Fluorometric Determination of Selenium in Water with 2,3=Diaminonaphthalene' John M. Rankin Division of Laboratories and Research, N e w York State Department of Health, New Scotland A v e n u e . Albanv, N . Y . 12201
w A simple method has been developed for determination of selenium in clean water. This method begins with hydrogen peroxide oxidation to convert inorganic selenium to selenate, followed by hydrochloric acid reduction to selenite. The selenite is then reacted with 2,3-diaminonaphthalene to form a n organic extractable piazselenol, naphtho-[2,3-d]-2 selena-1,3-diazole, which is determined fluorometrically. The method is free from common interferences and is capable of detecting selenium a t the level of 1 ppb. T h e use of 2,3-diaminonaphthalene (DAN) as an analytical reagent for the determination of trace amounts of selenium was reported by Lott et al. in 1963. Since that time, fluorometric procedures based on the formation of the selenium-DAN piazselenol have been applied to the determination of selenium in biological tissue and vegetation (Allaway and Cary, 1964), lake sediments (Wiersma and Lee, 1971), and waste water effluents (Raihle, 1972). The procedures described by Allaway and Cary (1964) and Wiersma and Lee (1971) are applicable to selenium in all its valence states, but due to the complexity of the matrices involved, they require careful digestion and separation procedures. The procedure described by Raihle (1972) employs a bromine-bromide redox buffer and is a p plicable to elemental selenium and selenite. The simple procedure described here makes it possible to determine all forms of inorganic selenium in clean water, such as potable water and fresh water t h a t receives waste effluent containing only small amounts of insoluble organic matter. The first step is hydrogen peroxide oxidation in order to destroy any organic matter present and to convert inorganic selenium to selenate. The sample is then treated with hydrochloric acid to reduce selenate to selenite (Dye et al., 1963). Finally, the selenite is reacted with DAN to form the piazselenol. which is extracted into cyclohexane and measured fluorometrically . Nitrite, which interferes, is removed by oxidation to nitrate. Metal interferences are masked by complexation (Lott et al., 1963).
A masking solution was prepared using the formulation described by Lott et al. (1963). Spectrometric-grade cyclohexane (J. T . Baker Chemical Co., Phillipsburg, N.J.) was used for all extractions. Analytical Procedure. Treat a 50-ml sample with 2 ml of 30% HzOz, and boil gently for 10 min. Add 25 ml of concd HC1, and boil the solution for a n additional 5 min. Remove the sample from the heat source, and adjust its volume to about 75 ml with distilled deionized water. Carefully add 20 ml of concd KH40H, and allow the sample to cool to room temperature. Then treat the sample with 5 ml of the masking solution, and adjust the pH to 1.8 k 0.2 with dilute NHIOH. After this adjustment, transfer the sample to a 125-ml separatory funnel, and add 5 ml of 0.1% DAN solution. Mix the sample by swirling, and store it in the dark for ll/z hr. Then add 10 ml of cyclohexane, and extract the piazselenol by shaking for 1 min. After discarding the aqueous phase, transfer the cyclohexane phase to a 15-ml centrifuge tube, and centrifuge for 5 min a t 225 x g. Determine the selenium concentration in the extract fluorometrically in a 1-cm cell, using excitation and emission wavelengths of 389 and 521 nm, respectively. Results and Discussion Analytical Range and Precision. Calibration curves constructed by plotting the emission intensity vs. the selenium concentration were linear in the range of 0.02-1.0 pg Se. Above this level the curves bent toward the concentration axis. Ten replicates each of a 0.5-pg and a 1.0-118 Se standard in 50 ml of distilled deionized water were analyzed. The results gave a relative standard deviation of 6.3%, with a mean value of 0.48 pg and 1.0 pg, respectively. Interferences. Standard solutions containing 1.0 pg of Table I . Recovery of Selenium Added (0.5 p G ) to 50-MI Samples of Potable Water Selenium recovered"
Experimental Apparatus. All fluorometric measurements were made with an Aminco-Bowman spectrofluorometer (American Instrument Co., Inc., Silver Spring, M d . ) . Reagents. All reagents were analytical reagent grade. unless otherwise specified. A stock selenium solution (1.0 g/l.) was prepared by dissolving 1.633 grams of HZSe03 in distilled deionized water and diluting to 1 liter with distilled deionized water. A 0.1% DAN solution was prepared by dissolving 100 mg of DAN in 100 ml of 0.1N HCl. Prior to use, the solution was extracted with 25 ml of cyclohexane. Address all correspondence to Alan Levensohn, Publications Editor. a t address above.
Sample
Lcs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0.48 0.48 0.50 0.52 0.50 0.50 0.50 0.53 0.50 0.47 0.52 0.59
0.50 0.54 0.50
Oh
96 96 100 104 100 100 100 106 100 94 104 118 100 108 100
Av recovery, 101.9% Std dev, 5.9 a The
background selenium content of each sample was
