Nitrate selective electrodes containing immobilised ion exchangers
A range of ion selective electrodes (ISEs) for the determination of nitrate has been produced using polymeric membranes containing immobilised quaternary ammonium salts as the ion exchanger. The use of the conventional solvent casting technique gave rise to a relatively low cross-link density, two sided response and poor thickness control. The development of a hot pressing technique resolved all the se problems and would readily suit large scale manufacturing. In this work poly(acrylonitrilebutadiene)(NBr) was selected as the membrane matrix and was robust, gave enhanced selectivity coefficients and extended membrane lifetime. The best electrode fabricated contained 6-5% m/m triallyloctylammonium bromide, 7-2% m/m dicumyl peroxide and 39-7% m/m2 -nitrophenyloctyl ether in a 50% ACN content N Br polymer matrix. The electrode had a lifetime in excess of 665 days and the response to nitrate was Nernstian in the range I x 10" to I x 10' mol dm-1 of nitrate. The limit of detection was 4-5 x IW mol dm-1 and the selectivity coefficient k Pot NO,,- - C1 was 5.3 xIW. The immobilised ion exchanger membrane electrode offered superior lifetime and mechanical strength. The cross-link density of the membranes was determined by solvent swelling and the use of the Flory-Rehner equation. The role of cross-link density upon the electroanalytical properties of membranes was investigated in the range 0-6 x 10' to 11-0 x 10' mol cross-link cm-'. The membranes were studied using electron microscopy and the conditioning process monitored using ion exchange chromatography. Two different response mechanisms were proposed dependent on the membrane composition and the inclusion of solvent mediator.