in analytical chemistry, procedure widely used for estimating the nitrogen content of foodstuffs, fertilizers, and other substances, invented in 1883 by a Danish chemist, Johan G.C.T. Kjeldahl. The method consists essentially of transforming all nitrogen in a weighed sample into ammonium sulfate by digestion with sulfuric acid, alkalizing the solution, and determining the resulting ammonia by distilling it into a measured volume of standard acid, the excess of which is determined by titration.