Glass Electrodes
This chapter is devoted to glass electrodes for the pH and metal ion measurements, as well as to RedOx-sensitive glass electrodes.
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Glass Electrodes
This chapter is devoted to glass electrodes for the pH and metal ion measurements, as well as to RedOx-sensitive glass electrodes.
5.1 Materials of the Glass Electrode Membranes Ion-selective electrodes with glass membranes are the oldest [1–5] (see also Sects. 1.2–1.4) and until now the most frequently used among other ISEs. This is because glass electrodes are by far the best sensors of the pH, and the pH is the most frequently measured parameter of the chemical composition of various types of samples. Besides measurements of the pH, measurements of activities of several metal cations (in the first place—Na+) can also be performed with the respective glass electrodes. Silicon dioxide (SiO2) is the main component of the electrode glasses. Silicate glass membranes are very stable against aggressive chemicals like concentrated acids (except of HF) or organic solvents. Therefore, electrodes with glass membranes can be used under harsh conditions, including those in chemical and biochemical industries. Phosphoric glasses are also known but only seldom in use because these kinds of glasses are relatively soluble in water and therefore unstable in most real applications. Although glasses are amorphous, a short-range ordering exists in glass membranes. In quartz, silicon dioxide forms tetrahedral lattice. A similar although less regular structure is characteristic also for the electrode glasses. As shown in Fig. 5.1, oxygen atoms are of two types: bridge and non-bridge atoms. The former are bound to two silicon atoms, the latter to only one silicon atom, while the other is a metal atom. The metal atoms in the structure originate from the respective metal oxides: Li2O, Na2O, and K2O. Electrode glasses are always doped with some of these metal oxides because pure SiO2 is non-conducting. Metal oxides are introduced into melted silicon dioxide, and the whole melt has to be cooled fast in order to maintain in the hard glass the uniform distribution of the components, which is characteristic of the melted oxide mixture.
K. N. Mikhelson, Ion-Selective Electrodes, Lecture Notes in Chemistry 81, DOI: 10.1007/978-3-642-36886-8_5, Ó Springer-Verlag Berlin Heidelberg 2013
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5 Glass Electrodes
Fig. 5.1 Silicon dioxide tetrahedral lattice in glass. Most of oxygen atoms are bound to silicon atoms (bridge oxygen atoms), but some of oxygens are bound to a silicon atom and to a metal atom
O
O
Si
Si
Na
O
O
O O
O
Bridge oxygen atom
When a glass membrane is in contact with an aqueous solution, water molecules enter glass and attack Si–O and also O–M bonds in :Si–O–Si: and :Si–O–M fragments. Hydrogen atoms partly replace silicon and metal atoms, producing :Si–O–H groups. These groups, in turn, are subjects for the water molecule attack as well and undergo the hydrolysis process, resulting in :Si–O- groups and H3O+ ions released into solution, see Fig. 5.2. These both processes are reversible (this feature is not shown in Fig. 5.2 to avoid overloading) and govern the glass electrode response to pH and me
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