ERRORS IN THE DETERMINATION OF LOW-LEVELS OF NITRATE IN LAKE-SEDIMENTS

Error sources associated with the spectrophotometric determination of low levels (eg. <2 mug g-1) of nitrate in sediments have been examined and problems identified included incomplete nitrate recovery (attributable in part to anion resorption) and light scattering by colloidal (<0.45 mum) matter in extract solutions (minimised by using uncoloured extract in the reference beam). Optimum retrieval (>90%) of nitrate from the marine lake sediments studied was achieved with 15 min mixing with 0.1 M NH4Cl, using a sediment to extractant ratio of 1:30. The nitrate in the extracts was determined by reducing it to nitrite (using Cd powder), with subsequent colour development based on the addition of sulfanilic acid and N-1-naphthyl-ethylenediamine dihydrochloride. The reduction step was sensitive to the experimental conditions used, but was near quantitative using 0.1 M NH4Cl extracts. (Much lower transformation levels were observed when the nitrate solutions contained KCl or CaSO4, or when Zn powder was used as the reductant). All the sediments tested sorbed nitrate ion from solution (some very avidly) and this sorbed ion was not readily retrieved by back extraction into NH4Cl solutions.