Article no. 3

Kjeldahl Chemistry:

A Brief Overview

NITROGEN ANALYSIS On March 7, 1883, Johan Kjeldahl presented his method of nitrogen analysis to the Danish Chemical Society. Since then, his method has been extensively studied, modified, and improved upon. Today, the Kjeldahl method for the determination of organic nitrogen is the worldwide standard for the purpose of calculating the protein content in both human food and animal food. Additionally, Kjeldahl has been adapted as a standard method of nitrogen analysis in water, wastewater, fertilizer, and fossil fuels, to name a few.   The Kjeldahl method for nitrogen analysis is composed of three distinct steps. These are digestion, distillation, and titration.  DIGESTION STEP The purpose of the digestion step is to break the intricate structure and chemical bonds that hold a chemical substance (piece of meat, cup of flour or quart of oil) down to simple chemicals and ionic structures. Specifically, proteins and other forms of nitrogen are broken down and converted to ammonia.     To accomplish this, one to two grams of the sample are placed on a digestion tube with 12-15 ml of concentrated sulfuric acid (H2SO4). Seven grams of potassium sulfate (K2SO4) and a metallic catalyst, usually copper, are then added. (The latter two chemicals are commercially available from Rose Scientific as the proper Kelmate NT™.) The digestion tube is laced into a digestion block where it is heated to the boiling temperature of the mixture. Digestion is usually completed after one hour at 370ºC to 400ºC. The Distillation Step Distillation involves separation of ammonia – nitrogen from the digestate. This is accomplished by raising the pH with sodium hydroxide (NaOH). This changes the ammonium (NH4+) ion to ammonia (NH3). Now it is possible to separate the nitrogen by distilling the ammonia and collecting the distillate in a suitable trapping medium. With Foss Tecator’s Kjeltec Systems, distillation takes less than five minutes.Today collection of ammonia is usually done by absorption into a solution of four percent boric acid. The ammonia is bound to the boric acid in the form of ammonium borate. The Titration Step Determination of the amount of nitrogen on the condensate flask can be accomplished by several methods. The most common is titration of the ammonia with a standard solution of one-tenth normal hydrochloric acid (0.1 H HCl) in the presence of mixed indicator. The mixed indicators (bromocresol green and methyl red) are available in the four percent boric acid solution. Calculation After all this chemistry it is now time to calculate the amount of nitrogen present in the sample. This calculation can either be performed as percent nitrogen or percent protein. For percent nitrogen: %  N = 14.01 x (ml titrant – ml blank) – (N of titrant) x 100                              Sample Wt. (grams) x 1000 It has been shown that protein is 16% nitrogen. (Wheat and dairy products are some exceptions.) By dividing 100 by 16, we get the conversion factor for nitrogen to protein of 6.25. Hence, the percent protein is calculated as follows: % Protein = 6.25 x %N