Construction of Nitrate-selective Electrodes and Monitoring of Nitrates in Hydroponic Solutions

A liquid-membrane type nitrate-selective electrode was constructed, in which the responding membrane contained polyvinylchloride, o-nitrophenyloctylether and tetraheptylammonium nitrate. The NO3−-selective electrode displayed a linear response to the concentration of NO3− with a Nernstian slope of −53.3 ± 1.0 mV decade−1, in the 10−5 – 10−1 mol dm−3 (M) NO3− concentration range. The NO3− detection limit was about 10−6 M. The electrochemical response of this electrode was stable for more than 30 days. Measurements performed using the NO3−-sensor indicated that in the presence of green plants, the concentration of NO3− in a hydroponic solution decreased from 0.20 to 0.05 mM over a three-day period.