Spectrophotometric Determination of Ammonia-Nitrogen After Preconcentration as Indothymol on a Glass-Fiber Filter in the Presence of a Cationic Surfactant Abstract: Ammonia-nitrogen, in the range 0.06 to 2 g in 20 ml of water sample, is treated with hypochlorite and thymol to form an intensely blue anionic dye called indothymol. The indothymol is collected on a glass-fiber filter as the ion-pair with benzyldimethyltetradecylammonium ion. The indothymol col- lected on the filter is dissolved in a small volume of dimethylformamide (DMF) and the absorbance due to the indothymol is measured at 675 nm against the reagent blank. An unexpectedly high ensitivity was obtained because of the increased molar absorptivity of indothymol in DMF (3.2 x 104 1 mo1-1 cm-1, max=675nm), pproximately 3 times larger than that in water(1.16x104 1 mo1-1cm-1, max=660nm). The detection limit, defined as three times the standard deviation of the blank, is better than 1g 1-1 of ammonia-nitrogen with 5-fold preconcentration. Key words: ammonia-nitrogen, preconcentration by filter, indothymol, spectrophotometry. Determination of ammonia-nitrogen, one of the decay products of nitrogenous organic compounds, is frequently needed to evaluate the drinking water safety and environmental pollution. Methods for the determination of ammonia-nitrogen were reviewed in a paper by Seale 1. Nesslers method was used at one time or the determination of ammonia-nitrogen, but this method lacks sensitivity and has a serious disadvantage of using a toxic mercury compound. In recent years, methods based on Berthelot reaction, the reaction between ammonia, chlorine and phenolic compounds to form blue compounds called indophenol dyes, have become more widely used2. Although the methods based on the Berthelot reaction are more sensitive than the Nessler method, their sensitivities are not sufficient for the determination of #g 1-1 levels of ammonia-nitrogen. We have proposed a simple, rapid and versatile preconcentration technique, based on collecting trace analytes on a solvent-soluble filter and dissolving the filter and the analytes collected on it in a small volume of solvent -3-8. The analytes may be retained on the filter mechanically or by sorption. The analytes in the concentrate are determined spectrophotometrically or by other means. This paper describes an application of a similar technique to the spectrophoto- metric determination of/g 1-1 levels of ammonia-nitrogen in water. Ammonia- nitrogen is converted into indothymol, an indophenol dye formed from thymol by the Berthelot reaction, and collected on a glass-fiber filter in the presence of Zephir- amine (benzyldimethyltetradecylammonium chloride) as the ion-pair with ben- zyldimethyltetradecylammonium ion. The ion-pair is eluted from the filter by placing the filter in a beaker containing 5 ml of dimethylformamide (DMF) and the absorbance due to indothymol in the DMF is measured at 675 nm. Since the molar absorptivity of indothymol in DMF is about 3 times greater than that in water, a 15-fold increase in sensitivity is achieved by 5-fold preconcentration. Our first attempt at collecting the indothymol on a soluble nitrocellulose filter failed because indothymo! is incompatible with nitrocellulose in organic solvents. Thus we had to use a glass-fiber filter as the collection medium, but the insolubility of glass-fiber filter causes little trouble, because indothymol is eluted from the glass-fiber filter in about 10 min without agitation. Experimental Apparatus A Hitachi Model 340 double-monochromator, double-beam spectrophotometer with matched 1.0-cm glass cuvettes was used for measurements of absorption spectra and absorbances. Measurements of pH were made with a Denki Kagaku Keiki Type HPH-22 pH meter. An Advantec Toyo KG-25 filter holder with an effective filtration area of 2.0 cm z was used as filter support. Color development was done in polypropylene centrifuge tubes. When not in use they should be filled with doubly distilled water to avoid contamination from air. Note that glassware readily absorbs ammonia from the atmosphere. Reagents (1) Buffer solution, pH 10. Dissolve 5.04 g of sodium hydrogencarbonate and 7.95 g of sodium carbonate in a sufficient volume of water to make 1 1 of solution. (2) Sodium hypochlorite solution, 0.3 available chlorine. Dilute standardized sodium hypochlorite solution with a sufficient volume of water to make a solution containing 0.3 available chlorine. No indicator is needed for the iodometric determination of available chlorine if 0.1 M sodium thiosulfate is used for the titration. (3) Thymol/acetone solution. Dissolve 5 g of thymol in 50 ml of acetone. Spectrophotometric Determination of Ammon