Potential differences, liquid junction potentials

As a result of a variable liquid-junction potential, the measured pH may be expected to differ seriously from the determined from cells without a liquid junction in solutions of high acidity or high alkalinity. Merely to affirm the proper functioning of the glass electrode at the extreme ends of the pH scale, two secondary standards are included in Table 8.14. In addition, values for a 0.1 m solution of HCl are given to extend the pH scale up to 275°C [see R. S. Greeley, Anal. Chem. 32 1717 (I960)] ... 

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... 

An electrode potential varies with the concentration of the ions in the solution. Hence two electrodes of the same metal, but immersed in solutions containing different concentrations of its ions, may form a cell. Such a cell is termed a concentration cell. The e.m.f. of the cell will be the algebraic difference of the two potentials, if a salt bridge be inserted to eliminate the liquid-liquid junction potential. It may be calculated as follows. At 25 °C ... 

Standard potentials Ee are evaluated with full regard to activity effects and with all ions present in simple form they are really limiting or ideal values and are rarely observed in a potentiometric measurement. In practice, the solutions may be quite concentrated and frequently contain other electrolytes under these conditions the activities of the pertinent species are much smaller than the concentrations, and consequently the use of the latter may lead to unreliable conclusions. Also, the actual active species present (see example below) may differ from those to which the ideal standard potentials apply. For these reasons formal potentials have been proposed to supplement standard potentials. The formal potential is the potential observed experimentally in a solution containing one mole each of the oxidised and reduced substances together with other specified substances at specified concentrations. It is found that formal potentials vary appreciably, for example, with the nature and concentration of the acid that is present. The formal potential incorporates in one value the effects resulting from variation of activity coefficients with ionic strength, acid-base dissociation, complexation, liquid-junction potentials, etc., and thus has a real practical value. Formal potentials do not have the theoretical significance of standard potentials, but they are observed values in actual potentiometric measurements. In dilute solutions they usually obey the Nernst equation fairly closely in the form ... 

Some commercial electrodes are supplied with a double junction. In such arrangements, the electrode depicted in Fig. 15.1(h) is mounted in a wider vessel of similar shape which also carries a porous disc at the lower end. This outer vessel may be filled with the same solution (e.g. saturated potassium chloride solution) as is contained in the electrode vessel in this case the main function of the double junction is to prevent the ingress of ions from the test solution which may interfere with the electrode. Alternatively, the outer vessel may contain a different solution from that involved in the electrode (e.g. 3M potassium nitrate or 3M ammonium nitrate solution), thus preventing chloride ions from the electrode entering the test solution. This last arrangement has the disadvantage that a second liquid junction potential is introduced into the system, and on the whole it is preferable wherever possible to choose a reference electrode which will not introduce interferences. 

Difference with respect to value calculated from accepted standard potential and activity coefficient is attributed to liquid junction potential. 

The influence of interfaeial potentials (diffusion or liquid junction potentials) established at the boundary between two different electrolyte solutions (based on e.g. aqueous and nonaqueous solvents) has been investigated frequently, for a thorough overview see Jakuszewski and Woszezak [68Jak2]. Concerning the usage of absolute and international Volt see preceding chapter. 

In the case of the cell with transference (1.20), the OCV includes an additional liquid-junction potential (a potential difference between electrolytes), and... 

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... 

The foregoing text highlights the fact that at the interface between electrolytic solutions of different concentrations (or between two different electrolytes at the same concentration) there originates a liquid junction potential (also known as diffusion potential). The reason for this potential lies in the fact that the rates of diffusion of ions are a function of their type and of their concentration. For example, in the case of a junction between two concentrations of a binary electrolyte (e.g., NaOH, HC1), the two different types of ion diffuse at different rates from the stronger to the weaker solution. Hence, there arises an excess of ions of one type, and a deficit of ions of the other type on opposite sides of the liquid junction. The resultant uneven distribution of electric charges constitutes a potential difference between the two solutions, and this acts in such a way as to retard the faster ion and to accelerate the slower. In this way an equilibrium is soon reached, and a steady potential difference is set up across the boundary between the solutions. Once the steady potential difference is attained, no further net charge transfer occurs across the liquid junction and the different types of ion diffuse at the same rate. 

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,... 

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... 

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. 

A liquid junction potential E-f forms when the two half-cells of a cell contain different electrolyte solutions. The magnitude of Ej depends on the concentrations (strictly, the activities) of the constituent ions in the cell, the charges of each moving ion, and on the relative rates of ionic movement across the membrane. We record a constant value of j because equilibrium forms within a few milliseconds of the two half-cells adjoining across the membrane. 

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]. 

Interesting results have been obtained from polarographic studies in various donor solvents. Measurements have been made of various metal perchlorates in solutions of donor solvents containing tetraalkylammonium perchlorate as supporting electrolyte against an aqueous saturated calomel electrode 113. In order to eliminate differences in liquid-liquid junction potentials bisbiphenylchromium (I) has been used as a reference ion 114 118). 

Yes The difference in potential between the two electrodes in a cell without transport is given by equation (3.3). In a cell with transport, there is generally an additional potential known as a liquid junction potential or just a junction potential. We shall give the liquid junction potential... 

To summarize, in this two-electron cell, a liquid junction potential of about 30 mV causes the activity of the ion to appear different by a factor of about 10. We... 

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... 

For accurate measurement, the liquid-junction potentials in the sample and in the standard must be identical or, at least, a correction must be made for the liquid-junction potential difference. 

The differential technique described under (a) has an advantage in removal of the liquid-junction potential and of mechanical faults often encountered in work with reference electrodes of the second kind. The procedure described under (b) suppresses the potential fluctuations, but difficulties can arise from the very high resistance of a cell containing two ISEs. A differential amplifier was designed for this prupose [15]. The two ISEs used can also exhibit different slopes electrode membranes were therefore prepared by cutting a single crystal into two halves, where each half contains a chaimel for passage of the solution and functions as an ISE [163]. 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

The sds (]) will include (Fig. 7.174) the liquid-junction potential that always arises whenever two solutions of different composition are in contact. But it will be assumed throughout this treatment that the liquid-junction potential has been minimized by the... 

Liquid junction Potentials between Different Solvents... 

Liquid Junction Potentials between Different Solvents I 197... 

Polarographic reductions have been studied for many kinds of metal ions and in a variety of lion-aqueous solvents. Large amounts of data on half-wave potentials are available and have been compiled in some books and reviews [18]. However, many of the old data were obtained using different reference electrodes or aqueous SCE, for which the problem of the liquid junction potential exists (Section 6.1.2). Gritzner [19] compiled half-wave potentials for metal ions as values referred to the BCr+/BCr system, which was recommended by IUPAC. Some of these are listed in Table 8.2. The potential of the BCr+/BCr system is not seriously affected either by the presence of water and other impurities or by differences in experimental conditions. Thus, although the determination of the half-wave poten-... 

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