Three main types of PVC-liquid membrane ISEs for diquat and paraquat are described. They are based on the crown ether DB30C10 and/or the diquat-paraquat ion pair with tetraphenylborate (DQT.2TPB or PQT.2TPB). With DB30C10-based ISEs (type 1), selectivity for diquat and paraquat over other cations is greatly improved after the addition of DQT.2TPB to the membrane to yield type 2 electrodes. Electrodes based entirely on DQT.2TPB or PQT.2TPB (type 3) also give excellent characteristics with appropriate solvent mediators.
Selectivity coefficient data (kpotDQT,B and kpotPQT,B) have been determined with respect to B = Li, Na, K, Mg, Ca, Ba, ammonium, di-methyl ammonium, anilinium and guanidinium. Although the selectivity coefficients frequently exceeded unity for interferents of singly charged B ions, the modifying influence of the square power term in the selectivity relation enhances the selectivity towards diquat and paraquat. For doubly charged B ions the selectivity coefficients for types 2 and 3 electrodes were normally <1 for diquat.
Type 2 electrodes with o-nitrophenyl phenyl ether as the plasticising solvent mediator in the PVC matrix membrane with DB30C10 and DQT.2TPB are recommended as diquat and paraquat ion-selective electrodes, whereas type 3 electrodes with either o-nitrophenyl phenyl ether or o-nitrophenyl octyl ether as the solvent mediator are suitable as paraquat ion-selective electrodes.
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