Analysis Techniques Using Ion-Selective Electrodes

This section describes the various calibration and evaluation methods which can be used with ion-selective electrodes. The specific procedure chosen determines whether the activity or concentration of the indicated ion is obtained as a result of the measurement Fundamental to all analytical methods is the Nernst equation. We have seen in Chap. 1.2 that the Nernst equation is a theoretically derived relationship. In practice it is usually the case that an electrode deviates somewhat from the Nernst equation. As long as this deviation is reproducible, the electrode can still be used for analytical applications. Testing an electrochemical cell for exact Nernstian behavior is quite painstaking, and is often impossible. Standard solutions of precisely known activities are needed. This, in turn, requires the individual activity coefficients of the corresponding measured ions, and these are not always known with sufficient accuracy. Instead, one usually uses an empirical form of the Nernst equation for analytical applications  

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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. 

The most popular device for fluoride analysis is the ion-selective electrode (see Electro analytical techniques). Analysis usiag the electrode is rapid and this is especially useful for dilute solutions and water analysis. Because the electrode responds only to free fluoride ion, care must be taken to convert complexed fluoride ions to free fluoride to obtain the total fluoride value (8). The fluoride electrode also can be used as an end poiat detector ia titration of fluoride usiag lanthanum nitrate [10099-59-9]. Often volumetric analysis by titration with thorium nitrate [13823-29-5] or lanthanum nitrate is the method of choice. The fluoride is preferably steam distilled from perchloric or sulfuric acid to prevent iaterference (9,10). Fusion with a sodium carbonate—sodium hydroxide mixture or sodium maybe required if the samples are covalent or iasoluble. 

Sodium and chloride may be measured using ion-selective electrodes (see Electro analytical techniques). On-line monitors exist for these ions. Sihca and phosphate may be monitored colorimetricaHy. Iron is usually monitored by analysis of filters that have had a measured amount of water flow through them. Chloride, sulfate, phosphate, and other anions may be monitored by ion chromatography using chemical suppression. On-line ion chromatography is used at many nuclear power plants. 

Techniques used to determine nitrates in soils include titration [17], spectrophotometry [18-26,29-31], flow injection analysis [20,21], ion selective electrodes [27,28], and ion chromatography [28,31-44],... 

The potentiometric determination of cyanide using ion selective electrodes has become yet another very popular technique, because it is convenient, rapid, and sensitive method of analysis (Frant et al, 1972). Microdiffusion of biological samples containing cyanide is recommended prior to potentiometric determination. The use of a cyanide ion-selective electrode in combination with the Conway microdiffusion method for the measurement of cyanide concentrations in human red blood cells and plasma was reported with remarkable recovery of cyanide (Yagi et al, 1990). Ion chromatographic determination of sulfide and cyanide in real matrices by using pulsed amperometric detection on a silver electrode was reported by Giuriati et al. (2004). 

The most utilized methods include X-ray fluorescence (XRF), atomic absorption spectroscopy (AAS), activation analysis (AA), optical emission spectroscopy (OES) and inductively coupled plasma (ICP), mass spectroscopy (MS). Less frequently used techniques include ion-selective electrode (ISE), proton induced X-ray emission (PIXE), and ion chromatography (IC). In different laboratories each of these methods may be practiced by using one of several optional approaches or techniques. For instance, activation analysis may involve conventional thermal neutron activation analyses, fast neutron activation analysis, photon activation analysis, prompt gamma activation analysis, or activation analysis with radio chemical separations. X-ray fluorescence options include both wave-length and/or energy dispersive techniques. Atomic absorption spectroscopy options include both conventional flame and flameless graphite tube techniques. 

FIA analyzers or FIA components. One company produces a series of instruments that are flow injection systems with atomic absorption spectrometric detection dedicated to determination of mercury. Some companies produce flow injection analyzers for a large number of ions. One supplier has an analyzer that comprises three separate units a basic analytical module, an automatic sample module, and a data capture module, all these units being completely automated. The instrument is capable of analyzing nutrients, ions, and metals. It offers a wide analytical choice using ion-selective electrodes (ISEs), chemiluminescence, or fluorescence. With analysis speeds up to 120 samples per hour and detection limits down to parts per billion levels, this flow injection analyzer performs determinations well compared with other techniques. 

The point analysis mode is used specifically to determine the concentration of the diffusible elements at the cellular level in complex tissues, in cells in culture, and at the subcellular level in individual cells. Under these circumstances information is provided that is not available from other techniques and that complements the information on ionic activities commonly obtained using ion-selective electrodes or fluorescent dyes. In addition, mapping techniques allow the investigation of 2D spatial distributions of elements at the nanometer level in intracellular compartments, yielding information that again cannot be provided by other techniques. 

Methods for iodine deterrnination in foods using colorimetry (95,96), ion-selective electrodes (94,97), micro acid digestion methods (98), and gas chromatography (99) suffer some limitations such as potential interferences, possibHity of contamination, and loss during analysis. More recendy neutron activation analysis, which is probably the most sensitive analytical technique for determining iodine, has also been used (100—102). 

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). 

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. 

Prior to the introduction of ion-selective electrode techniques, in situ monitoring of free copper (II) in seawater was not possible due to the practical limitations of existing techniques (e.g., ligand competition and bacterial reactions). Ex situ analysis of free copper (II) is prone to experimental error, as the removal of seawater from the ocean can lead to speciation of copper (II). Potentially, a copper (II) ion electrode is capable of rapid in situ monitoring of environmental free copper (II). Unfortunately, copper (II) has not been used widely for the analysis of seawater due to chloride interference that is alleged to render the copper nonfunctional in this matrix [288]. 

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 Cu 64.18%, Cl 35.82%. Copper(I) chloride is dissolved in nitric acid, diluted appropriately and analyzed for copper by AA or ICP techniques or determined nondestructively by X-ray techniques (see Copper). For chloride analysis, a small amount of powdered material is dissolved in water and the aqueous solution titrated against a standard solution of silver nitrate using potassium chromate indicator. Alternatively, chloride ion in aqueous solution may be analyzed by ion chromatography or chloride ion-selective electrode. Although the compound is only sparingly soluble in water, detection limits in these analyses are in low ppm levels, and, therefore, dissolving 100 mg in a liter of water should be adequate to carry out aU analyses. 

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