Activity 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... 

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. 

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 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. 

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-... 

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. 

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,... 

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. 

The difference between free and total concentrations of ions can be significant in biological samples and is particularly important for calcium and magnesium [23]. ISEs are known to respond to free ion activity (Eq. 3), but it has recently been shown that direct potentiometric measurements can lead to total ionic concentrations of calcium if the composition of the inner solution induces a strong zero-current flux of the primary ion toward the inner compartment [24]. 

The determination of an ion or molecule by direct potentiometric measurement is rapid and simple, requiring only a comparison of the potential developed by the indicator electrode in the test solution with its potential when immersed in one or more standard solutions of the analyte. Because most indicator electrodes are selective, preliminary separation steps are seldom required. In addition, direct potentiometric measurements are rapid and readily adapted to the continuous and automatic monitoring of ion activities. 

In this chapter we examine the general problem of determining the amount of a specific ion in solution. Direct potentiometric measurement of ion activity is based upon the cell... 

Standard potentials are determined with full consideration of activity effects, and are really limiting values. They are rarely, if ever, observed directly in a potentiometric measurement. In practice, measured potentials determined under defined concentration conditions (formal potentials) are very useful for predicting the possibilities of redox processes. Further details are given in Section 10.90. 

Two types of methods are used to measure activity coefficients. Potentiometric methods that measure the mean activity coefficient of the dissolved electrolyte directly will be described in Section 3.3.3. However, in galvanic cells with liquid junctions the electrodes respond to individual ion activities (Section 3.2). This is particularly true for pH measurement (Sections 3.3.2 and 6.3). In these cases, extrathermodynamical procedures defining individual ion activities must be employed. 

Potentiometric measurements with ISEs can be approached by direct potentiometry, standard addition and titrations. The determination of an ionic species by direct potentiometry is rapid and simple since it only requires pretreatment and electrode calibration. Here, the ion-selective and reference electrodes are placed in the sample solution and the change in the cell potential is plotted against the activity of the target ion. This method requires that the matrix of the calibration solutions and sample solutions be well matched so that the only changing parameter allowed is the activity of the target ion. 

The equivalent circuit corresponding to this interface is shown in Fig. 6.1b. The charge-transfer resistances for the exchange of sodium and chloride ions are very low, but the charge-transfer resistance for the polyanion is infinitely high. There is no direct sensing application for this type of interface. However, it is relevant for the entire electrochemical cell and to many practical potentiometric measurements. Thus if we want to measure the activity of an ion with the ion-selective electrode it must be placed in the same compartment as the reference electrode. Otherwise, the Donnan potential across the membrane will appear in the cell voltage and will distort the overall result. 

Table 2.1 summarizes a number of redox couples that are well behaved in aqueous solutions and provide a means for monitoring the indicated species by potentiometric measurements. This can be either in the form of monitoring a titration or as a direct absolute measurement of activity. Although the tabulations of standard potentials12 17 imply that the listing should be much more comprehensive, most of the couples tabulated are not well behaved in an electrochemical sense and do not provide a Nemstian response under normal laboratory conditions. The vast majority of the data tabulated is based on other than electrochemical measurements. 

Potentiometry has found extensive application over the past half-century as a means to evaluate various thermodynamic parameters. Although this is not the major application of the technique today, it still provides one of the most convenient and reliable approaches to the evaluation of thermodynamic quantities. In particular, the activity coefficients of electroactive species can be evaluated directly through the use of the Nemst equation (for species that give a reversible electrochemical response). Thus, if an electrochemical system is used without a junction potential and with a reference electrode that has a well-established potential, then potentiometric measurement of the constituent species at a known concentration provides a direct measure of its activity. This provides a direct means for evaluation of the activity coefficient (assuming that the standard potential is known accurately for the constituent half-reaction). If the standard half-reaction potential is not available, it must be evaluated under conditions where the activity coefficient can be determined by the Debye-Hiickel equation. 

Several of the procedures described in the previous sections can be advantageously carried out with double barrel tips. Such a probe consists of two capillaries (see Sec. V.B), one of which acts as the potentiometric sensor, while the other is used to determine the tip-substrate distance. For example (79), a gallium microdisk was combined with an ion-selective (K+) potentiometric probe to image K+ activity near the aperture of a capillary (see Fig. 7). Similarly (77), a double barrel tip with one channel as an open Ag/ AgCl micropipette for solution resistance measurement and the other channel as an ion-selective neutral carrier-based microelectrode for potentiometric measurements was successfully used to image concentration distributions for NH4 (Fig. 8) and Zn2+ (Fig. 9). While dual-channel tips facilitate the approach of the substrate and permit a direct determination of the absolute tip-substrate distance, their difficult fabrication severely limits their use. Reference 80 compares the above methods. 

Binding constants in solution are usually determined from potentiometric titrations or plots of cyclic voltammetry peak potentials vs. CyD concentration after assessing the guest/host ratio in the complex [3, 4]. Potentiometric measurements are more frequently used, and ion-selective electrodes are employed, for the direct measurement of the guest activity in the solution. Measurements of pH allow the evaluation of concentrations of several reaction components. 

In potentiometry, the potential of a suitable indicator electrode is measured versus a reference electrode, i.e. an electrode with a constant potential. Whereas the indicator electrode is in direct contact with the analyte solution, the reference electrode is usually separated from the analyte solution by a salt bridge of various forms. The electrode potential of the indicator electrode is normally directly proportional to the logarithm of the activity of the analyte in the solution. Potentiometric methods have been and are still frequently used to indicate the end point of titrations. This use has been known since the end of the nineteenth century. Direct potentiometric determinations using ion-selective electrodes have been mainly developed in the second half of the twentieth century. 

During the last years in the clinical laboratory potentiometric methods using ion-sensitive electrodes (ISEs) have been introduced. It must be considered that with these methods ion activities are measured and not ion concentrations [18-21]. The methodic differences resulting from this fact have to be considered in the interpretation of the results or have to be eliminated in the process of evaluation of the method [22]. Ion-sensitive electrodes are practicable in single apparatuses as well as in automatic analyzers. Even in analyzers using the technique of dry reagent chemistry, slides for direct potentiometric determination of sodium and potassium are used [23]. 

Potentiometric titrations provide a high degree of accuracy and total concentration measurement. When making a direct pH measurement, only the free active hydrogen ion is sensed and the value obtained is relative to another solution, the pH buffer. With titrations, the calculated answer provides total hydrogen ion concentration provided the reaction between the titrant and the hydrogen ion has a sufficiently fast rate, is in one direction without... 

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