Calibration of ion-selective electrodes

It has been emphasized repeatedly that the individual activity coefficients cannot be measured experimentally. However, these values are required for a number of purposes, e.g. for calibration of ion-selective electrodes. Thus, a conventional scale of ionic activities must be defined on the basis of suitably selected standards. In addition, this definition must be consistent with the definition of the conventional activity scale for the oxonium ion, i.e. the definition of the practical pH scale. Similarly, the individual scales for the various ions must be mutually consistent, i.e. they must satisfy the relationship between the experimentally measurable mean activity of the electrolyte and the defined activities of the cation and anion in view of Eq. (1.1.11). Thus, by using galvanic cells without transport, e.g. a sodium-ion-selective glass electrode and a Cl -selective electrode in a NaCl solution, a series of (NaCl) is obtained from which the individual ion activity aNa+ is determined on the basis of the Bates-Guggenheim convention for acr (page 37). Table 6.1 lists three such standard solutions, where pNa = -logflNa+, etc. 

Calibration of ion-selective electrodes for use in quantitative analysis is usually done by preparing a series of standards as in most other instrumental analysis methods (see Chapter 7), since the measured potential is proportional to the logarithm of the concentration. The relationship is... 

R.P. Buck, V.V. Cosofret, Recommended Procedures for Calibration of Ion-Selective Electrodes. PureAppL Chem. 65 (1993) 1849. 

This Is representative of a falling exponential curve. Pungor et al. [89] found analogous expressions In developing a procedure for the calibration of Ion-selective electrodes. 

On the calibration of ion-selective electrodes, an early and simple recommendation [30] was based on their general (but not always, because of com-plexation and other phenomena) independence of the nature of the co-ion and standardisation in solutions of the completely dissociated chloride salts for cation response and in solutions of the completely dissociated sodium salts for anion response. Among the exceptions was the recommended calibration of fluoride ion-selective electrodes with potassium fluoride rather than sodium fluoride, which is likely to become associated in moderately concentrated solutions [31]. 

There have been complaints over expensive and cumbersome membrane replacement with liquid ion-exchanger electrodes, but such technical inconveniences can be minimised by confining the liquid ion-exchanger or carrier sensors in PVC matrix membranes [6,24,54], Such practical considerations with the above attention to principles and proposals [9] for concentration calibration of ion-selective electrodes for use in biological fluids can all facilitate the biological scientist s confident use of ion-selective electrodes. 

Metal buffers are used for the calibration of ion-selective electrodes at low ion levels (below lO moll where the preparation of standard... 

The response of ion-selective electrodes in biological fluids may be affected by a wide variety of physical and chemical factors. These may influence the indicator electrode directly or may affect the liquid junction of the reference electrode. A brief discussion is presented of the various sources of error and uncertainty in electrode measurements in biologic media especially with microelectrodes. A serious need exists for the development of practical standards for the calibration of ion-selective electrodes in physiologic media in order to ensure the consistency of interlaboratory measurements. 

K.R. Beebe and B.R. Kowalski, Nonlinear calibration using projection pursuit regression Application to an array of ion-selective electrodes. Anal. Chem., 60, 2273-2278 (1988). 

Fig. 1 Calibration graphs of ion-selective electrodes and evaluation of selectivity coefficients. 1) Calibration response against —log in solution of free determinand i. The practical limit of detection may be taken as the activity (or the concentration) at the point of intersection of the extrapolated lines as shown. 2) Calibration against —log a in the presence of the interferant), the activity of which is of a known and constant value. Response ) is obtained for —log a = —log The resulting interference can restrict the measuring range, b) The abscissa approximately equal to 60/z mV.

A variety of electrochemical methods have been incorporated into automated systems. The most widely used Mec-trochemical approach involves ion-selective electrodes. These electrodes have replaced flame photometry for the determination of sodium and potassium in many analyzers and have lately found direct application in the measurement of other electrolytes and indirect application in the analysis of several other serum constituents. The operating principle of ion-selective electrodes is given in some detail in Chapter 4. The relationship between ion activity and the concentration of ions in the specimens must be established with calibrating solutions, and frequent recalibration must be done to compensate for alterations of electrode response. 

In Chapter 12, we mentioned measurement of the potential of a solution and described a platinum electrode whose potential was determined by the half-reaction of interest. This was a special case, and there are a number of electrodes available for measuring solution potentials. In this chapter, we hst the various types of electrodes that can be used for measuring solution potentials and how to select the proper one for measuring a given analyte. The apparatus for making potentiomet-ric measurements is described along with limitations and accuracies of potentio-metric measurements. The important glass pH electrode is described, as well as standard buffers required for its calibration. The various kinds of ion-selective electrodes are discussed. The use of electrodes in potentiometric titrations is described in Chapter 14. 

Much of the uncertainty concerning activity coefficients and liquid junctions can be minimised by calibrating the ion-selective electrode with calcium chloride standards (approx. 0.5—0.2 mmol dm ) in 0.150 mol dm sodium chloride solutions. This corresponds to an emf span of just approx. 18 mV, thus emphasising the need for using precise emf-measuring instruments and attention to electrode care and solution details. 

