Phosphorus hexafluoride (PF6), anthraquinone-2-sulphonate (AS), octylsulphate (OS), picrate (PIC), dipicrylaminate (DPA), Diamine Green B (DGB), tetraphenylborate (TPB) and tetrakis-4-chlorophenylborate (T4CIPB) have been studied as possible ion-pairing agents for use in PVC-type liquid membrane ion-selective electrodes (ISEs) for diquat (DQT), paraquat (PQT) and 4,4′-dipyridinium (4,4′-DPy). Of these, only PF6, PIC, DGB, TPB and T4CIPB formed precipitates with DQT, PQT and 4,4′-DPy in aqueous solution, and only PF6, TPB and T4CIPB gave good electrodes; PF6-based electrodes were not selective.
The analogy between ISE function and ion-pair extraction into an organic phase system is explored in detail. ISE characteristics, e.g., selectivity and minimum detectable activity of sample ion [a(min)] are quantitatively related to the sample ion membrane site association equilibrium and the lipophilicity of the ion pair formed.
In agreement with the proposed theory, PIC is not sufficiently lipophilic to form the basis of a functional ISE for DQT, PQT or 4,4′-DPy. ISEs with T4CIPB as the membrane site show the best sensitivity for DQT (3 × 10–8M) or PQT (2 × 10–7M). The experimental selectivity coefficients for these ISEs in both instances are shown to be near to those calculated. Electrodes for 4,4′-DPy, studied as model systems, show an anomalously poor selectivity or sensitivity.
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