Ionophore-Based ISEs
This chapter describes ISEs with membranes based on ionophores. Ionophores are organic lipophilic substances which selectively bind ions. The nature of these interactions makes the basis of the potentiometric selectivity of ISEs with membranes containing
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Ionophore-Based ISEs
This chapter describes ISEs with membranes based on ionophores. Ionophores are organic lipophilic substances which selectively bind ions. The nature of these interactions makes the basis of the potentiometric selectivity of ISEs with membranes containing ionophores. A large variety of ionophores enables selective sensing of various analytes, mostly ions but sometimes also neutral species. The fundamentals of the ionophore-based potentiometric and optical sensors, as well as brief characterization of a large number of ionophores, are presented in review papers [1, 2]. Although published more than a decade ago, these reviews remain highly relevant. Currently, most of the progress in ISEs theory and its applications is related to ionophore-based membranes. This makes these membranes, probably, the most important kind, and therefore, we start our in-depth discussion of ISEs with this particular kind of sensor membranes: ionophore-based electrodes. Originally, ionophore-based membranes were comprised of liquids, namely solutions of ionophores in suitable organic solvents. However, already for several decades, solvent-polymeric membranes with polymeric matrixes normally containing plasticizers, and doped with ionophores and ion exchangers, strongly predominate over liquid membranes in most applications. The chapter starts with description of the membrane materials, followed by a brief description of the theory of the response and the selectivity of this kind of ISEs.
4.1 Ion Exchangers and Charged Ionophores The type of the electrode response (cationic or anionic) and the selectivity of the electrode are determined by ionophores and ion exchangers contained by the electrode membrane. Among the first ion exchangers were potassium salts of the tetraphenylboric acid derivatives (lipophilic anions) [3] and also salts of tetraalkylammonium, tetraalkylphosphonium, and tetraalkylarsonium (lipophilic cations) [4], see Fig. 4.1. Generally speaking, ion exchangers are lipophilic salts (sometimes acids or bases) which, at least to some extent, dissociate in the membrane phase. The products of the
K. N. Mikhelson, Ion-Selective Electrodes, Lecture Notes in Chemistry 81, DOI: 10.1007/978-3-642-36886-8_4, Springer-Verlag Berlin Heidelberg 2013
51
52 Fig. 4.1 Typical ion exchangers: potassium tetrakis(p-Cl-phenyl)borate (left) and tetradecylammonium bromide (right)
4 Ionophore-Based ISEs
Cl +
_
Cl
Cl
B
N
Br
_
+
K Cl
dissociation are RzR : a lipophilic organic cation or anion and IzI : a hydrophilic ion. The lipophilicity is a measure of the affinity of the species to organic phases. Quantitatively, the lipophilicity is defined as decimal log of the partition coefficient of the species between water and normal octanol [5]. Partition coefficients of individual ions (see below) cannot be measured. However, partition coefficients of salts formed by a lipophilic RzR anion or cation and a water-soluble cation or anion are determined primarily by the lipophilicity of RzR . The latter must be enough to
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