NITRATE-SELECTIVE ELECTRODES CONTAINING IMMOBILIZED ION-EXCHANGERS WITHIN A RUBBERY MEMBRANE WITH CONTROLLED CROSS-LINK DENSITY

A range of ion-selective electrodes have been produced for the determination of nitrate using polymeric membranes containing immobilized quaternary ammonium salts as the ion exchanger. The use of acrylonitrile-butadiene copolymer with a 19% acrylonitrile content as the polymer matrix produced electrodes with excellent K(NO3-, Cl-)pot values of 1.1 x 10(-3) but with short lifetimes. Varying the cross-link density of the membranes from 0.6 x 10(-5) to 11.0 x 10(-5) mol cross-link cm-3 did not produce any significant difference in electrochemical behaviour. Two novel ion exchangers, triallyloctylammonium bromide and triallyldodecylammonium bromide, were successfully synthesized and fabricated into membranes. Triallyloctylammonium bromide gave the longest lifetime membranes and the best electrode was fabricated from the acrylonitrile-butadiene copolymer with a 50% acrylonitrile content and 6.5% m/m triallyloctylammonium bromide, 7.2% m/m dicumyl peroxide and 39.7% 2-nitrophenyl octyl ether. The electrode had a lifetime in excess of 500 d and the response to nitrate was Nernstian in the range 1 x 10(-1)-1 x 10(-4) mol dm-1 of nitrate. The limit of detection was 4.5 x 10(-5) mol dm-3 and the selectivity coefficient for chloride over nitrate (K(NO3-, Cl-)pot) was 5.3 x 10(-3). The immobilized ion exchanger membrane electrode offered superior lifetime and mechanical strength.