Mobility, liquid junction potentials

Liquid Junction Potentials A liquid junction potential develops at the interface between any two ionic solutions that differ in composition and for which the mobility of the ions differs. Consider, for example, solutions of 0.1 M ITCl and 0.01 M ITCl separated by a porous membrane (Figure 11.6a). Since the concentration of ITCl on the left side of the membrane is greater than that on the right side of the membrane, there is a net diffusion of IT " and Ck in the direction of the arrows. The mobility of IT ", however, is greater than that for Ck, as shown by the difference in the... 

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... 

The potential developed is determined by the chloride concentration of the inner solution, as defined by the Nemst equation. As can been seen from the above reaction, the potential of the electrode remains constant as long as the chloride concentration remains constant. Potassium chloride is widely used for the inner solution because it does not generally interfere with pH measurements, and the mobility of the potassium and chloride ions is nearly equal. Thus, it minimizes liquid-junction potentials. The saturated potassium chloride is mainly used, but lower concentrations such as 1M potassium chloride can also be used. When the electrode is placed in a saturated potassium chloride solution, it develops a potential of 199 mV vs the standard hydrogen electrode. 

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]. 

The only process occurring in a Hquid junction is the diffusion of various components of the two solutions in contact with it. The various mobilities of the ions present in the Hquid junction lead to the formation of an electric potential gradient, termed the diffusion potential gradient. A potential difference, termed the liquid-junction potential, A0x,. is formed between two solutions whose composition is assumed to be constant outside the Hquid junction. 

Another less precise but frequently used method employs a liquid bridge between the analysed solution and the reference electrode solution. This bridge is usually filled with a saturated or 3.5 m KCl solution. If the reference electrode is a saturated calomel electrode, no further liquid bridge is necessary. Use of this bridge is based on the fact that the mobilities of potassium and chloride ions are about the same so that, as follows from the Henderson equation, the liquid-junction potential with a dilute solution on the other side has a very low value. Only when the saturated KCl solution is in contact with a very concentrated electrolyte solution with very different cation and anion mobilities does the liquid junction potential attain larger values [2] for the liquid junction 3.5 M KCl II1 M NaOH, A0z, = 10.5 mV. 

When a constant ionic strength of the test solution is maintained and the reference electrode liquid bridge is filled with a solution of a salt whose cation and anion have similar mobilities (for example solutions of KCl, KNO3 and NH4NO3), the liquid-junction potential is reasonably constant (cf. p. 24-5). However, problems may be encountered in measurements on suspensions (for example in blood or in soil extracts). The potential difference measured in the suspension may be very different from that obtained in the supernatant or in the filtrate. This phenomenon is called the suspension (Pallmann) effect [110] The appearance of the Pallmann effect depends on the position of the reference electrode, but not on that of ISE [65] (i.e. there is a difference between the potentials obtained with the reference electrode in the suspension and in the supernatant). This effect has not been satisfactorily explained it may be caused by the formation of an anomalous liquid-junction or Donnan potential. It... 

Table 15-1 shows mobilities of several ions and Table 15-2 lists some liquid junction potentials Saturated KC1 is used in a salt bridge because K+ and Cl- have similar mobilities. Junction potentials at the two interfaces of a KC1 salt bridge are slight. 

Here D, is the diffusion coefficient of species i, and Ej is the liquid junction potential that develops along the channel. The electrolytic mobility n, is the limiting velocity of an ion in the electric field of unit strength. It has dimensions of cm2 s 1 V 1 and... 

That equation explicitly shows the dominating influence of the ion mobilities on the value of the liquid junction potential. Although its application is limited, the Henderson junction is a significant experimental achievement because it allows the measurement of one electrode against another with known contribution of the liquid junction potential. 

In the half-cell of Eq. (5.24), the concentration of AgClj" must be small compared to that of Cl-, or a liquid-junction potential will result because the mobilities of AgClJ and Cl- are not the same. Thus, for a reference electrode of the second kind to be elfective in cells without appreciable junction potentials, the equilibrium constant for the reaction of Eq. (5.25) must be smaller than unity (preferably <0.1). In water, methanol, formamide, and V-methyl-formamide, this criterion is met, but in most organic solvents the equilibrium constant for the reaction of Eq. (5.25) ranges from 30 to 100. The silver chloride electrode is not recommended for general use in organic solvents.27... 

When liquid junctions exist, liquid junction potentials (LJPs) can arise due to differing ion mobilities across the interface, leading to charge separation and the development of a potential difference across the liquid junction. These can amount to some tens of millivolts and add a corresponding uncertainty in any voltammetric measurement. It follows that systems that avoid LJPs are generally preferable otherwise some consideration of their likely magnitude is desirable (see below). 

Finally, one should note that the mobilities of K+ and Cl are almost equal. It is for this reason that potassium chloride is frequently used in salt bridges in an attempt to avoid the contribution of liquid junction potentials to the cell potential. 

Liquid junction potentials are the result of different cation and anion mobilities under the influence of an electric field. The potential manifests itself in the interface between two different solutions separated by a porous separator or by a membrane. These junctions can be classified into three distinct types ... 

Minimization of the liquid junction potential is commonly carried out using a salt bridge in which the ions have almost equal mobilities. One example is potassium chloride (t+ = 0.49 and t =0.51) and another is potassium nitrate (t+ = 0.51 and / = 0.49). If a large concentration of electrolyte is used in the salt bridge this dominates the ion transport through the junctions such that the two values of have the same magnitude but opposing polarities. The result is that they annul each other. In this way values of E can be reduced to 1-2 mV. 

Ah already stated the liquid junction potential results from the different mobility of ions. Consequently no diffusion potential can result at the junction of the electrolyte solution the ions of which migrate with the same velocity. It is just this principle on which the salt bridge, filled by solutions of those salts the ions of which have approximately the same mobilities, is based (the equivalent conductivities of ions Kf and Cl- at infinite dilution at 25 °C are 73.5 and 70.3 respectively and the conductivities of ions NH+ and NOg are 73.4 and 71.4 respectively). Because ions of these salts have approximately the same tendency to transfer their charge to the more diluted solution during diffusion, practically no electric double layer is formed and thus no diffusion potential either. The effect of the salt bridge on t he suppression of the diffusion potential will be better, the more concentrated the salt solution is with which it is filled because the ions of the salt are considerably in excess at the solution boundary and carry, therefore, almost exclusively the eleotric current across this boundary. 

Liquid junction potential — Table. Mobility of ions in water (T = 298 K)... 

Due to the different mobilities, concentration gradients and thus potential gradients will be established. In actual measurements these potentials will be added to the electrode potentials. A calculation of liquid junction potential is possible with the -> Henderson equation. As liquid junction potential is an undesired addition in most cases, methods to suppress liquid junction potential like -> salt bridge are employed. (See also -> diffusion potentials, -> electrolyte junction, -> flowing junctions, and -> Maclnnes.)... 

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.

In order to minimize liquid junction potentials, Parker and coworkers [32, 38, 39, 44, 45] advocated the use of 0.1 M tetraethylammonium picrate in acetonitrile for the construction of a salt bridge. This approach, confirmed by Izutsu [37], is based on the finding that both of the ions Et4N and picrate have similar mobilities in many solvents, but not in other solvents such as methanol. Also, in the case of mixed solvents formed by water and a solvent of high acidity this bridge does not work properly [46]. 

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