Kjeldhal Method - A Method for Analyzing the Nitrogen Content

The Kjeldhal method is a quantitative analytical technique that is used to analyze the Nitrogen content of a sample. This method was developed by a Danish chemist, Johan Kjeldahl in 1883.

Using the Kjeldhal method it can be analyzed,

• Nitrogen content in organic compounds,
• NH₃ and NH₄⁺ content (in inorganic compounds).

But without any modifications/ catalysts, other forms of inorganic nitrogen such as nitrates (NO₃^-), and nitrites (NO₂^-) cannot be analyzed in this method.

The Kjeldhal method

A. Reagents needed

• Concentrated Sulfuric acid
• Catalyst - 3 types (Hg-based, Se-based, Cu based)
• 8M NaOH solution
• 4% boric acid (H₃BO₃)
• 0.2 M HCl solution
• Indicator- Tashiro indicator

B. Procedure

There are three steps involved in a chemical analysis carried out by the Kjeldhal method.

Three steps of the Kjeldhal method

1. Digestion
2. Distillation
3. Titration

01. Digestion

In the digestion step, all the Nitrogen present in the sample is converted into Ammonium sulfate. The sample is treated with conc. Sulfuric acid (H₂SO₄) in 360 - 410 0C.

This process is very slow and requires a lot of heat energy. Therefore, the sample needs to be added a catalyze. There are three types of catalyzes that are used in the digestion process in the Kjeldhal method.

I. Mercury based catalyzes

Mercury-based catalyzes have the highest rate of reaction. But when Hg-based catalyzes are used, all the Nitrogen present in the sample is not converted into Ammonium sulfate. Because some amount of Nitrogen undergoes side reactions and results in cyclic compounds that contain Nitrogen. To release nitrogen from these cyclic compounds, Sodium thiosulfate is added to the final mixture.

II. Selenium based catalyzes

Selenium-based catalyzes have a moderate reaction rate. But when selenium-based catalyzes are used, some amount of Nitrogen is converted to Nitric acid.

III. Copper-based catalyzes

The reaction rate is very low when the Copper-based catalyzes are used. Also, all the Nitrogen is not converted into Ammonium sulfate. Only a 98% recovery can be expected. Here, CuSO₄ is formed. Therefore, a light blue color solution is obtained.

After the digestion is completed, a colorless solution is obtained. If there is color present, that indicates an incomplete digestion.

02. Distillation

The solution of Ammonium sulfate that is obtained from the digestion process is transferred to a distillation unit. Before the distillation starts, 8M NaOH solution is added to the above solution. Then the solution is heated using steam. When heating Ammonium sulfate (NH₄OH) is reacted with Sodium Hydroxide (NaOH) as follows.

In this reaction, Ammonia gas (NH₃) is released. This ammonia is trapped in 4% Boric acid.

03. Titration

The collected solution from the distillation is then titrated with 0.2 M HCl solution in the presence of a Tashiro indicator.

Tashiro indicator is a mixture of two indicators. It is composed of 0.1% methylene blue and 0.03% methyl red dissolved in ethanol or methanol. At the endpoint, it gives a pinkish-red endpoint.

C. Calculations

Where,

• V = Burette reading (Volume of HCl)
• N = Normality of the HCl solution
• M = Molar mass of Nitrogen
• W = Mass of the sample

Therefore, the above equation can be arranged as follows.

Use cases of the Kjeldhal method in Nitrogen content analysis

• Analysis of proteins in a food sample - Proteins are made from polymerizing amino acids that contain nitrogen. So analyzing nitrogen content using the Kjeldhal method will provide the amino acid content in the sample. This is not a direct method to calculate the protein content so to get the protein content the result has to be multiplied by a conversion factor.
• Analyzing the "Total nitrogen content" of soil
• In agrochemical analysis
• Water and wastewater analysis