Simultaneous spectrophotometric determination of nitrate and nitrite in water and some vegetable samples by column preconcentration

A direct spectrophotometric method for the simultaneous determination of nitrate and nitrite by column preconcentration has been developed. Nitrate is reduced to nitrite in a copper-coated cadmium column which is then treated with azo dye reagent by column preconcentration, and the absorbance due to the sum of nitrite and nitrate is measured; nitrate is determined from the difference in absorbance values. Nitrite ion reacts with sulfanilamide (SAM) or sulfamethoxazole (SMZ) in the pH range 2.0-5.0 in hydrochloric acid medium to form a diazonium cation, which is subsequently coupled with 1-naphthol-4-sulfonic acid sodium (NS) in the pH ranges 8.9-11.8 for the SAM-NS system and 10.0-12.0 for the SMZ-NS system to form a stable azo dye to be retained on a naphthalene-tetradecyldimethylbenzylammonium (TDBA)-iodide (I) packed column. The solid mass is dissolved from the column with 5 ml of final dimethylformamide (DMF) solution and the absorbance is measured at 543 nm for SAM-NS and 532 nm for SMZ-NS. The effects of various analytical parameters, such as acidity (pH), flow rate, reaction time, temperature, reagent concentration, and interfering species, are studied. The calibration curve is linear over the concentration ranges 30-600 ng NO2-N and 22-450 ng NO3-N in 15 ml of final aqueous solution (i.e., 2-40 ng NO2-N/liter and 1.5-30 ng NO3-N/liter in aqueous samples) and the concentration factor is 8 for both systems. The method has detection limits of 1.4 ng NO2-N/liter and 1.1 ng NO3-N/liter for SAM-NS and 1.2 ng NO2-N/liter and 0.9 ng NO3-N/liter for SMZ-NS, respectively. The method is successfully applied to water and some vegetable samples.