Direct potentiometric measurement

The equipment required for direct potentiometric measurements includes an ion-selective electrode (ISE), a reference electrode, and a potential-measuring device (a pH/millivolt meter that can read 0.2mV or better) (Figure 5-1). Conventional voltmeters cannot be used because only very small currents are allowed to be drawn. The ion-selective electrode is an indicator electrode capable of selectively measuring the activity of a particular ionic species. Such electrodes exhibit a fast response and a wide linear range, are not affected by color or turbidity, are not... 

One of the most fruitful uses of potentiometry in analytical chemistry is its application to titrimetry. Prior to this application, most titrations were carried out using colour-change indicators to signal the titration endpoint. A potentiometric titration (or indirect potentiometry) involves measurement of the potential of a suitable indicator electrode as a function of titrant volume. The information provided by a potentiometric titration is not the same as that obtained from a direct potentiometric measurement. As pointed out by Dick [473], there are advantages to potentiometric titration over direct potentiometry, despite the fact that the two techniques very often use the same type of electrodes. Potentiometric titrations provide data that are more reliable than data from titrations that use chemical indicators, but potentiometric titrations are more time-consuming. 

Applications Potentiometry finds widespread use for direct and selective measurement of analyte concentrations, mainly in routine analyses, and for endpoint determinations of titrations. Direct potentiometric measurements provide a rapid and convenient method for determining the activity of a variety of cations and anions. The most frequently determined ion in water is the hydrogen ion (pH measurement). Ion chromatography combined with potentiometric detection techniques using ISEs allows the selective quantification of selected analytes, even in complex matrices. The sensitivity of the electrodes allows sub-ppm concentrations to be measured. 

A direct potentiometric measurement of N02 in air appeared possible by means of a properly activated Fe-chalcogenide glass electrode158 with the approximate composition SegoGe Sbu doped with 1.7 -2% of Fe it yields a positive potential shift when equilibrated with N02 in the presence of air, being insensitive to NO, S02, CO and CH4. 

Electrode Systems. Direct Potentiometric Measurements. Potentiometric Titrations. Null -point Potentiometry. Applications of Potentiometry. 

Whenever dissimilar electrolyte solutions are in contact, a voltage difference called the junction potential develops at their interface. This small voltage (usually a few millivolts) is found at each end of a salt bridge connecting two half-cells. The junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements, because we usually do not know the contribution of the junction to the measured voltage. 

In biological systems, most of the elements are present as complexes, rather than as free ions, hence direct potentiometric measurements provide little information except for major electrolytes such as Fl+, Na+, K+, Cl- and possibly Ca2+, Mg2+. The introduction of ion-selective microelectrodes (e.g. tips < 1 jtm diameter) has allowed these major electrolyte ions to be determined in single cells. 

Direct potentiometric measurement an Agl membrane electrode with a double junction reference electrode system must be used to quantify CN-. 

Direct potentiometric measurement by Oxidation of sulfide by dissolved 50 using an Ag/AgS/ISE 02 must be prevented and ionic... 

Table 5.10 summarizes the presently available electrodes categorized as glass, ion-exchange membrane, crystal membrane, and liquid membrane. These electrodes can be used either for direct potentiometric measurements of ionic activity after calibration of the Nemst expression for the particular electrode or to monitor a potentiometric titration when a selected reaction that involves the monitored ion is available. Table 5.10 also indicates the common interfering ions. Several instrument companies are endeavoring to develop potentiometric-membrane electrodes to monitor directly ions in body fluids. 

The direct potentiometric measurement of a junction potential is not possible because of the impossibility of directly measuring a single-electrode potential. 

The number of reversible metal-metal ion electrodes is limited so that the accurate direct potentiometric measurement of the activity of a metal ion with an electrode of the same metal usually is not feasible, except perhaps with the Ag/Ag,(OH2)4 system. However, a number of metal ion-metal half-reactions are sufficiently reversible to give a satisfactory potentiometric titration with a precipitation ion or complexing agent. These couples include Cuu(OH2>6+/Cu, Pbn(OH2>4+/Pb, Cdu(OH2)l+/Cd, and Znn(OH2)i+/Zn. However, all these metals can be determined by EDTA titration and the mercury electrode that is described in the preceding section. 

W26. Wise, W. M., Kurey, M. J., and Baum, G., Direct potentiometric measurement of K in blood serum with liquid ion exchange electrode. Clin, Chem. 16, 103-106 (1970). 

Direct potentiometric measurements are used to complete chemical analyses of species for which an indicator electrode is available. The technique is simple, requiring only a comparison of the voltage developed by the measuring cell in the test solution with its voltage when immersed in a standard solution of the analyte. If the electrode response is specific for the analyte and independent of the matrix, no preliminary steps are required. In addition, although discontinuous measurements are mainly carried out, direct potentiometry is readily adapted to continuous and automatic monitoring. 

The result obtained by a titration is usually more precise than that obtained in a direct potentiometric measurement. It is usually not too difficult to create an end point with a precision of better than 5 %. In the direct potentiometric determination the relative error Frei is given by... 

El 32 Aguanno, J. and Bunata, C. (1984). Direct potentiometric measurement of sodium and potassium in whole blood using the Kodak Ektachem 4(H). Clin. Chem. 30,961-962, Abstr. III. 

The potential of liquid-membrane electrodes develops across the interface between the solution containing the analyte and a liquid-ion exchanger that selectively bonds with the analyte ion. These electrodes have been developed for the direct potentiometric measurement of numerous polyvalent cations as well as certain anions. 

Direct potentiometric measurements provide a rapid and convenient method to determine the activity of a variety of cations and anions. The technique requires only a comparison of the potential developed in a eell containing the indicator elec-... 

The sign convention for potentiometry is consistent with the convention described in Chapter 18 for standard electrode potential. In this convention, the indicator electrode is always treated as the right-hand electrode and the reference electrode as the left-hand electrode. For direct potentiometric measurements, the potential of a cell can then be expressed in terms of the potentials developed by the indicator electrode, the reference electrode, and a junction potential, as described in Section 21 A ... 

Give several advantages of a potentiometric titration over a direct potentiometric measurement. 21-14. What is the operational definition of pH Why is it used ... 

Potentiometric electrodes measure activity rather than concentration, a unique feature, and we will use activities in this chapter in describing electrode potentials. An understanding of activity and the factors that affect it are important for direct potentiometric measurements, as in pH or ion-selective electrode measurements. You should, therefore, review the material on activity and activity coefficients in Chapter 6. 

Expanded-scale pH meters are available that will measure the potential to a few tenths of a millivolt, about 10 times more closely than conventional pH meters. They are well suited for direct potentiometric measurements with ion-selective electrodes. 

In potentiometric measurements, a cell of the type shown in Figure 13.5 is set up. For direct potentiometric measurements in which the activity of one ion is to be calculated from the potential of the indicating electrode, the potential of the reference electrode will have to be known or determined. The voltage of the cell is described by Equation 13.7, and when a salt bridge is employed, the liquid-junction potential must be included. Then,... 

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