Ion-Selective Electrodes with Crystalline Membranes
Crystalline membranes are made of polycrystalline or monocrystalline materials. Most of the polycrystalline membranes comprise mixed crystals of low-soluble silver salts and heavy metal sulfides.
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Ion-Selective Electrodes with Crystalline Membranes
6.1 Materials of Crystalline Electrode Membranes Crystalline membranes are made of polycrystalline or monocrystalline materials. Most of the polycrystalline membranes comprise mixed crystals of low-soluble silver salts and heavy metal sulfides. Due to the similarity in the chemical compositions, also chalcogenide glasses will be briefly discussed in this chapter although these materials comprise amorphous phases. Some activity related to potentiometric sensing using crystalline electrodes has been reported already in the 1920s and 1930s [1–4]. However, the first really working electrode with crystalline sensing element was proposed by Pungor in the early 1960s [5]. This electrode was very different from the modern crystalline electrodes. The sensing material, solid silver iodide powder, was embedded in inert polyethylene matrix. Since that time, a large number of ISEs based on lowsoluble salt precipitates embedded in polyethylene or polypropylene matrixes were invented. The next and very important step was proposed by Frant and Ross in 1966 [6]. These authors invented a fluoride-selective electrode with a homogeneous solid membrane made of a monocrystalline LaF3. The selectivity of this ISE to its target analyte—F- anion is extremely high, so that only the pH glass electrodes are more selective to their target ion. Among possible interferences, only hydroxyl anion interferes significantly with the fluoride electrode response. Although the fluoride electrode with a monocrystalline membrane shows such impressive properties, this ISE comprises rather exception than a rule. Crystalline membranes of electrodes for measuring other ions are made of mixed polycrystalline materials. These are mostly pressed polycrystalline pellets [7–9]. The pellets typically consist of silver sulfide and another low-soluble silver salt (a halide or a thiocyanate). Without silver sulfide, the electrodes have relatively high resistance and show significant interference from the ambient light so that stable readings require maintaining constant illumination. Silver sulfide is not purely ionic conductor, having also some electronic conductivity. This is why the presence of Ag2S allows for decrease in the membrane resistance. Otherwise, silver
K. N. Mikhelson, Ion-Selective Electrodes, Lecture Notes in Chemistry 81, DOI: 10.1007/978-3-642-36886-8_6, Ó Springer-Verlag Berlin Heidelberg 2013
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6 Ion-Selective Electrodes with Crystalline Membranes
sulfide is often considered as inert matrix since it is much less soluble than other silver salts used in membranes [7]. Electrodes with such membranes show anionic response and are suitable for measuring the activities of S2-, Cl-, Br-, I-, SCN-. Another combination of lowsoluble salts—silver sulfide mixed with a low-soluble metal sulfide—allows for sensing of a number of heavy metals like Hg2+, Ag+, Cu2+, Pb2+, Cd2+, and some others. Thus, crystalline membranes provide a basis for potentiometric measurements of a number of analytes, inclu
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