Development of Novel Sensors for Anions of Environmental Interest

Thierry Le Goff

January 2000

Department of Environmental Sciences, University of Plymouth, Plymouth, U.K.

A range of ion-selective electrodes (ISEs) for the determination of nitrate has been produced based upon rubbery membranes having covalently bound betaine salt sensor molecules. The best performing electrode contained N,N,N-triallyl leucine betaine (6.5 % m/m) covalently bound to polystyrene-block-polybutadiene-block-polystyrene (SBS) (43.5% m/m), with 2-nitrophenyloctyl ether (2-NPOE) as solvent mediator (40 % m/m) and dicumyl peroxide (DCP) as free radical initiator (10% m/m). The Nernstian slope was -59.1 mV per decade over a linear range of 1 x 10-1-5 x 10-6 mol dm-3 nitrate, a limit of detection of 0.34 mmol dm-3 nitrate and a selectivity coefficient for nitrate against chloride (kpotNO3-, Cl-) of 3.4 x 10-3. The speed of response was less than 1 minute over the linear Nernstian range. The lifetime in the laboratory exceeded 5 months with no potentiometric drift over the linear Nernstian range. Temperature dependency (0-25°C), pH range (2-12) and a selection of interfering anions (F-, Cl-, Br-, I-, SCN-, ClO4-, HCO3-, NO2-, SO42-, phthalate) were studied.

A field evaluation by continuous immersion in both agricultural drainage weirs and a river were undertaken. The nitrate results obtained with the ISEs compared very favourably (R2=0.99) with those obtained with a segmented-flow instrument in a concentration range 0.47-16 ppm nitrate-N. The electrodes performed continuously for over 5 months in run-off water from a field and over 2 months in river water. The ISEs did not require re-calibration and no deterioration in performance or fouling of the membrane surface was observed.

A preliminary investigation of a phosphate ionophore based upon a heterocyclic macrocycle was also undertaken. This work, based on previous literature, resulted in a dibasic phosphate electrode having a linear Nernstian range from 3 x 10-3 to 1 x 10-6 mol dm-3, a slope of -27 mV per activity decade and a limit of detection of 1 x 10-6 mol dm-3 HPO42-.

© 2000 by Thierry Le Goff. All Rights Reserved

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