Liquid junction potential, negligible

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. 

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... 

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... 

Another proposed procedure of finding the ionic data is the application of a special salt bridge, which provides practically constant or negligible liquid junction potentials. The water-nitrobenzene system, containing tetraethylammonium picrate (TEAPi) in the partition equilibrium state, has been proposed as a convenient liquid junction bridge for the liquid voltaic and galvanic cells. 

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. 

Here, C(s) is a solid reduced form of A, and the double line denotes a liquid junction of negligible potential. Also, A is a thermally equilibrated excited (or thexi) state and, as such, is essentially a different chemical species from A. We suppose, therefore, that it is possible to find an electrode M that is reversible to the reduction of A to C(s), but completely polarized with respect to the reduction of A to C(s). Operation of this cell under steady state conditions should then give the desired reversible work available from the photoproduction of A. ... 

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... 

The calibration of ISEs using the tabulated activity values is especially simple when the dependence of the ISE potential on the determinand activity is Nemstian (3.1.5). If the liquid-junction potential is negligible or constant, the determinand activity flj+(X) can be found, using the standard activity [Pg.80]

When the condition of negligible or constant liquid-junction potential is not satisfied, a correction must be made for the change in this potential,... 

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 precision and accuracy of the measurement also depend strongly on the reference electrode, which affects the results through fluctuations in its own potential and through the liquid-junction potential at the test solution-liquid bridge interface. This subject is extensively treated in [158]. Common electrodes of the second kind have sufficiently stable potentials at a constant temperature, but difficulties can be encountered due to temperature hysteresis. Silver chloride electrodes are preferable to calomel electrodes, because their temperature hysteresis is substantially smaller with a calomel electrode, potential stabilization after a change in the temperature may even take several hours. Negligible temperature hysteresis is exhibited by the thallamide reference electrode [26,... 

The results obtained with ISEs have been compared several times with those of other methods. When the determination of calcium using the Orion SS-20 analyser was tested, it was found that the results in heparinized whole blood and serum were sufficiently precise and subject to negligible interference from K and Mg ([82]), but that it is necessary to correct for the sodium error, as the ionic strength is adjusted with a sodium salt [82], and that a systematic error appears in the presence of colloids and cells due to complexa-tion and variations in the liquid-junction potential [76]. Determination of sodium and potassium with ISEs is comparable with flame photometric estimation [39, 113, 116] or is even more precise [165], but the values obtained with ISEs in serum are somewhat higher than those from flame photometry and most others methods [3, 25, 27, 113, 116]. This phenomenon is called pseudohyponatremia. It is caused by the fact that the samples are not diluted in ISE measurement, whereas in other methods dilution occurs before and during the measurement. On dilution, part of the water in serum is replaced by lipids and partially soluble serum proteins in samples with abnormally increased level of lipids and/or proteins. 

The electrode on the left of the junction (jj) is the standard hydrogen electrode (SHE), the potential of which is defined as zero at all temperatures. p° is the standard pressure and is equal to 105 Pa (lbar) by the IUPAC recommendation (1982). The liquid junction potential at jj is kept negligible by an appropriate salt bridge. The potential of the electrode on the right of the junction is expressed by the Nernst equation ... 

LIQUID JUNCTION. To avoid the unknow n liquid junction potential in measuring the potential of a half-cell against a reference electrode, the two hall-cells are frequently connected via a sail bridge, usually a concentrated solution of potassium chloride. Since its anion and cation have almost the same velocity, a negligible ddlusittn potential is set up across the liquid junctions at the ends of llte bridge. 

In addition, a solution of pure hydrochloric acid might be present between the two saturated solutions. In this particular case, the considerations outlined in previous sections would lead us to believe that the liquid-junction potential in such a cell is negligible. If, however, hydrogen and silver chloride were more soluble, the resulting liquid-junction potential might not be negligible. 

Abbreviations extr. = extrapolation (methods) n.Lj.p. = negligible liquid junction potential... 

Izutsu et al. studied the compiexing of Na" in acetonitrile solution with various protic and aprotic solvents using an ion-sensitive glass electrode. Parker s assumption of negligible liquid junction potential with an tetraethylammonium picrate salt bridge was adopted and found to be valid, even when water was added. The formation constants increased in the order methanol < H2O < DMF < NJ -dimethylace-tamide DMSO < HMPA. 

The authors claim that the 5 values could be directly used with other electrode systems or by other laboratories, given that the residual liquid junction potential of the respective system is negligible [74-76], This can be a convenient way to convert from the pH scale to spH scale as Espinosa et al. have described [73],... 

Reviewing all the results, we may say that there was reasonable agreement between theory and experiment except when the electrolyte concentration in the oil phase was very low. It was concluded that the variations in liquid junction potential had been made negligible, and when the same oil phase was used throughout the cell, the liquid-junction potentials themselves were probably very small. 

Non-aqueous Solvation.—Structural radii and electron-cloud radii, together with lattice enthalpies and enthalpies of solvation of ionic crystals, have been reviewed.84 The free energies of transfer, AGtr(K+), of potassium ions from water to 14 non-aqueous solvents have been reported, and they were derived from measurements in an electrochemical cell assumed to have a negligible liquid-junction potential. The essentially electrostatic nature of its solvation allows K+ to be used as a model for non-specific solvent-ion interactions. A... 

The potential of a cell such as that shown in Figure 18-4a is the difference between two half-cell or single-electrode potentials, one associated with the half-reaction at the right-hand electrode (f,the other associated with the half-reaction at the left-hand electrode ( eft)- According to the lUPAC sign convention, as long as the liquid-junction potential is negligible or there is no liquid junction, we may write the cell potential as... 

In this cell all liquid liquid junction potentials are negligible, so that the measured potential HgAng t i reflects the difference in potentials of the two reference electrodes which are common to each cell. As a result... 

We could measure the cell e.m.f. (E) by matching it with an exactly equal, but opposite e.m.f. from a potentiometer. The cell e.m.f. will be a measure of the free energy for the cell reaction if the electrodes are truly reversible and if potentials at the liquid junction are negligible. Reversibility implies that an external e.m.f. infinitesimally greater than E causes the cell reaction to be reversed (cf. external pressure for gaseous work). This is not true of all cells. 

As the concentration of the (dissimilar) electrolyte on the other side of the boundary (in the test solution) increases, or as the ions are made differenf the liquid-junction potential will get larger. Very rarely cm the liquid-junction potential be considered to be negligible>The liquid-junction potential with neutral salts... 

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