Classic, ion-selective electrodes

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

Fig. 30.8. (A) Classical ion-selective electrode. (B) Coated-wire electrode.

Figure 8.2 Construction of liquid contact classic ion-selective electrode.

Ketoprofen can also be determined using another classic, ion-selective electrode [76] with very similar parameters in between those of the two electrodes described above. The active ingredient of the polymer membrane is a complex of ketoprofen and rhoda-mine 6G. The electrode shows a deviation of (58.0 1.0 mV decade Hn the concentration range of 1.0 x 10 to 1.0 x 10 S detection limit of 6.3 x 10" mol L short response time of 3-4 s and lifetime of approx. 4 months. By using this electrode, ketoptofen was determined with a standard deviation of s 0.21 and s 0.27 in the following preparations Fastum gel (Berlin-Chemie/Menarini) and Ketonal injections (Sandoz). 

More recendy, two different types of nonglass pH electrodes have been described which have shown excellent pH-response behavior. In the neutral-carrier, ion-selective electrode type of potentiometric sensor, synthetic organic ionophores, selective for hydrogen ions, are immobilized in polymeric membranes (see Membrane technology) (9). These membranes are then used in more-or-less classical glass pH electrode configurations. 

Controlled potential methods have been successfully applied to ion-selective electrodes. The term voltammetric ion-selective electrode (VISE) was suggested by Cammann [60], Senda and coworkers called electrodes placed under constant potential conditions amperometric ion-selective electrodes (AISE) [61, 62], Similarly to controlled current methods potentiostatic techniques help to overcome two major drawbacks of classic potentiometry. First, ISEs have a logarithmic response function, which makes them less sensitive to the small change in activity of the detected analyte. Second, an increased charge of the detected ions leads to the reduction of the response slope and, therefore, to the loss of sensitivity, especially in the case of large polyionic molecules. Due to the underlying response mechanism voltammetric ISEs yield a linear response function that is not as sensitive to the charge of the ion. 

The classic potentiometric enzyme electrode is a combination of an ion-selective electrode-based sensor and an immobilized (insolubilized) enzyme. Few of the many enzyme electrodes based on potentiometric ion- and gas-selective membrane electrode transducers have been included in commercially available instruments for routine measurements of biomolecules in complex samples such as blood, urine or bioreactor media. The main practical limitation of potentiometric enzyme electrodes for this purpose is their poor selectivity, which does not arise from the biocatalytic reaction, but from the response of the base ion or gas transducer to endogenous ionic and gaseous species in the sample. 

Potentiometry is one of the versatile electroanalytical techniques widely applied for sensing. A classical example of its application is potentiometric determination of pH, or activity of other inorganic and organic ions by using a pH electrode or ion-selective electrodes (ISEs), respectively. Generally, potential difference across a... 

Figure 2.3 depicts comparison of the theoretical predictions and experimental observations of the potential response of a silver-selective electrode based on o-xylylenebis(/V,/V-diisobutyldithiocarbamate. Figure 2.3A demonstrates the potential response of an electrode that utilizes a classical experimental setup, i.e. concentrated inner solution (open circles) compared with theoretical prediction based on Eq. (2.2) (full line). The experimentally observed LOD of 10 7M corresponds poorly with the optimistic theoretical prediction of 4 x 10 15M. On the other hand, after optimization of the inner solution [19], the potential response is extended (Fig. 2.3B closed circles) and the detection limit is improved by almost three orders of magnitude to 3 x 10 10M. At the same time, an excellent correspondence between experimental observation and theoretical prediction was achieved by employing the extended Nikolskii-Eisenman equation (Eq. (2.4)—full line). This demonstrates the essential role of membrane fluxes in the potential response of ion-selective electrodes. (For all experimental and calculations parameters see the figure caption.)... 

Other analytical methods can also be applied for the detection of F in archaeological artefacts, especially when it is possible to take a sample or to perform microdestructive analysis. These are namely the electron microprobe with a wavelength-dispersive detector (WDX), secondary ion mass spectrometry (SIMS), X-ray fluorescence analysis under vacuum (XRF), transmission electron or scanning electron microscopy coupled with an energy-dispersive detector equipped with an ultrathin window (TEM/SEM-EDX). Fluorine can also be measured by means of classical potentiometry using an ion-selective electrode or ion chromatography. 

Using the proper choice of separation column, ion chromatography appears to be applicable for the sequential multicomponent analysis of other anions such as sulfate, chloride, fluoride and nitrate. The detection limits will be substantially lower than the classical gravimetric and potentiometric methods currently used. Ion selective electrodes are available for chloride and fluoride. 

Constructional principles of ion-selective electrodes are similar to the classical glass electrode, namely ... 

A wide variety of both organic and inorganic ions has been determined by this type of inexpensive, simple and durable ion-selective electrode [73]. All-solid types of wire-based electrodes can also be included in this category. For example, the classical Ag/AgCl electrode is made by applying a 0.02-mm-thick layer of AgCl to a melt-coated Ag wire. 

Similar classical countercations for the detection of anion antiport in U-tubes do not exist. However, most standard probes including lucigenin (see below) will work for this purpose (unpublished results). Many alternative methods for the detection of U-tube activity are conceivable (ion-selective electrodes, probes, etc). For the transport of charged molecules, standard probes such as 8-hydroxy-l,3,6-pyrenetrisulfonate (HPTS) and 5(6)-carboxyfluorescein (CF) have been used directly to detect anion transport, and p-xylene bis(pyridinium) bromide (DPX), or safranin O has been used to detect cation transport (see below for structures). ... 

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