ISE Constructions
The schematic sketch of a conventional ISE (an ISE with internal aqueous solution) is presented in Fig. 8.1, left. The main sensing element—the electrode membrane—is fixed in the end of a tubular body. The material of the body depends on the type of the m
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ISE Constructions
8.1 Conventional ISEs with Internal Filling Solution The schematic sketch of a conventional ISE (an ISE with internal aqueous solution) is presented in Fig. 8.1, left. The main sensing element—the electrode membrane—is fixed in the end of a tubular body. The material of the body depends on the type of the membrane. For glass membrane electrodes, the body constitutes a glass tubing, and the membrane is formed in the end of the body by glassblowing technique. Thereto, the body tubing is immersed for a short while into the electrode glass melt, and then, the drop of the melt is blown into the typical spherical glass membrane. ISEs with crystalline and polymeric membranes, normally, have plastic bodies. Membranes are fixed in the end of the body with a suitable glue or with a clamping nut. In the latter case also, an O-ring made of an inert material (e.g., silicon rubber) is used for a hermetic seal. Typical constructions of the conventional ISEs are shown in Fig. 8.2. Micro-ISEs for cellular studies (see Sect. 8.4) and electrodes in flow-through cells, in particular in clinical analyzers (see Sect. 8.5), are often of the conventional type, although solid-contact setup (see Sect. 8.2) appears more promising for these miniature devices. Conventional ISEs contain an internal solution. This can be an ordinary liquid solution or a gel. For stable and reproducible electrical potential at the interface between the membrane and the internal solution, the latter must contain the ion to which the membrane is selective. For instance, for a glass pH electrode, this must be H+; for a Pb2+ crystalline electrode, this is Pb2+; for a K+ and NO3- polymeric electrodes, these are K+ and NO3-, etc. The membrane and the internal solution are ionic conductors, while the wire is electronic conductor. For a reversible transduction from the ionic conductivity in the internal solution to the electronic conductivity in the wire, an internal electrode is needed. To this end, one can use well-known classical first- and second-kind electrodes, as well as RedOx electrodes. Most often, a second-kind electrode, in particular—Ag/AgCl, is used as the internal electrode in the conventional ISEs. Then, the internal solution must also contain Cl- ions. For the above listed
K. N. Mikhelson, Ion-Selective Electrodes, Lecture Notes in Chemistry 81, DOI: 10.1007/978-3-642-36886-8_8, Ó Springer-Verlag Berlin Heidelberg 2013
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8 ISE Constructions
Wire
Internal electrode Body
Internal solution
Membrane
Wire
Electronically conducting material Body
Transducer layer Membrane
Fig. 8.1 Schematic sketch of a conventional ISE (left) and a solid-contact ISE (right)
Fig. 8.2 Left the Denver Instruments ISEs with crystalline membrane (M), PVC membrane (N), and glass membrane (O). Right the Thermo Fisher ISEs
examples, the most typical internal solutions are as follows: 0.01 M HCl, 0.0001 M PbCl2, 0.01 M KCl, and 0.01 M NaNO3 ? 0.01 M NaCl, respectively. Potassium ion is controlled mostly in clinical and agricultural samples where th
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