Kjeldahl method
The Kjeldahl method is a widely used analytical procedure for measuring nitrogen content in various substances, which also allows for the estimation of protein levels. Developed by Danish chemist Johan Kjeldahl in the late 19th century, this method is particularly valuable in fields such as agriculture, food science, and environmental analysis. The process consists of three main steps: digestion, distillation, and titration.
During digestion, a strong sulfuric acid is introduced to the sample, converting its nitrogen content into ammonium ions. In the distillation phase, sodium hydroxide is added to convert these ions into ammonium gas, which is then captured in a trapping solution. Finally, titration is performed to measure the amount of ammonium, from which the nitrogen content can be calculated.
Since its introduction, the Kjeldahl method has maintained its relevance and accuracy, being endorsed by numerous scientific organizations. While alternative nitrogen detection methods exist, the Kjeldahl method is noted for its precision and versatility, making it a staple in laboratories around the world.
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Kjeldahl method
The Kjeldahl method is a means for chemists to calculate the amount of nitrogen in a substance. Calculating the amount of nitrogen in a substance can also tell chemists the amount of protein in the substance. While other processes for detecting the amount of nitrogen inherent in a substance exist, none are as precise or versatile as the Kjeldahl method.
The process involves three steps: digestion, distillation, and titration. During digestion, chemists use a powerful acid to alter the substance. In distillation, they heat, separate, and trap ammonium gas. In the final step, they isolate the ammonium gas from the trapping substance, then calculate the amount of nitrogen indicated by the amount of ammonium detected.
Background
Johan Gustav Christoffer Thorsager Kjeldahl was born in Jagerpris, Denmark, in 1849. He studied at the Roskilde Gymnasium, then at the Technical University of Denmark, Copenhagen. After graduating, Kjeldahl found work at the Royal Veterinary and Agricultural University. However, he soon moved to the Carlsberg Laboratory in Copenhagen, Denmark.
The Carlsberg Laboratory was an advanced facility founded in 1875 by J.C. Jacobsen. It was created with the intention of advancing biochemical knowledge. For this reason, it featured both a department of chemistry and a department of physiology. Jacobsen was also the founder and owner of the Carlsberg brewery, which focused on brewing and selling beer. For this reason, Jacobsen utilized the Carlsberg Laboratory's resources to advance the chemistry of brewing, allowing the Carlsberg brewery to produce higher quality product.
When Kjeldahl was hired by Carlsberg Laboratory, he was tasked with improving the brewing process in the brewery's chemistry department. Kjeldahl excelled in that position. In 1876, he was promoted to head of the Carlsberg Laboratory chemistry department.
Kjeldahl made several advancements at the Carlsberg Laboratory. He is best known for developing a method for accurately estimating the nitrogen content of compounds. Prior to his innovations, the prevailing method for estimating the nitrogen content was the combustion tube method developed by Jean Dumas in 1826. Dumas's method involves combusting a sample of known mass in a high temperature chamber containing oxygen. Kjeldahl's method was more accurate, faster to conduct, and could be operated on a much larger scale than the combustion tube method.
Kjeldahl is also known for the creation of the Kjeldahl flask. The Kjeldahl flask is a specialized glass container with a long, narrow neck and a wide bottom. It was created to prevent dangerous materials from splashing out of the flask and onto the chemist when heating the materials. It is still in use today.
Overview
The Kjeldahl method, also called Kjeldahl analysis, is a method for measuring the nitrogen and protein content in drinks, meat, feeds, cereals, and forages. It may also be used to find the protein and nitrogen content in soil, wastewater, fertilizers, oilseeds, and other agricultural materials. The method was originally designed to measure the nitrogen and protein content in beer for the Carlsberg Brewery. The completed method was presented to the Danish Chemical Society on March 7, 1883. It has remained in consistent use throughout the world since its development.
The concept behind nitrogen analysis is to measure the amount of nitrogen in a substance. Nitrogen is one of the five major elements found in most organic materials, including protein. For that reason, measuring the amount of nitrogen in a substance allows scientists to predict the amount of protein inherent in that substance. To accomplish this, they use a strong sulfuric acid to oxidize an organic compound. As the substance is oxidized, its carbon is changed into carbon dioxide, and its hydrogen is converted to water. However, the nitrogen undergoes a different reaction. Any nitrogen in the reaction is converted into ammonium ion, which can later be translated into ammonium gas. The ammonium gas is vital in measuring the nitrogen content, and thus the protein content, of the substance.
A Kjeldahl analysis is carried out with three steps: digestion, distillation, and titration. The first step, digestion, refers to the introduction of the substance into a strong sulfuric acid. In most cases, scientists also include a catalyst, which speeds the chemical reaction. This is conducted in a Kjeldahl flask, which is heated by an external source. At this stage, the nitrogen is changed to an ammonium ion.
In the second step, distillation, scientists add a sodium hydroxide solution. This converts the ammonium ions to ammonium gas, which is led to a trapping solution. Common trapping solutions include boric acid solutions, sulfuric acid solutions, or hydrochloric acid solutions. Once the ammonium gas reaches the trapping solution, it is again converted into an ammonium ion. During this process, the nitrogen is stripped from the gas through the distillation process, and the resulting chemicals are contained in the trapping flask.
The third and final step, titration, involves distilling any remaining ammonia and calculating the amount of nitrogen left in the solution. It is usually performed by adding an indicator substance to the solution, which changes color to show the presence of unwanted chemicals. The chemist then introduces very small amounts of sodium hydroxide to the solution. As the chemist continues to add sodium hydroxide, the acid will be neutralized by the base. Once all the acid has been neutralized, the indicator changes color, showing that only desired substances remain. At this point, the chemist can calculate the amount of ammonia that came from the original sample. The amount of ammonia corresponds to the amount of nitrogen.
The Kjeldahl method is still commonly used in chemistry labs throughout the world. It is approved by the Association of Official Analytical Chemists, the Association of American Cereal Chemists, the American Oil Chemist Society, the Environmental Protection Agency, and many other organizations and agencies. While other methods for detecting the amount of nitrogen have been devised, and some can be performed at a significantly more rapid pace than the Kjeldahl method, none are as versatile or precise.
Bibliography
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