Nitrate-selective electrodes containing an amino acid betaine as sensor
A novel type of nitrate-selective electrode is reported in which the sensor is glycine betaine hydrochloride. A membrane, hot-pressed from a mixture of glycine betaine hydrochloride (sensor 5% m/m), dicumyl peroxide (initiator, 7.5% m/m), 2-nitrophenyloctyl ether (mediator, 41.5% m/m) and the acrylonitrile polymer Krynac (50.75)(46% m/m), gave a Nernstian response to nitrate from 0.1 to 1 × 10–5 mol dm–3 with a selectivity coefficient, kpotNO3–,Cl–, of 9 × 10–4 for 0.1 mol dm–3 chloride. The figures of merit and the limit of detection, 1 × 10–5 mol dm–3, were comparable to those for an established commercial electrode. Glycine betaine also functioned well as the sensor, provided that the membrane was treated with 5 mol dm–3 hydrochloric acid for 24 h. Possible correlations between nitrate selectivity and the role of betaines in marine plants are suggested.