Ion-selective electrodes with unusual response functions: Simultaneous formation of ionophore-primary ion complexes with different stoichiometries

Masafumi Miyake, Li D. Chen, Gianluca Pozzi, Philippe Bühlmann

Research output: Contribution to journalArticlepeer-review

14 Scopus citations

Abstract

It is well known that the selectivity of an ion-selective electrode (ISE) depends on the stoichiometry of the complexes between its ionophore and the target and interfering ions. It is all the more surprising that the possibility for the simultaneous occurrence of multiple target ion complexes with different complex stoichiometries was mostly ignored in the past. Here, we report on the simultaneous formation of 1:1 and 1:2 complexes of a fluorophilic crown ether in fluorous ISE membranes and how this results in what looks like super-Nernstian responses. These increased response slopes are not caused by mass transfer limitations and can be readily explained with a phase boundary model, a finding that is supported by experimentally determined complex formation constants and excellent fits of response curves. Not only Cs + but also the smaller ions Li +, Na +, K +, and NH 4 + form 1:1 and 1:2 complexes with the fluorophilic crown ether, with cumulative formation constants of up to 10 15.0 and 10 21.0 for of the 1:1 and 1:2 complexes, respectively. Super-Nernstian responses of the type observed with these electrodes are probably not particularly rare but have lacked in the past an adequate discussion in the literature, remaining ignored or misinterpreted. Preliminary calculations also predict sub-Nernstian responses and potential dips of a similar origin. The proper understanding of such phenomena will facilitate the development of new ISEs based on ionophores that form complexes of higher stoichiometries.

Original languageEnglish (US)
Pages (from-to)1104-1111
Number of pages8
JournalAnalytical chemistry
Volume84
Issue number2
DOIs
StatePublished - Jan 17 2012

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