Abstract
Abstract A study of inorganic interferences with the 2,4-xylenol spectrophotometric method for nitrate and their elimination is reported. Fifty-three substances do not interfere with the original method. Nitrite interferes somewhat by producing a faint yellow color. Certain reducing agents (Fe 2+ , S 2- , S 2 O 3 2- , and SCN - ) cause low results by reducing the nitrate in the strong sulfuric acid solution, while some oxidizing agents (Mn 7+ , Cr 6+ , V 5+ , and ClO 3 - ) cause low results by inactivating or destroying the 2,4-xylenol. Persulfate and small amounts of H 2 O 2 produce a slight deepening of the color; larger amounts of H 2 O 2 ; cause low results, as do Cl - , Br - , I - , and metals. The recommended maximum permissible limits (mg per 10-ml aliquot) for the original method are NO 2 - -N, Fe 2+ , S 2- , SCN - , V 5+ , ClO 3 - , Cl - , I - , 0.2; Mn 7+ , Cr 6+ , S 2 O 8 2- , 5; H 2 O 2 , 0.02; S 2 O 3 2- , Br - , 0.1; metals, none. Procedures for the elimination of most of the interferences are described. Nitrite is destroyed with sulfamic acid. The interferences of reductants (Fe 2+ , S 2- , S 2 O 3 2- , and SCN - ) and oxidants (Mn 7+ and Cr 6+ ) are eliminated with hydrogen peroxide, the excess of which (and S 2 O 8 2- ) is destroyed by boiling in the presence of Fe 3+ . The interference of Cl - , Br - , and I - is eliminated by precipitation with silver sulfate. An alternative to the sulfamic acid procedure is to oxidize nitrite to nitrate with peroxide and deduct NO 2 - -N from the total NO 3 - -N. After elimination of interferences, a 10-ml aliquot of sample solution is treated with 17.0 ml of sulfuric acid and 2,4-xylenol, the 6-nitro-2,4-xylenol is steam-distilled into an ammonia—water—isopropanol mixture, and the yellow color is measured.
Published Version
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