An automated method for determining nitrate-nitrogen in soil extracts

Abstract

A method is described in which nitrate is determined automatically in 2N KCl extracts of calcareous soil. Nitrate is reduced to nitrite with hydrazine and copper catalyst under alkaline conditions at 37°C. The nitrite formed is reacted with sulphanilamide in acid solution to produce a diazonium salt. This salt is then coupled with N-1-naphthylethylenediamine dihydrochloride to form a soluble coloured compound. The absorbance of the coloured solution is measured at 520 nm. The method has advantages over current published methods in that problems associated with precipitation of calcium and magnesium ions were overcome by buffering the sample stream at a pH level that allowed for the conversion of substantial amounts of nitrate to nitrite but prevented precipitation. In addition, the method has removed the need for a dialyser to overcome interfering cations. The method is precise, accurate, sensitive and rapid. The operating range is 0 to 4 p.p.m. N03-N (solution) with a coefficient of variation of 0·7%.