THE DETERMINATION OF NITRATES IN COLORED SOIL EXTRACTS

Abstract

In highly colored soil extracts, it was found impossible to determine nitrates directly by the phenoldisulphonic acid color reaction. Attempts to remove the coloring matter by oxidation or absorption were not successful. The most satisfactory method was found to be reduction of the nitrate to ammonia with Devarda's alloy in 0.1 N sodium hydroxide solution in the cold after the removal by steam distillation, of any ammonia that might be present in the extract. The ammonia formed from the nitrate was steam distilled into 0.01 N hydrochloric acid solution and determined colorimetrically by Nessler's reagent using a Klett–Summerson photoelectric colorimeter. The recovery of nitrates from the colored solutions and from standards was satisfactory. The aliquot taken for the determination should contain not more than 0.125 mgm. nitrogen as nitrate.