The key difference between Dumas and Kjeldahl method is that Dumas method is an automated and instrumentalized method, whereas Kjeldahl method is a manual method.
Both Dumas method and Kjeldahl method are important in determining the nitrogen content of chemical substances quantitatively. These two processes differ from each other depending on the techniques used for the determination.
What is Dumas Method?
Dumas method is an analytical technique which is helpful in the determination of the nitrogen content in chemical substances through an automated system. This method was first developed by the scientist Jean-Baptiste Dumas in 1826. Compared to other nitrogen-quantifying techniques, the specificity of this technique is that this method is fully automated and instrumentalized, which allows us to get rapid measurements of crude protein content in food samples. Therefore, this technique has replaced the Kjeldahl method.
In the Dumas method, there is a combusting sample of known mass in a high-temperature range (typically about 800-900 Celsius) chamber in the presence of oxygen. This combustion leads to the release of carbon dioxide, water, and nitrogen. These compounds are released in the form of gases, and these gases then pass over a special column (e.g. potassium hydroxide aqueous solution) that can absorb carbon dioxide and water in the sample.
The detector of this system is a column containing a thermal conductivity detector at the end of the process. It can separate nitrogen from any residual carbon dioxide and water, which allows us to determine the remaining nitrogen content in the released gas mixture.
However, there are advantages and limitations to the Dumas method. This technique is easy and fully automatable. It is considerably faster than other methods, and it may take only a few minutes per measurement. This technique also does not involve any toxic chemicals. The major disadvantage of the Dumas method is the high initial cost.
What is Kjeldahl Method?
Kjeldahl method is an analytical technique for the determination of the nitrogen content in organic substances and inorganic substances. Here, inorganic substances refer to ammonia molecules and ammonium ions. However, other forms of nitrogen, such as nitrate ions are not included in this technique. The Kjeldahl method was developed by Johan Kjeldahl in 1883.
Kjeldahl method involves the heating of a sample at 360-410 Celsius with concentrated sulfuric acid. This reaction decomposes the organic substances in the sample by oxidation to liberate the reduced nitrogen as ammonium sulfate. Catalysts such as selenium, mercuric sulfate and copper sulfate are added to make this digestion occur faster. Sometimes, we can add sodium sulfate to increase the boiling point of sulfuric acid. When the liquor clarifies after releasing fumes, we can say that the digestion is complete. Then we need a distillation system to get the final value.
The distillation system has a condenser at the end of it. This condenser is dipped into a known volume of the standard boric acid. Then the sample solution is distilled with a small amount of sodium hydroxide. Here, sodium hydroxide reacts with ammonium or ammonia, which boils off the solution. Thereafter, we can determine the amount of nitrogen in the sample via titrating this final solution. An acid-base titration is suitable because we are using a boric acid sample.
What is the Difference Between Dumas and Kjeldahl Method?
Dumas method and Kjeldahl method are important in the quantitative determination of nitrogen content in chemical substances. The key difference between Dumas and Kjeldahl method is that Dumas method is an automated and instrumentalized method, whereas Kjeldahl method is a manual method. Because of this, the Dumas method is very fast, while the Kjeldahl method is time-consuming.
Moreover, Dumas method D does not use any toxic chemicals while Kjeldahl method uses toxic chemicals such as boric acid.
Below infographic presents more details of the difference between Dumas and Kjeldahl method.
Summary – Dumas vs Kjeldahl Method
Dumas method and Kjeldahl method are important in the quantitative determination of nitrogen content in chemical substances. The key difference between Dumas and Kjeldahl method is that Dumas method is an automated and instrumentalized method whereas Kjeldahl method is a manual method.
1. “11B: The Dumas Method (Experiment).” Chemistry LibreTexts, Libretexts, 14 July 2020, Available here.
1. “Apparatus for determining the vapour density by Dumas’ method” By Mendeleyeff D. (1834—1907) – The Principles of Chemistry. Volume I (of 2) Mendeleyeff D. The Principles of Chemistry. Volume I (of 2). — 2th ed. (translated from the russian sixth edition). — London e. a.: Longmans, Green, and Co, 1897. — xviii + 621 p. (Public Domain) via Commons Wikimedia