Automatic continuous river monitoring of nitrate using a novel ion-selective electrode

On-site continuous measurement of nitrate in the River Taw situated in Devon (UK), as performed automatically for two months using a recently developed ion-selective electrode (ISE). A non-submersible electrode support system was developed and a field instrument was constructed comprising the nitrate-ISEs, a reference electrode and a temperature probe connected through a pre-amplifier to a data-logger and batters supply. The nitrate-ISE was constructed using a commercially available electrode body, with a membrane incorporated into the tip. This rubbers membrane contained the sensor molecule (N,N,N-triallylleucine betaine chloride, 6.5% m/m) covalently bound to polystyrene-block-polybutadiene-block-polystyrene, SBS. (43.5% m/m), with 2-nitrophenyloctyl ether (2-NPOE) as mediator (40% m/m). The Nernstian slope was -59.1 mV per decade over a linear range of 1400-0.07 mg L-1 nitrate-N. The limit of detection was 0.007 mg L-1 nitrate-N with a selectivity coefficient for nitrate against chloride (k(NO3-. Cl-)(pot)) of 3.4 x 10(-3). The speed of response was less than a Minute over the linear Nernstian range. The lifetime in the laboratory exceeded 5 months kith no potentiometric drift over the linear Nernstian range. A temperature correction algorithm was also used to accomodate the temperature changes encountered in the river. The river nitrate results obtained with the ISEs at hourly intervals compared very favourably (R-2 = 0.9), with those obtained with laboratory automated chemical determinations made on contemporaneous river samples over a concentration range 0.55 2.00 mg L-1 nitrate-N. The ISEs did not require re-calibration and no deterioration in performance or fouling of the membrane surface was observed during 40 days.