[Chemistry Class Notes] Kjeldahl Method Pdf for Exam

This method was specifically developed by Danish chemist, Johan G.C.T. Kjeldahl, in 1883 to determine the nitrogen contents in organic and inorganic substances  (like foodstuffs, fertilizers, wastewater, soil, feed, grain, and other substances). This method is also used for estimating the protein content in food.

 

Kjeldahl method is an analytical chemistry method that helps in the quantitative measurement and determination of nitrogen present in organic substances as well as inorganic compounds. The method was first developed by Johan Kjeldahl in the year 1883. This process plays an important role in the method of analyzing proteins. This method was developed to determine the nitrogen contents in organic and inorganic substances  (like foodstuffs, fertilizers, wastewater, soil, feed, grain, and other substances). This method is also used for estimating the protein content in food.

Applications of Kjeldahl Method

This method is used internationally as it is subjected to many applications which include precision and reproducibility. It is not however able to measure the true protein content. But still, it has a number of advantages in the field of chemical analysis.

The major applications of Kjeldahl nitrogen are as follows:

1. Total kjeldahl nitrogen:

The total nitrogen found in organic substances is known as total Kjeldahl nitrogen. It is mostly used in sewage treatment plants effluent as it is helpful in the chemical analysis of soil water or wastewater. This method is the most accurate, versatile and efficient strategy.

Till now this particular treatment is used in many areas including treatment plants and as a means of monitoring plant operations.

2. Conversion factors:

The total Kjeldahl nitrogen depends on the type of protein also. To work with total Kjeldahl nitrogen, it’s important to note what fraction of the protein is composed of nitrogenous amino acids.

The range of conversion is very low. For food, it generally ranges from 6.38 for meat, egg, maize; it is 6.25 for sorghum; it is 5.83 for rice, and so on.

3. Sensitivity :

The Kjeldahl method is not at all sensible in its original version. It needs to be made sensible by different processes which include potentiometric titration, zone capillary electrophoresis, and ion chromatography.

The Procedure involved in the Kjeldahl Method

The working principle of Kjeldahl analysis is three steps process as described below:

1. Digestion: 

The organic sample provided or taken is firstly treated with a concentrated acid solution, mostly H₂SO₄. The solution is boiled at an extremely high temperature. The acid solution digests the sample to produce ammonium sulfate solution.

2. Distillation:

The particular process is a combination of boiling and condensation. An excess of base is added to the formed solution to convert the ammonium sulfate solution to NH3 gas.

3. Titration:

To finally quantify the nitrogen present in the sample, the obtained product from the previous process is titrated in order to give the final required results.

This particular method left a huge impact. Furthermore, studies have improved the quality of the method. Various scientific associations approved this method to be one of the most versatile methods. These associations include AOAC(Association Official Analytical Chemists), AACC(Association of American Cereal Chemists), AOCS(American Oil Chemist Society), EPA(Environmental Protection Agency), ISO(International Standards Organization). The following are the names of some of the associations which lead the scientific equipment for conducting the Kjeldahl method which is known as VELP.

Kjeldahl Method Reaction

1. Digestion:

Organic (C, H, N) + H₂SO₄ → digest Cu²⁺ + (NH₄)₂SO₄

2. Distillation: 

(NH₄)₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O + 2NH₃

And, NH₃ + HCl → NH₄Cl

3. Titration:

B(OH)₂ + H₂O + Na₂CO₃ → NaHCO₃ + CO₂ + H₂O 

Note: Same steps are involved for protein estimation by the Kjeldahl method.

Kjeldahl Method Formula

The equivalent weight of NH₃ is 17g/eq. And, 14 gm of Nitrogen is contained one equivalent weight of NH₃. So, the percentage of nitrogen can be determined using the following formula:

Kjeldahl method of nitrogen estimation = 1.4V × NW

Where,

W = Weight of the sample used (in grams)

V = Acid used in titration (in ml)

N = Normality of standard acid

Limitations of Kjeldahl Method

This method measures only nitrogen bound to organic components (proteins, amino acids, nucleic acids) and ammonium in the sample. This method is not suitable for compounds containing nitrogen in azo and nitro groups or in rings (quinoline, pyridine, nitrate, and nitrite, etc). In these compounds, the nitrogen cannot be converted to ammonium sulfate following the Kjeldahl method.

Did You Know?

Protein quantification (total protein content in a sample) is necessary to understand the total protein content in a formulated product. Accurate protein quantification is important because a range of other evaluations require precise total protein content results in order to generate accurate data.