Residual liquid-junction potential

What is the liquid-junction potential Residual liquid-junction potential How can these be minimized ... 

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... 

Undoubtedly, the mercury/aqueous solution interface, was in the past, the most intensively studied interface, which was reflected in a large number of original and review papers devoted to its description, for example. Ref. 1, and in the more recent work by Trasatti and Lust [2] on the potentials of zero charge. It is noteworthy that in view of numerous measurements of the double-layer capacitance at mercury brought in contact with NaF and Na2S04 solutions, the classical theory of Grahame [3] stiU holds [2]. According to Trasatti [4], the most reliable PZC value for Hg/H20 interface in the absence of specific adsorption equals to —0.433 0.001 V versus saturated calomel electrode, (SCE) residual uncertainty arises mainly from the unknown liquid junction potential at the electrolyte solution/SCE reference electrode boundary. 

If the response of an ISS in the standards and samples is fast, the reference electrode potential and the liquid-junction potential are constant ( —Ehef+Ed — constant) and do not change during the calibration and the sample measurement (i.e., residual liquid-junction potential is constant), a simple equation then applies over the course of an analysis with the ISS ... 

The list of error sources continues, just to mention a few the ionic strength of the sample, the liquid-junction and residual liquid-junction potentials, temperature effects, instabilities in the galvanic cell, carryover effects, improper use of available corrections (e.g., for pH-adjusted ionized calcium or magnesium). An error analysis goes beyond the limited scope of this paper more details are presented elsewhere [10]. 

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],... 

With the ion-selective electrode standardized at an ionic strength of 0.01, and for cells involving silver-silver chloride and saturated calomel half-cells, the residual effect of liquid-junction potential is estimated to amount to about —0.02 and —0.07 pNa units at ionic strengths of 0.1 and 1.0 for chloride, -1-0.02 and -t-0.07 pCl units at 0.1 and 1.0 for calcium, —0.03 and — 0.22pCa units at 0.1 and 1.0. 

The pH scale has been defined operationally, and standard reference solutions based on a conventional scale of hydrogen ion activity have been selected (i, 2). Measurements of the pH of seawater made with different electrodes and instruments are satisfactorily reproducible when standardized in the same way (3). The results obtained, however, do not always have a clear interpretation. Formally, this diflSculty can be attributed to the residual liquid junction potential involved in the measurement. The primary standards are necessarily dilute buffer solutions (ionic strength, I 0.1) whereas seawater normally has an ionic strength exceeding 0.6. This difference in the concentrations and mobilities of the ions coming in contact with the concentrated solution of potassium chloride of which the salt bridge-liquid junction is composed gives rise to a potential difference that is indeterminate. Consequently, the meas-m ed pH is in error by an unknown amount and does not fall exactly on the scale fixed by the primary standards. 

The operational definition was formulated by omitting the last term of Equation 2, that is, j, the residual liquid junction potential expressed in pH units ... 

The analogy with the operational definition of pH (Equation 3) is evident. It remains to explore the effectiveness of seawater in rendering yn constant and in nullifying the residual liquid junction potential, Ej. 

The presence of erythrocytes in the sample may also affect the magnitude of the residual liquid junction potential in a less predictable manner. For example, erythrocytes in blood of normal hematocrit are estimated to produce approximately 1.8mmol/L positive error in the measurement of sodium by ISEs when an open, unrestricted liquid-liquid junction is used. This bias may be minimized if a restrictive membrane or frit is used to modify the liquid-liquid junction. 

Czaban JD, Cormier AD, Legg KD. Establishing the direct potentiometric normal range for Na/K residual liquid junction potential and activity coefficient effects. Chn Chem 1982 28 1936-45. 

If the liquid-junction potentials of the calibrating and test solutions are identical, no en-or results (the residual Ej = 0). Our goal is to keep residual Ej as small as possible. 

We have assumed above in Equations 13.32 and 13.34 that k is the same in meas urements of both standards and samples. This is so only if the liquid-junction potential at the reference electrode is the same in both solutions. But the test solution will usually have a somewhat different composition from the standard solution, and the magnitude of the liquid-junction potential will vary from solution to solution. The difference in the two liquid-junction potentials is called the residual liquid-junction potential, and it will remain unknown. The difference can be kept to a minimum by keeping the pH of the test solution and the pH of the standard solution as close as possible, and by keeping the ionic strength of both solutions as close as possible. The former is particularly important. 

A l-mV error results in an error in Up,g+ of 4%. This is quite significant in direct potentiometric measurements. The same percent error in activity will result for all activities of silver ion with a 1-mV error in the measurement. The error is doubled when n is doubled to 2. So, a 1-mV error for a copper/copper(II) electrode would result in an 8% error in the activity of copper(II). It is obvious, then, that the residual liquid junction potential can have an appreciable effect on the accuracy. 

It should be poiated out that if a glass electrode-SCE cell is calibrated with one standard buffer and is used to measure the pH of another, the new reading will not correspond exactly to the standard value of the second because of the residual liquid-junction potential. 

A second limitation in the accuracy is the residual liquid-junction potential. The cell is standardized in one solution, and then the unknown pH is measured in a solution of a different composition. We have mentioned that this residual liquid-junction potential is minimized by keeping the pH and compositions of the solutions as near as possible. Because of this, the cell should be standardized at a pH close to that of the unknown. The error in standardizing at a pH far removed from that of the test solution is generally within 0.01 to 0.02 pH unit but can be as large as 0.05 pH unit for very alkaline solutions. 

The residual liquid-junction potential, combined with the uncertainty in the standard buffers, limits the absolute accuracy of measurement of pH of an unknown solution to about 0.02 pH unit. It may be possible, however, to discriminate between the pH of two similar solutions with differences as small as 0.004 or even 0.002 pH units, although their accuracy is no better than 0.02 pH units. Such discrimination is possible because the hquid-junction potentials of the two solutions will be virtually identical in terms of true a. For example, if the pH values of two blood solutions are close, we can measure the difference between them accurately to 0.004 pH. If the pH difference is fairly large, however, then the residual hquid-junction potential will increase and the difference cannot be measured as accurately. For discrimination of 0.02 pH unit, large changes in the ionic strength may not be serious, but they are important for smaller changes than this. 

The residual liquid-junction potential limits the accuracy of pH measurement. Always calibrate at a pH close to that of the test solution. 

Measurements made by calibration of electrodes with lUPAC aqueous RVS or PS standards to obtain pH(X) are perfectly valid. However, the interpretation of pH(X) in terms of the activity of hydrogen ion is complicated by the non zero residual liquid junction potential as well as by systematic differences between electrode pairs, principally attributable to the reference electrode. For 35%o salinity seawater (S = 0.035) calculated from pH(X) is typically 12% too low. Special seawater pH scales have been devised to overcome this problem ... 

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