Sir: Voltammetric procedures are described in the chemical literature for the quantitative determination of NOs™ and N02-in aqueous media based on their electrocatalyzed reductions at mercury electrodes in the presence of various diand trivalent cations (1-13). The mechanisms of these electrochemical reactions involve (i) the direct reduction of a complex formed between the cation and the nitrogen species or (ii) the enhancement of the voltammetric wave for the electroreduction of the cation when the reduced form of the cationic species is rapidly oxidized by NO3or N02~transported to the surface of the electrode by convective-diffusional processes. The uncatalyzed reductions of N03-and N02_ at mercury electrodes are sufficiently irreversible that the large cathodic current due to evolution of H2 obscures any cathodic wave for NO3or N02~.Direct electrochemical reductions of NO3-have been studied at a variety of other electrode materials, including Cu, Ti, Zn, Pb, Sn(Hg), Cu(Hg), Cr, and Pt (14-20). For none of these electrodes in dilute solutions of NO3can analytically useful cathodic waves be resolved from the wave due to evolution of H2. Well developed anodic waves for the oxidation of N02~t o NO3' at platinum electrodes are observed in the pH range 0.5-8 (21, 22). The process is strongly influenced by the extent of oxidation of the electrode surface (23). A quantitative method for determining N02~by oxidation at a platinum electrode in an acetic acid-acetate buffer was reported ( 23) in which controlledpotential coulometry was employed. We describe here cathodic current-potential curves recently obtained with a rotating cadmium disk electrode (RCDE) in weakly acidic solutions of NO3-and N02-. At low rotational velocities and bulk concentration, the cathodic currents are limited by convective-diffusional mass transport.
The determination of N03~a nd N02~by forced-flow liquid chromatography with a Cd tubular detector is described. Dissolved 02, N03~, and N02~w ere separated with Amberlite IRA-900, a strongly basic anion-exchange resin. The eluent was 0.010M HCIO4, and the time required for the analysis was 7-9 min. The areas of the chromatographic peaks for N03~a nd N02~w ere proportional to concentration over the working range of 0.05-1.0 m (0.7-14 ppm N) In a 0.33-ml sample. The detection limit was approximately 0.01mM (0.1 ppm N). A Cd coulometric detector was used to demonstrate that both N03_ and N02_ are electrochemically reduced to NH2OH in acidic media. Nitrate was rapidly reduced to N02_, and N02~m ore slowly reduced to NH2OH at a rotating Cd disk electrode.
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