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Apparently “Non‐Nernstian” Equilibrium Responses Based on Complexation Between the Primary Ion and a Secondary Ion in the Liquid ISE Membrane
Author(s) -
Bühlmann Philippe,
Umezawa Yoshio
Publication year - 1999
Publication title -
electroanalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.574
H-Index - 128
eISSN - 1521-4109
pISSN - 1040-0397
DOI - 10.1002/(sici)1521-4109(199907)11:10/11<687::aid-elan687>3.0.co;2-c
Subject(s) - chemistry , divalent , ion , ionophore , membrane , ion selective electrode , inorganic chemistry , analytical chemistry (journal) , ion exchange , standard electrode potential , electrode , chromatography , electrolyte , organic chemistry , selectivity , biochemistry , catalysis
We have previously reported on a phase boundary model explaining the apparently “twice‐Nernstian” responses to divalent cations as they have been observed for some liquid‐membrane ion‐selective electrodes (ISEs) based on acidic ionophores. The rationalization of those responses, which are characterized by slopes that are two times larger than Nernstian response slopes for divalent cations, took into account the equilibrium distribution of two types of ions, i.e., the divalent cations as primary ions and H + ions as “secondary ions”, across the sample–membrane interface. In the present work, we have intentionally chosen an extremely simple type of electrodes to show that apparently “non‐Nernstian” responses can also occur if the primary and the secondary ion have opposite charge signs. It is shown that apparently “non‐Nernstian” equilibrium responses can occur in the case of ionophore‐free ion‐exchanger electrodes if the sample solutions contain besides the primary ion also a secondary ion, and if 1: n or n :1 complexes between these two types of ions are formed in the electrode membrane phase. In contrast to the well‐known responses of ion‐exchanger electrodes in solutions that contain metal ion complexes, an interaction between the primary and secondary ion in the sample solution is not required for the observation of apparently “non‐Nernstian” equilibrium responses. A set of very simple equations allows the determination of the response slopes that are expected for particular sets of complex stoichiometries and charges of the primary and secondary ion. Due to the very general formulation of the theory, these results are applicable both for electrodes with primary and secondary ions of opposite charge signs and for electrodes with primary and secondary ions of the same charge sign.