Liquid junction potentials measurement

In fact, some care is needed with regard to this type of concentration cell, since the assumption implicit in the derivation of A2.4.126 that the potential in the solution is constant between the two electrodes, caimot be entirely correct. At the phase boundary between the two solutions, which is here a semi-pemieable membrane pemiitting the passage of water molecules but not ions between the two solutions, there will be a potential jump. This so-called liquid-junction potential will increase or decrease the measured EMF of the cell depending on its sign. Potential jumps at liquid-liquid junctions are in general rather small compared to nomial cell voltages, and can be minimized fiirther by suitable experimental modifications to the cell. 

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)] ... 

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... 

Electrochemical methods covered in this chapter include poten-tiometry, coulometry, and voltammetry. Potentiometric methods are based on the measurement of an electrochemical cell s potential when only a negligible current is allowed to flow, fn principle the Nernst equation can be used to calculate the concentration of species in the electrochemical cell by measuring its potential and solving the Nernst equation the presence of liquid junction potentials, however, necessitates the use of an external standardization or the use of standard additions. 

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 ... 

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. 

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. 

A typical set of experimental data290a,290b is shown in Fig. 11. All measurements converge to the value measured by Grahame.286 At present, the of Hg in water can be confidently indicated5 as -0.433 0.001 V (SCE), i.e., -0.192 0.001 V (SHE). The residual uncertainty is related to the unknown liquid junction potential at the boundary with the SCE, which is customarily used as a reference electrode. The temperature coefficient of of the Hg/H20 interface has been measured and its significance discussed.7,106,1 8,291... 

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-... 

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 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 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]. 

Keeping in view the above serious anomalies commonly encountered with direct potentiometry, such as an element of uncertainty triggered by liquid junction potential (E.) and high degree of sensitivity required to measure electrode potential (E), it promptly gave birth to the phenomenon of potentiometric titrations,... 

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). 

So the activity of the copper ion is 9.5 times greater than in the cell where no liquid junction potential was measured. 

An electrode potential Eaq+.aq is measured as 0.670 V. In fact, this value is too high because the value of Eaq+.aq also incorporates a liquid junction potential of 22 mV. Calculate two values of a(Ag ), first by assuming that Eaq+.aq is accurate, and secondly by taking account of E. What is the error in a(Ag ) caused by the liquid junction potential Take E + = 0.799 V at 298 K. 

When considering potentiometric errors, it is necessary to appreciate how a liquid junction potential, Ej, arises, and appreciate how such potentials can lead to significant errors in a calculation. In addition, we saw how the IR drop can affect a potentiometric measurement (and described how to overcome this). Finally, we discussed the ways that potentiometric measurements are prone to errors caused by both current passage through the cell, and by the nature of the mathematical functions with which the Nemst equation is formulated. 

While the Planck liquid-junction model corresponds to a junction with restrained flow , for example in a porous diaphragm, fig. 2.2, the Hendersoii model approaches a liquid junction with free diffusion (fig. 2.3). Ives and Janz [13] give inaccuracies in measuring liquid-junction potentials between 1 and 2 mV. 

Ideal potentiometric measurements, especially in analytical chemistry, would require that the potential of the reference electrode be fixed and known, and that the composition of the studied solution affect only the potential of the indicator electrode. This would occur only if the liquid-junction potential could be completely neglected. In practice this situation can be attained only if the whole system contains an indifferent electrolyte in a much larger concentration than that of the other electrolytes, so that the concentration of a particular component in the analysed solution, which is not present in the reference electrode solution, has only a negligible effect on the liquid-junction potential Such a situation rarely occurs, so that it is necessary to know or at least fix the liquid junction potential... 

An acceptable method quite frequently used in practice depends on the cell whose EMF is being measured having a liquid junction with a constant potential value. Such a situation is attained in the determination of the activity of fluoride ions, by adding a constant amount of quite concentrated buffer, for example TISAB, to the studied solution this buffer also fulfills other functions in the analysis (see p. 146). Then the liquid junction potential is a function of the composition of the reference electrode electrolyte and of the buffer composition alone, and not of the concentrations of the other components of the studied solution. 

In the presence of colloidal solutions in contact with a liquid junction, anomalous liquid-junction potentials are often measured. This suspension or Palmarm effect [14] has not yet been satisfactorily explained. It is probably a Donnan-type potential with the electrically-charged colloidal species acting as indiffusible ions (cf. section 5.1.3). 

The temperature coefficient of the ISE potential has received relatively little attention. As follows from (3.1.7), the constant term (the ISE standard potential), the determinand and interferent activity coefficients and the selectivity coefficient, liquid-junction potentials and, of course, also the RTIZfF coefficient, depend on the temperature [118]. When the internal reference electrode and the reference electrode in the test solution are identical, the interferent activity sufficiently low and the liquid-j unction potentials negligible, then the constant term depends on the determinand activity in the electrode internal solution alone and thus the temperature coefficient of the measured EMV depends only on the temperature coefficient of the determinand activity coefficient and on the/ 77z,F coefficient. Measuring instruments are usually... 

The ionic strength of the solution influences the activity coefficients of the species present and the values of the liquid-junction potentials in the system and thus must be held constant in all related measurements, within a range of about 0.1 to 2m. 

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