Ion selective electrodes Measurement of the free metal ion is an important goal of metal speciation studies in natural waters, because of the tenet of the free ion activity model, that bioavailability is proportional to the free metal ion concentration. Ion selective electrodes respond to the free metal ion activity. A range of ion selective electrodes are available, but only the copper electrode has sufficient sensitivity for use in measurement at realistic environmental concentrations in natural waters. The correct use of the copper ion selective electrode remains a matter of debate, especially its calibration at the very low free metal ion concentrations using metal ion buffers and the applicability of this procedure to natural samples. 

The examples of unified approach to diffusion potential modeling for solution/ solution (immiscible liquids) and solutirai/membrane boundaries can be found in refs. [25-27] and [91, 92]. There are already no doubts that diffusion potential contributes to the apparent membrane potential and affects strongly the calibration curves of ion-selective electrodes [26, 93, 94]. This can take place for glass electrode as well (see refs, related to pH measurements in Sect. 3.3.3). 

Preparation of lead standard solntions to calibrate the Ion Selective Electrode (ISE) Pipette 1, 3, and 5 mL of 1000 ppm lead perchlorate stock solution into three different 100 mL volnmetric flasks and make up to the mark with distilled water to make 10, 30, and 50 mg/L lead standards. 

With the help of ion-selective electrodes and the proper calibration, the ion activity at the indicating electrode position can be monitored directly and continuously. According to Friedman [249], who was the first to carry out continuous blood pNa and pK measurements, one can distinguish between static and dynamic flow measurements. Static measurements are those such as ion activity measurements in the stomach or on the surface of the skin. In vivo flow measurements are primarily ion activity measurements in bloodstreams, either in the flow circuit of a heart-lung machine, or also post-operatively by means of vein shunts. 

Ion-selective electrodes are a relatively cheap approach to analysis of many ions in solution. The emf of the selective electrode is measured relative to a reference electrode. The electrode potential varies with the logarithm of the activity of the ion. The electrodes are calibrated using standards of the ion under investigation. Application is limited to those ions not subject to the same interference as ion chromatography (the preferred technique), e.g. fluoride, hydrogen chloride (see Table 10.3). 

By calibration against solutions containing known activities of the ion being determined, measurement of the current can be used to ascertain the activity of the ion in the test solution. Such measurements can be carried out with very small volumes of liquid, and find application in biochemical analyses.36 37 However, the simpler ion-selective electrodes discussed above can be readily adapted for dealing with small volumes, and even for intracellular measurements. 

As an alternative to plotting a calibration curve, the method of standard addition may be used. The appropriate ion-selective electrode is first set up, together with a suitable reference electrode in a known volume (Ft) of the test solution, and then the resultant e.m.f. ( t) is measured. Applying the usual Nernst equation, we can say... 

Situation Suppose a (monovalent) ionic species is to be measured in an aqueous matrix containing modifiers direct calibration with pure solutions of the ion (say, as its chloride salt) are viewed with suspicion because modifier/ion complexation and modifier/electrode interactions are a definite possibility. The analyst therefore opts for a standard addition technique using an ion-selective electrode. He intends to run a simulation to get a feeling for the numbers and interactions to expect. The following assumptions are made ... 

De Marco R, Mackey DJ (2000) Calibration of a chalcogenide glass membrane ion-selective electrode for the determination of free Fe in seawater I. Measurements in UV photooxidised seawater. Mar Chem 68 283-294 Van den Berg CMG (2000) Mar Chem 71 331-332... 

It has been long believed that a lithium ion-selective electrode would render obsolete the flame photometer in the clinical laboratory. Lithium is administered to manic depressive psychiatric patients. Since the therapeutic range (0.5-1.5 mM) is quite close to the toxic range (>2 mM), it must be closely monitored. Most of the iono-phores propo d to date have not met the Li" /Na selectivity required for an interference-free assay. However, it has been reported that calibration in the presence of 140 mMNa permitted the analysis of Li in serum The errors observed are due to fluctuations in the Na concentrations in the sample. More selective ionophores would certainly improve the accuracy of this method. 

Ion-selective electrodes are particularly useful for monitoring the disappearance of an ion during a titration. In many cases it is not necessary to calibrate the instrument because there is often a significant change in the potential at the end-point of a titration. However, some electrodes have a slow response time and care must be taken to ensure that titration is not performed too quickly. 

The following is an important point if a constant ionic strength can be assumed, then a calibration graph can be constructed of emf or electrode potential against concentration, rather than against activity. Most commercial ion-selective electrodes (see Section 3.5) would be effectively useless without such calibration graphs. 

Cd ion-selective electrodes The usual version of the Cd " ISE contains a membrane of a sintered or pressed mixture of CdS and Ag2 S [121, 325,408]. Membranes from sintered Ag2S, CuS and CdS mixtures [157] have also been proposed, similarly as forPb ISEs. CdS precipitate in a polyethylene matrix [250] or a CdS-Ag2S precipitate mixture in a silicone rubber matrix [153] can also be used for Cd ISEs. Cd ISEs can be calibrated using a metal diethylenetriamine buffer [66]. Similar substances interfere in the response of the Cd ISE as for the Hg, Ag and Cu electrodes. 

A Ca2+ ion-selective electrode was calibrated in metal ion buffers whose ionic strength was fixed at 0.50 M. Using the following electrode readings, write an equation for the response of the electrode to Ca2+ and Mg2+. 

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