Determination of nitrogen in soil and plant materials: Use of boric acid in the micro‐kjeldahl method

Abstract

AbstractThe use of boric acid for trapping ammonia in the micro‐Kjeldahl determination is examined. A suitable procedure, together with a modified type of micro‐distillation apparatus, is described.The buffer capacity of boric acid solutions was examined in relation to the sensitivity in the titration of ammonia dissolved in them. Two grades of boric acid, ‘pure’ (technical) and AnalaR, were compared; the former showed appreciable buffering on the acid side.The pH values of solutions of both grades decreased with increasing concentration. The sensitivity of the three indicators examined for the titration of ammonia diminished with increasing concentration of boric acid as a result of the increased buffering. With concentrated boric acid, all three indicators showed wide transitional periods at the end‐point, but in solutions of concentration up to 1‐0% of AnalaR grade the end‐points shown by all three indicators were sharp. The methylene blue‐methyl red indicator, however, was generally preferable.The ammonia‐fixing capacity of boric acid was studied in air‐bubbling tests and by recovery of a known amount of ammonia by distillation. A volume of 10 ml. of 1‐0% boric acid fixed 5 mg. of nitrogen as ammonia sufficiently firmly to afford accurate analyses. For macro‐determinations, 100 ml. of 2‐0% boric acid held firmly up to 90 mg. of nitrogen.