Kjeldahl method

 ( Titration ) 

Hot concentrated sulfuric acid oxidizes carbon (as bituminous coal) and sulfur (see sulfuric acid's reactions with carbon):

C + 2 H₂SO₄ → CO₂ + 2 SO₂ + 2 H₂O

S + 2 H₂SO₄ → 3 SO₂ + 2 H₂O

Most of organic carbon and sulfur are decomposed and eliminated as gaseous and .

In contrast to organic carbon and sulfur, the digested organic nitrogen remains preserved in the concentrated sulfuric acid as stable ammonium cation (). Ammonium does not further oxidize to gaseous , or a higher oxidized form of nitrogen, such as, e.g., . If it was the case, the Kjeldahl method would not work.

Catalysis like selenium, Hg₂SO₄ or CuSO₄ are often added to accelerate the digestion. Na₂SO₄ or K₂SO₄ is also added to increase the boiling point of H₂SO₄. Digestion is complete when the liquor clarifies with the release of fumes.

After complete digestion of the sample, to recover ammonia () from the ammonium sulfate, sodium hydroxide (NaOH) is first added to the residual sulfuric acid to neutralize it and to convert the soluble ammonium ion into volatile ammonia:

Then, ammonia is recovered by distillation using the system below (right side of the figure).

The end of the Liebig condenser is dipped into a known volume of standard acid (i.e. acid of known concentration). A weak acid like boric acid (H₃BO₃) in excess of ammonia is often used. Standardized HCl, H₂SO₄ or some other strong acid can be used instead, but this is less commonplace. The sample solution is then distilled with a small excess of sodium hydroxide (NaOH). NaOH can also be added with a dropping funnel. NaOH converts dissolved ammonium (NH₄⁺) to gaseous ammonia (NH₃), which boils off the sample solution. Ammonia bubbles through the standard acid solution and reacts back to ammonium salts with the weak or strong acid.

Ammonium ion concentration in the acid solution, and thus the amount of nitrogen in the sample, is measured via titration. If boric acid (or some other weak acid) was used, direct acid–base titration is done with a strong acid of known concentration. HCl or H₂SO₄ can be used. Indirect back titration is used instead if strong acids were used to make the standard acid solution: a strong base of known concentration (like NaOH) is used to neutralize the solution. In this case, the amount of ammonia is calculated as the difference between the amount of HCl and NaOH. In the case of direct titration, it is not necessary to know the exact amount of weak acid (e.g. boric acid) because it does not interfere with the titration (it does have to be in excess of ammonia to trap it efficiently). Thus, one standard solution (e.g., HCl) is needed in the direct titration, while two are needed (e.g., HCl and NaOH) in the back-titration. One of the suitable indicators for these titration reactions is Tashiro's indicator.

In practice, this analysis is largely automated; specific accelerate the decomposition. Originally, the catalyst of choice was mercuric oxide. However, while it was very effective, health concerns resulted in its replacement with cupric sulfate. Cupric sulfate was less efficient than mercuric oxide and yielded lower protein results. It was soon supplemented with titanium dioxide, the approved catalyst in all protein analysis methods in the Official Methods and Recommended Practices of AOAC International.AOAC International

Today, TKN is a required parameter for regulatory reporting at many treatment plants and for monitoring plant operations.

• Dumas method, another nitrogen analysis method
• Devarda's alloy, a powerful reducing agent for nitrate analysis
• Bicinchoninic acid assay, a colorimetric assay for protein-nitrogen
• Combustion analysis another carbon, hydrogen and nitrogen analysis method

• Wastewater Engineering: Treatment and Reuse, Metcalf & Eddy, McGraw-Hill Higher Education; 4th edition, 1 May 2002,

• Solutions for automation of the Kjeldahl method
• Kjeldahl Proficiency guide