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Abstract: Nitrate-selective electrodes have been developed by electropolymerizing pyrrole onto glassy-carbon electrodes in the presence of NaNO3. Electrochemical variables were used to optimize the potentiometric response of the electrodes and to maximize the selectivity for nitrate over potentially interferent anions. Calibration plots with near-Nernstian slopes for nitrate were observed, -56 +/- 1 mV/decade (n = 18), over a linear range of four decades of concentration (5.0 x 10(-5)-0.50 M nitrate). The electrodes had detection limits of (2 +/- 1) x 10(-5) M nitrate (n = 18). The commercially available nitrate-selective electrodes suffer from interference by lipophilic anions, such as perchlorate and iodide. Compared to these electrodes, the polypyrrole-based nitrate electrodes demonstrated improved selectivity coefficients for perchlorate and iodide by as much as 4 orders of magnitude (K-NO3- ,ClO4-(pot) = 5.7 X 10(-2); K-NO3-,I-(pot) = 5.1 X 10(-2)) An electrochemically mediated templating mechanism is proposed to explain the observed high selectivity for nitrate