Determination of Low Levels of Nitrates in Natural Waters by Direct Potentiometry Using an Ion Selective Electrode of Improved Sensitivity

Abstract

This work describes the construction and the evaluation of a nitrate potentiometric transducer with improved sensitivity and it application to the determination of low levels of nitrate in natural waters. The results obtained in these determinations were evaluated comparatively with those obtained by the conventional ion-selective electrodes and by the reference methods which include colourimetric measurement. The construction of the developed units closely follows that used for the conventional ones, but using a circuit that performs the summing of the potentials supplied by the two sensor membranes. The quality of these, units response was assessed evaluating their lower limit of response and detection, sensitivity, potential stability, selectivity coefficients, response time and pH effect. The results obtained by potentiometric methodologies, using ISEIS, offer precision values higher to those given by units of common sensitivity. A comparative study of the results obtained by potentiometry, using both types of units, and the conventional method was made in the determination of nitrate in natural waters. It showed that, in samples with concentrations of nitrate anions comprised between 0.24 and 2.8 μg/1, the results given by the improved sensitivity units (relative deviation less than 2.6 %) were more precise than those given by the common sensitivity electrodes (relative deviation less than 11 %).