A STUDY OF CONDITIONS FOR KJELDAHL DETERMINATION OF NITROGEN IN PROTEINS

Abstract

The publications on Kjeldahl’s method for determining nitrogen that have appeared since Kjeldahl’s (1) original paper in 1883 perhaps outnumber those on any other analytical method in the same period of time.1 The diversity of papers is attributable to the immense usefulness of the method, to its need for modifications for applications to various types of organic and inorganic compounds, and to the search for catalysts to provide such modifications and to accelerate the digestion. Kjeldahl himself used a digestion mixture of sulfuric and phosphoric acids which he found was adequate for many organic substances. For alkaloids, however, he found the addition of an oxidizing agent necessary and added potassium permanganate crystals to the hot concentrated digest. The substances that have been added to the sulfuric acid digest may be divided into three classes: (1) Potassium sulfate to increase the boiling point and thereby accelerate the digestion process. (2) Oxidizing agents. The requirement for these is that they must assist the digestion of organic compounds without destroying any of the ammonia formed. Permanganate was thus used by Kjeldahl. At present the oxidizing agents in most general use are hydrogen peroxide and potassium persulfate. (3) Metallic catalysts and other substances that act as accelerators. Phosphoric acid employed by Kjeldahl is such an accelerator; although its boiling point is lower than that of concentrated sulfuric acid, the presence of phosphoric acid makes digestion go faster. Phosphoric acid has the disadvantage of etching the digestion flask. The metallic catalysts used to accelerate digestion include mercury, copper, and selenium.