Determination of Trace Amounts of Nitrite in Water by Coagulated lon-exchanger Colorimetry

Abstract

A 50ml (1 ml =1 cm³) sample solution containing less than 0.5μg of nitrite ion is placed in a 100 ml beaker and kept at 25°C. One milliliter of p-aminobenzenesulfonamide solution (0.5% in 2 mol⋅dm^-3 HCl), 0.5ml of N-(1-naphthyl)-ethylenediamine dihydrochloride solution (0.1% in 2mol⋅dm^-3 HCl), and 2ml of 4 mol⋅dm^-3 hydrochloric acid are added. After 10min stirring, 3.0 ml of anion exchange resin suspension (Amberlyst A-27, particle size: less than 30μpm, 8.97pequiv. ⋅ ml^-1) and 2.0 ml of cation exchange resin suspension (Amberlyst 15, 15.40, μequiv. ⋅ ml^-1) are added. The mixture is stirred for 10min. During this period, the red colored azo dye is fixed on the cation exchanger and the resin is coagulated bulky with the anion exchanger. The mixed resins are then collected on a filter paper (No.5B) by filtration with suction for preparing a colored thin layer of 17 mm in diameter and about O.3mm in thickness. The thin layer is dipped in 0.2 mol⋅dm^-3 hydrochloric acid for 15 min to stabilize the color developed. The absorbance is measured against a white paper at 555nm and at 670nm where the azo dye has no absorption. The reagent blank value is also obtained and the net absorbance is determined. The calibration curve deviates slightly from the proportionality, the relative standard deviations (n=6) being 2.5% for 0.05 pg of nitrite ion and 1.8% for 0.5 pg. The absorbance for 1. Opg NO₂^- dm^-3 is about O.1: the sensitivity is approximately 100 times that of the conventional solution method using a 1 cm cell. The method is successfully applied to the determination of nitrite in water samples.