The Liquid Junction Potential

For a hydrogen ion activity of an = 1 (thus pH = 0) the following equality applies  

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 above example illustrates the reason why saturated KCl is usually used in the calomel and silver silver chloride electrodes. As a result, the liquid junction potential is kept small. Nevertheless, there is an interest in estimating the value of the liquid junction potential for the general case. This subject is considered in more detail in the following section. 

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. 

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 

Both ions move in the same direction from the solution of higher concentration to that of lower concentration. If the concentration Cj is higher than Ci then the ions on the left-hand side move across the liquid junction from the left to the right. The movement is governed by the law of electroneutrality which requires that the local concentrations of cation and anion be the same throughout the system. This condition is met by requiring that the local flux of the two ions be equal, that is. 

If the ions move in the x direction only, this means that 

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When the potential of an electrochemical cell is measured, the contribution of the liquid junction potential must be included. Thus, equation 11.1 is rewritten as... 

Assuming that the glass electrode shows an ideal hydrogen electrode response, the emf of the cell still depends on the magnitude of the liquid junction potential j and the activity coefficients y of the ionic species ... 

Fig. 1.21 Concentration cell in which flAg+.il < Ag+.i that charge transfer occurs spontaneously and proceeds until the activities are equal (fj is the liquid junction potential at the sintered glass plug that is used to minimise mixing of the two solutions)...

To avoid contamination of the solution under study, and to minimise the liquid-junction potential, it is usual to use a salt bridge, but in many cases this can be dispensed with thus if corrosion in a chloride-containing solution is being studied a Ag/AgCl electrode immersed directly in the solution could be used similarly a Pb/PbOj electrode could be used for studies of corrosion in H2SO4. 

Although in certain cells the liquid junction can be eliminated by appropriate choice of electrolyte solution, this is not always possible. However, the liquid junction potential can be minimised by the use of a salt bridge (a saturated solution of KCl of about 4-2m), and the liquid junction potential is then only 1-2 mV this elimination of the liquid junction potential is indicated... 

Since the small interfacial potentials at the junctions of the electrodes and copper leads are equal and opposite, they cancel out, and if the liquid junction potential is assumed to be small, or is reduced to a negligible value by using a salt bridge, then equation 20.216 reduces to... 

An element of uncertainty is introduced into the e.m.f. measurement by the liquid junction potential which is established at the interface between the two solutions, one pertaining to the reference electrode and the other to the indicator electrode. This liquid junction potential can be largely eliminated, however, if one solution contains a high concentration of potassium chloride or of ammonium nitrate, electrolytes in which the ionic conductivities of the cation and the anion have very similar values. 

One way of overcoming the liquid junction potential problem is to replace the reference electrode by an electrode composed of a solution containing the same cation as in the solution under test, but at a known concentration, together with a rod of the same metal as that used in the indicator electrode in other words we set up a concentration cell (Section 2.29). The activity of the metal ion in the solution under test is given by... 

In view of the problems referred to above in connection with direct potentiometry, much attention has been directed to the procedure of potentio-metric titration as an analytical method. As the name implies, it is a titrimetric procedure in which potentiometric measurements are carried out in order to fix the end point. In this procedure we are concerned with changes in electrode potential rather than in an accurate value for the electrode potential with a given solution, and under these circumstances the effect of the liquid junction potential may be ignored. In such a titration, the change in cell e.m.f. occurs most rapidly in the neighbourhood of the end point, and as will be explained later (Section 15.18), various methods can be used to ascertain the point at which the rate of potential change is at a maximum this is at the end point of the titration. 

These figures include the liquid junction potential.29... 

Japaridze et al.m 323 have studied the interface between Hg and a number of vicinal and nonvicinal diols such as 1,2-, 1,3-, 2,3- and 1,4-butanediol (BD), ethanediol (ED), and 1,3-propanediol. KF and LiC104 were used as surface-inactive electrolytes. The potential of zero charge was measured by the capacitance method against an SCE in water without correction for the liquid junction potential at the solvent/H20 contact (such a potential drop is estimated to be in the range of 20 to 30 mV). The potential of the capacitance minimum was found to be independent of the electrolyte concentration while capacitance decreased with dilution. Therefore, Emin was taken to measure E . These values are reported in Table 4. 

Capacitance and interfacial tension measurements were used to study the interface between Hg and mixtures of acetone + nitromethane.330 The potential was measured against an SCEin H20 and corrected for the liquid junction potential by measuring the half-wave potential of the ferrocene-... 

Studies of pzc in mixed solvents were also carried out by Blaszczyk etal n using the dipping method. They worked in mixtures offormamide and NMF and estimated the shift of the standard potential of the hydrogen electrode, of the surface dipole potential atHg, and of the liquid junction potential. 

The liquid junction potential from the organic side may be negligible, owing to the use of a nitrobenzene-water partition system containing tetraethylammonium picrate as the salt bridge. The mobilities of both ions in nitrobenzene are similar, and they have similar Gibbs energies of... 

