An Overview of the Kjeldahl Method of Nitrogen Determination. Part I. Early History, Chemistry of the Procedure, and Titrimetric Finish

In 1883 Kjeldahl devised a method for the determination of nitrogen, which has become a classical measurement in analytical chemistry and has been used extensively over the past 130 years. In the original method, sulfuric acid alone was used as a digestion medium. The use of a catalyst in Kjeldahl digestion accelerates oxidation and completes the digestion to allow the subsequent determination of nitrogen. Mercury (its use being in decline because of environmental concerns), selenium, and copper are the catalysts of choice, though for certain applications titanium has found some usage. Short digestion times in association with maximum nitrogen recovery may be achieved by using a methodology based on experimental design and response surfaces, with microwave digestion processes, and with the aid of the couple sulfuric acid-hydrogen peroxide without catalyst. The quantification of distilled ammonia is generally achieved by titration; the ammonia is absorbed in an excess of boric acid, followed by titration with standard acid in the presence of a suitable indicator. The Kjeldahl method can be done with limited resources; nitrogen determination with the Kjeldahl method does not require expensive devices nor specialized techniques and is precise and accurate. The Kjeldahl method is used for calibrating other protein assays; it is still the primary reference method for protein analysis today. The original method as presented by Kjeldahl has been continuously improved. Today's digestion systems offer safety both from a personal perspective and from an environmental point of view. The determination of nitrogen content is a frequently conducted analysis in industry and commerce, and numerous organizations have official methods. The use of instrumental finish in Kjeldhal applications will be the subject of the second part of this review.