Nitrate-selective electrodes with covalently bound sensors.

Nitrate determination and ion-selective electrodes, with particular emphasis on the nitrate selective electrode, are reviewed. A new electrode for nitrate has been developed, by covalent attachment of appropriate sensor groups to a robust polymer matrix, with a view to improving the lifetime and the applications of the electrode to include hostile environments. The QAS (I) were prepared as sensor materials.(1) R' = R" = R''' = ethyl(2) R' = allylR" = R''' = methyl, ethyl, propyl(3) R' = R" = allylR''' = methyl, ethyl, propyl, butyl(4) R' = R" = R''' = allylX- = Cl, Br. (I) [diagram]. The production of membranes by cross-linking the sensors (I) with styrene-b-butadiene-b-styrene triblock elastomer (SBS) using a free radical initiated cyclopolymerisation is described.Various methods of membrane preparation have been investigated, and a convenient one-step solvent casting procedure identified and optimised. A range of membranes has been prepared and their physical and electroanalytical properties evaluated. The cross-linked density of the polymers was calculated from swelling measurements and the Flory-Rehner equation, the extent of covalent attachment has been derived from Kjeldahl analyses. The performance of the membranes as ion-selective electrodes is presented and discussed.Several electrodes showed fast response, long lifetime and Nernstian behaviour in the range 10-1 - 10-4 mol dm-3, but their selectivity was inferior to present commercial electrodes. A number of materials were introduced as mediators in order to improve the selectivity of the membranes. A possible mediator (O-nitrophenyloctyl ether) was identified and an analogue (O-nitrophenyl-w-undecylenyl ether) prepared which had suitable functionality for covalent attachment to the polymer. Unfortunately the inclusion of this material did not improvethe selectivity to chloride and the reasons for this are discussed.