurements of Cells Without Liquid Junction (Cell A) and With Liquid Junction (Cell B)...

Propose cells, without liquid junction, which might be used for determining the activities (or activity coefficients) of (i) H Oi, (ii) KCl, (iii) NaOH, (iv) CdBr2, in solution. Why is it not posrable to determine the activity of a nitrate in this manner ... 

The electrodes in some cells share a common electrolyte these are known as cells without liquid junction. For an example of such a cell, see Figure 1 9-2 and Example 19-7. 

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). 

Figure 1 9-2 Cell without liquid junction for Example 19-4.

Challenge Problem. As we saw in Problem 19-16, as a preliminary experiment in their effort to measure the dissociation constant of acetic acid, Harned and Ehlers- measured ° for the cell without liquid junction shown. To complete the study and determine the dissociation constant, these workers also measured the potential of the following cell. 

Workers at NIST and elsewhere have used cells without liquid junctions to study primary-standard buffers extensively. Some of the properties of these buffers are discussed in detail elsewhere. Note that the NIST buffers are described by their molal concentrations (mol solute/kg solvent) for accuracy and precision of preparation. For general use, the buffers can be prepared from relatively inexpensive laboratory reagents for careful work, however, certified buffers can be purchased from the NIST. 

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