Electrodes with solid state membranes

The development of these sensors has recently been summarized7. 

In this type of selective electrode, the membrane is an ionic solid which must have a small solubility product in order to avoid dissolution of the membrane and to ensure a response that is stable with time. Conduction through the membrane is principally ionic and is due to point defects in the crystal lattice, relying on the fact that no crystal is perfect. 

Point defects in crystal lattices can be classified into two essential types (Fig. 13.58)  

Other defects are combinations of those mentioned above, the most common being the formation of ion-pair vacancies. 

These defects are natural, or intrinsic, defects, the total charge of the solid remaining unaltered. In certain cases we can alter the structure of the solids and introduce defects externally by doping, in interstitial positions or by substitution of ions in the lattice by others with a different charge. This latter procedure can increase the electronic conductivity and turn an insulator into a semiconductor. There is also the possibility of creating defects by electromagnetic radiation. 

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Table 13.1. Examples of electrodes with solid state membranes...

Fig. 13.7. Forms of ion-selective electrodes with solid state membranes (a) with internal reference electrode (b) with ohmic contact (c) with ohmic contact and combined reference electrode.

The application of potentiometric detection in ion-chromatography is favoured by the progress in the field of membrane ion-selective electrodes (ISE). The electrodes with solid-state membranes were mostly employed for determination of halides, pseudohalides and some other anions binding silver ions. The use of fluoride electrode in multidetector, chromatographic system offers very low detection limit of 1.2 ng fluoride in injected samples. Application of bromide electrode in the same system provides even five-fold better detectability. The same level of detectability was reported by Butler and Gershey for iodide with iodide ISE. In the system with preconcentration step the detectability can be lowered by an order... 

Gel of collodion, PVC, etc. Solution (with low ) of ion exchangers or complex in suitable solvent (apparently dry) "solid-state membrane electrode K, nh , Ca bf4, no3... 

Apart from interference the greatest problem in the use of ion-selective electrodes is that of contamination. Any insoluble material deposited on the surface of the electrode will significantly reduce its sensitivity and oil films or protein deposits must be removed by frequent and thorough washing. It is possible to wipe membranes with soft tissue but they can be easily damaged. Solid-state membranes are more robust but they must not be used in any solution which might react with the membrane material. 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

The integration of chemically sensitive membranes with solid-state electronics has led to the evolution of miniaturized, mass-produced potentiometric probes known as ion-selective field effect transistors (ISFETs). The development of ISFETs is considered as a logical extension of coated-wire electrodes (described in Section 5.2.4). The construction of ISFETs is based on the tech-... 

Fifty-Microliter Samples. For small sample volumes. Durst (23) showed that with a gelled fluoride electrode in an inverted position and a conventional calomel reference electrode, the fluoride electrode could be made the sample container, and measurements can be made in samples as small as 50 juL. This concept was applied here with the chloride electrode in the inverted position because all connections in the electrode are solid state no liquids are inside the body. The fluoride electrode (Orion Model 94-09), used here as the reference, was directly opposed to the chloride electrode with an air gap of about I mm (Figure 4). Both flat membranes were lightly coated with silicone oil to make them hydrophobic, and the test solution was pipetted into the gap. 

In the present chapter, the relationship between the electrode potential and the activity of the solution components in the cell is examined in detail. The connection between the Galvani potential difference at the electrode solution interface and the electrode potential on the standard redox scale is discussed. This leads to an examination of the extrathermodynamic assumption which allows one to define an absolute electrode potential. Ion transfer processes at the membrane solution interface are then examined. Diffusion potentials within the membrane and the Donnan potentials at the interface are illustrated for both liquid and solid state membranes. Specific ion electrodes are described, and their various modes of sensing ion activities in an analyte solution discussed. The structure and type of membrane used are considered with respect to its selectivity to a particular ion over other ions. At the end of the chapter, emphasis is placed on the definition of pH and its measurement using the glass electrode. 

Another solid-state membrane of extremely high selectivity and appKcability is the single crystal LaFs membrane electrode [26]. The crystal is doped with europium to lower its electrical resistance... 

M = gas-permeable membrane electrode. Glass and solid-state membranes are more resistant to certain organic solvents (acetone, methanol, benzene, dioxane. etc.). PVC-membrane electrodes must not be allowed to come into contact with any organic solvent. becau.se this dramatically reduces the lifetime of the electrode. 

The membrane electrodes should reach equiUbrium rapidly, response only to the ion of interest, and change linearly with different concentrations of the ion. Different types of membranes have been developed to fabricate ISEs. The major ones are glass membranes, solid-state membranes, liquid membranes, and polymeric membranes [2]. 

Solid-state membrane electrodes include H+, F , CN , Cl , 1, Br, H2S, CN , thiourea, Pb, Cu, and Cd selective electrodes. In this category, silver halide and silver sulphide electrodes are found. The macroscopic ion-selective electrodes made with these systems have been applied in many fields, such as medicine, food industry, and environmental studies (56, pp. 93,96,103,104) but their full potential has not yet been exploited in terms of SECM studies. 

An article by Kormosh et al. compares the properties of various electrodes for determination of indomethacin a classic electrode with a polymer membrane, a solid-state electrode, and a Wood alloy in PVC tube, in which membranes were prepared using PVC and graphite [81]. The active ingredient applied was an ion-pair type complex of indomethacin with Rhodamine B. The optimum sensor turned out to be the solid-state one based on PVC. By comparing it with another solid-state sensor (BMSA type electrode)... 

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