Measuring Techniques with Ion-Selective Electrodes

Contents Introduction. - Fundamentals of Potentiometry. -Electrode Potential Measurements. - Ion-Selective Electrodes. - Measuring Techniques with Ion-Selective Electrodes. - Analysis Techniques Using Ion-Selective Electrodes. - implications of Ion-Selective Electrodes. - Outlook. - Appendix. - Literature. - Subject Index. - Index of Symbols Used. 

Methods used successfully for stability constant determinations are calorimetry (14—16), potentiometric measurements with ion-selective electrodes (3, 17), and certain spectroscopic techniques where favorable spectral properties are found (13, 18). Values of log K have also been deduced indirectly from conductance (19, 20) and potential measurements on phospholipid bilayers (21). Descriptions of these procedures... 

The simplest method of measurement with ion-selective electrodes is direct potentiometry by use of the Nemst equation. However, this makes extreme demands on the reproducibility of the junction potential, and there is the problem of variation of activity with ionic strength. Concentration-cell techniques have proved to be very precise, especially in terms of null-point potentiometi... 

Ion-selective electrodes use membranes which are permeable only to the ion being measured. To the extent that this can be done, the specificity of the electrode can be very high. One way of overcoming a lack of specificity for certain electrodes is to make multiple simultaneous measurement of several ions which include the most important interfering ones. A simple algorithm can then make corrections for the interfering effects. This technique is used in some commercial electrolyte analyzers. A partial list of the ions that can be measured with ion-selective electrodes includes H+ (pH), Na, K, Lff, Ca +, Cl , F , NHt, and CO. ... 

The above implies simple measurement of the ion-selective electrode potential, E, with respect to a suitable reference electrode and relating the potential to activity or concentration by a calibration graph as the means of using ion-sele-tive electrodes. Use in potentiometric titrations depends on the change in E with volume of added titrant. There are, however, other techniques by which ion-selective electrodes may be used these include differential and null-point poten-tiometry. 

When using ion-selective electrodes, and indeed any measuring device, proper technique and an appreciation of principles, scope and limitations are prerequisites to the best results, otherwise there is the dismay expressed by some investigators. With ion-selective electrodes as with pH electrodes, the property measured (the potential) is proportional to the logarithm of the level of active species so that high precision is rarely achieved in direct measurements. In addition, the problems of liquid junction phenomena, laboratory conditions and lack of stability with many electrodes may make readings to within 0.1 mV difficult. With bivalent ions such as calcium, even readings to within 0.1 mV lead to uncertainties of 0.8 per cent (0.4 per cent for univalent ions) under Nernstian conditions. If the error is random it can, in principle, be reduced with replicate determinations [Ij. 

Typical biological fluids include blood and blood serum, blood plasma, urine and saliva. Measurement of calcium in serum was the first analysis to which the technique of AAS was applied and is an obvious example of how FAAS is useful for biomedical analysis. Other specimens e.g. dialysis fluids, intestinal contents, total parenteral nutrition solutions, may be analysed on rare occasions. Elements present at a sufficiently high concentration are lithium and gold when used to treat depression and rheumatoid arthritis respectively, and calcium, magnesium, iron, copper and zinc. Sodium and potassium can be determined by FAAS but are more usually measured by flame atomic emission spectroscopy or with ion selective electrodes. Other elements are present in fluids at too low a concentration to be measured by conventional FAAS with pneumatic nebulization. With other fluids, e.g. seminal plasma, cerebrospinal fluid, analysis may just be possible for a very few elements. 

As is the case with any analytical technique, interferences can arise when working with ion-selective electrodes. These interferences can generally be classified as interferences due to direct indication of other ions or interferences due to chemical influences on the measured ion, such as complexation, which only alter the activity of free measured ion indicated by the electrode. This second type of interference is not due to the electrode itself, but rather to the solution chemistry of the corresponding measured ion in the particular matrix employed, and as such will not be considered in any further detail here. 

Sources of Error. pH electrodes are subject to fewer iaterfereaces and other types of error than most potentiometric ionic-activity sensors, ie, ion-selective electrodes (see Electro analytical techniques). However, pH electrodes must be used with an awareness of their particular response characteristics, as weU as the potential sources of error that may affect other components of the measurement system, especially the reference electrode. Several common causes of measurement problems are electrode iaterferences and/or fouling of the pH sensor, sample matrix effects, reference electrode iastabiHty, and improper caHbration of the measurement system (12). 

The sodium hydroxide is titrated with HCl. In a thermometric titration (92), the sibcate solution is treated first with hydrochloric acid to measure Na20 and then with hydrofluoric acid to determine precipitated Si02. Lower sibca concentrations are measured with the sibcomolybdate colorimetric method or instmmental techniques. X-ray fluorescence, atomic absorption and plasma emission spectroscopies, ion-selective electrodes, and ion chromatography are utilized to detect principal components as weU as trace cationic and anionic impurities. Eourier transform infrared, ft-nmr, laser Raman, and x-ray... 

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). 

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 ... 

Principles and Characteristics Combustion analysis is used primarily to determine C, H, N, O, S, P, and halogens in a variety of organic and inorganic materials (gas, liquid or solid) at trace to per cent level, e.g. for the determination of organic-bound halogens in epoxy moulding resins, halogenated hydrocarbons, brominated resins, phosphorous in flame-retardant materials, etc. Sample quantities are dependent upon the concentration level of the analyte. A precise assay can usually be obtained with a few mg of material. Combustions are performed under controlled conditions, usually in the presence of catalysts. Oxidative combustions are most common. The element of interest is converted into a reaction product, which is then determined by techniques such as GC, IC, ion-selective electrode, titrime-try, or colorimetric measurement. Various combustion techniques are commonly used. 

Elemental composition Ce 56.85%, Cl 43.15%. In the aqueous phase following acid digestion, cerium may he analyzed by various instrumental techniques (see Cerium). Chloride ion in the solution may be measured by ion chromatography, chloride ion-selective electrode or titration with silver nitrate using potassium chromate indicator. The solution may require appropriate dilution for analysis of both the metal and the chloride anion. 

Elemental composition Co 45.39%, Cl 54.61%. Aqueous solution of the salt or acid extract may be analyzed for cobalt by AA, ICP, or other instrumental techniques following appropriate dilution. Chloride anion in the aqueous solution may be measured by titration with silver nitrate using potassium chromate indicator, or by ion chromatography, or chloride ion-selective electrode. 

Elemental composition Na 27.08%, N 16.48%, 0 56.47%. An aqueous solution of the salt is analyzed for sodium by various instrumental techniques (See Sodium). Nitrate ion in solution can readily be measured by ion chromatography, nitrate-ion selective electrode, or various colorimetric methods, such as its reduction with cadmium to nitrite followed by diazotization. 

The section Analysis starts with elemental composition of the compound. Thus the composition of any compound can be determined from its elemental analysis, particularly the metal content. For practically all metal salts, atomic absorption and emission spectrophotometric methods are favored in this text for measuring metal content. Also, some other instrumental techniques such as x-ray fluorescence, x-ray diffraction, and neutron activation analyses are suggested. Many refractory substances and also a number of salts can be characterized nondestructively by x-ray methods. Anions can be measured in aqueous solutions by ion chromatography, ion-selective electrodes, titration, and colorimetric reactions. Water of crystallization can be measured by simple gravimetry or thermogravimetric analysis. 

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