The Kjeldahl method is a rapid method for measuring the proportion of nitrogen in an organic compound, named after the Danish chemist Johan Kjeldahl (1849–1900). Nitrogen in the sample is converted into ammonium sulfate by heating it and a concentrated sulfuric acid. Sodium hydroxide is added and the whole is boiled, liberating ammonia. This is dissolved in acid and the quantity of ammonia, and therefore the nitrogen, is determined by titration.