New PVC Nitrate-Selective Electrode:  Application to Vegetables and Mineral Waters

The paper refers the analytical potentialities of the oxa-azamacrocycles as potentiometric ionophores for the construction of electrodes selective to nitrate. Afterward, the membrane selective to nitrate is designed and optimized using a [3.3.3.3]oxazane as an ionophore. The membrane was prepared using dibutylphthalate as a solvent mediator, tetraoctylammonium as a lipophilic membrane additive, and poly(vinyl chloride (PVC) as a polymeric matrix, applied directly onto a conductive graphite/epoxy resin support. The electrodes presented a slope of 60 ± 0.1 mV log-1, a low limit of linear response of 4.2 × 10-6 mol L-1, a useful lifetime of 1 year, and improved selectivity characteristics when compared with other nitrate electrodes. The good working characteristics of this electrode, constructed without inner-reference solution, made possible its application to the determination of nitrate in different types of vegetables and bottled mineral waters without the use of a conditioning solution. The application of a significant F test proved that the results obtained were similar to those attained by application of the brucine spectrophotometric method adopted as a reference technique. Linear regression analysis also showed good agreement between the results obtained by the proposed method and the reference one. Keywords: Oxa-azamacrocycles; ionophore; nitrate-selective electrode; vegetables; water