A representative ISE is shown schematically in Fig. 1. The electrode consists of a membrane, an internal reference electrolyte of fixed activity, (ai)i , ai and an internal reference electrode. The ISE is immersed in sample solution that contains analyte of some activity, (ajXampie and into which an external reference electrode is also immersed. The potential measured by the pH/mV meter (Eoe,) is equal to the difference in potential between the internal (Eraf.int) and external (Eref.ext) reference electrodes, plus the membrane potential (E emb), plus the liquid junction potential... 

It can be shown that the liquid junction potential, E]y between two concentrations, c, and c2, of a uni-univalent electrolyte (e.g., KC1, HC1) is provided by the following special case of an equation derived as ... 

The two equations show that the nearer the cationic transport is to 0.5, the smaller is the liquid junction potential (other factors being unchanged). Among common electrolytes one of the highest numerical values of the factor (2 t+ - I) is given by hydrochloric acid, at 0.65. Hence a potential difference of about 39 mV develops at 25 °C across the junction between 0.001 N and 0.01 N hydrochloric acid. In the case of potassium chloride solution,... 

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 ... 

Here, x is the coordinate normal to the diaphragm, so that d — q—p. The liquid junction potential A0L is the diffusion potential difference between solutions 2 and 1. The liquid junction potential can be calculated for more complex systems than that leading to Eq. (2.5.31) by several methods. A general calculation of the integral in Eq. (2.5.30) is not possible and thus assumptions must be made for the dependence of the ion concentration on x in the liquid junction. The approximate calculation of L. J. Henderson is... 

Although the Henderson formula depends on a number of simplifications and also employs ion concentrations rather than activities, it can nonetheless be used for estimation of the liquid junction potential up to moderate electrolyte concentrations, apparently as a result of compensation of errors. 

The general Planck s solution of the liquid-junction potential is based on the assumption that the fluxes of ions in the liquid junction are in a steady state. As the mathematical treatment is rather space-consuming the reader is recommended to inspect a more recent treatment of this by W. E. Morf... 

So far, a cell containing a single electrolyte solution has been considered (a galvanic cell without transport). When the two electrodes of the cell are immersed into different electrolyte solutions in the same solvent, separated by a liquid junction (see Section 2.5.3), this system is termed a galvanic cell with transport. The relationship for the EMF of this type of a cell is based on a balance of the Galvani potential differences. This approach yields a result similar to that obtained in the calculation of the EMF of a cell without transport, plus the liquid junction potential value A0L. Thus Eq. (3.1.66) assumes the form... 

When eliminating the liquid junction potential by one of the methods described in Section 2.5.3, we obtain a concentration cell without transport. The value of its EMF is given simply by the difference between the two electrode potentials. More exactly than by the described elimination of the liquid junction potential, a concentration cell without transport can be obtained by using amalgam electrodes or electrodes of the second kind. 

It would appear from Eq. (3.2.8) that the pH, i.e. the activity of a single type of ion, can be measured exactly. This is not, in reality, true even if the liquid junction potential is eliminated the value of Eref must be known. This value is always determined by assuming that the activity coefficients depend only on the overall ionic strength and not on the ionic species. Thus the mean activities and mean activity coefficients of the electrolyte must be employed. The use of this assumption in the determination of the value of Eref will, of course, also affect the pH value found from Eq. (3.2.8). Thus, the potentiometric determination of the pH is more difficult than would appear at first glance and will be considered in the special Section 3.3.2. 

For current practice, the described method of pH measurement is too tedious. Moreover, not hydrogen but glass electrodes are used for routine pH measurements (see Section 6.3). Then the expression for the EMF of the cell consisting of the glass and reference electrodes contains a constant term from Eq. (6.3.10), in addition to the terms present in Eq. (3.3.3) this term must be obtained by calibration. Further, a term describing the liquid junction potential between the reference electrode and the measured solution must also be included. 

The membrane phase m is a solution of hydrophobic anion Ax (ion-exchanger ion) and cation Bx+ in an organic solvent that is immiscible with water. Solution 1 (the test aqueous solution) contains the salt of cation Bx+ with the hydrophilic anion A2. The Gibbs transfer energy of anions Ax and A2 is such that transport of these anions into the second phase is negligible. Solution 2 (the internal solution of the ion-selective electrode) contains the salt of cation B with anion A2 (or some other similar hydrophilic anion). The reference electrodes are identical and the liquid junction potentials A0L(1) and A0L(2) will be neglected. 

In most texts, the liquid junction potential is given the symbol Ej. In some books it is written as E(ij) or even E(ijp). 

Henderson or Plank formalisms. Mobilities for several ions can be seen in Table 18a. 1. Liquid junction potentials can become more problematic with voltammetric or amperometric measurements. For example, the redox potentials of a given analyte measured in different solvent systems cannot be directly compared, since the liquid junction potential will be different for each solvent system. However, the junction potential Ej can be constant and reproducible. It can also be very small (about 2-3 mV) if the anion and cation of the salt bridge have similar mobilities. As a result, for most practical measurements the liquid junction potential can be neglected [9]. 

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