Kjeldahl Method in Pharmaceutical Analysis

Experts use the Kjeldahl method for determining the nitrogen content in samples. These samples can be both organic and inorganic. The Kjeldahl method has remained relevant for over a century for determining nitrogen in a broad category of samples. This Kjeldahl nitrogen determination takes place in drugs, chemicals, meals, drinks, cereals, forages, soils, and other samples. As such, there is tremendous usage of the Kjeldahl Method in pharmaceutical analysis. Keep on reading to know about the application of the Kjeldahl Method in pharmaceutical analysis. Here, we shall also see how nitrogen estimation is performed through the Kjeldahl method. In the end, we see Kjeldahl’s method and its limitations.

Kjeldahl Method in Pharmaceutical Analysis

One of the five major elements that are available in organic materials is nitrogen. Johan Kjeldahl, a Danish chemist, was the first one to recognise this. He then began to use it as a method for determining the amount of protein in organic samples. These organic samples were collected from various types of organisms. 

Johan Kjeldahl kept on improving this method for the determination of nitrogen amount in mixtures of substances. These substances involved:

• Ammonium salts
• Nitrate
• Organic nitrogen compounds

This method is what is known as the Kjeldahl method and it has wide usage in pharmaceutical analysis.

In this method, the most important basis is the oxidation of the organic compound. This oxidation takes place with strong sulfuric acid. When oxidation takes place, the contained carbon is converted to carbon dioxide. Also, the conversion of hydrogen takes place into water.

The Kjeldahl method of pharmaceutical nitrogen analysis has been accepted globally as the standard for measuring the protein content in a huge category of materials. These materials can be:

• Human food 
• Animal food
• Fertiliser
• Wastewater 
• Fossil fuels
• Others 

How is nitrogen estimation performed through the Kjeldahl method?

There are three sequential steps in the Kjeldahl method. The Kjeldahl Method in pharmaceutical analysis strictly involves these three steps. The sequence of these steps is as follows:

Digestion: The first step involves the digestion of the sample in strong sulfuric acid. This process takes place in the presence of a catalyst. This way, the amine nitrogen gets converted to ammonium ions.

Distillation: In the second step, the conversion, heating, and distillation of the ammonium ions take place into ammonia gas. Afterwards, the ammonia gas goes into a trapping solution and becomes an ammonium ion after dissolving in it.

Titration: In the third step, the determination of the trapped ammonia takes place by titration with a standard solution. Afterwards, the experts are able to make calculations. 

Kjeldahl Method Reactions

The Kjeldahl Method in pharmaceutical analysis involves the following reactions:

Digestion:

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

Distillation: 

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

And, NH3 + HCl → NH₄Cl

Titration:

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

The formula for the Estimation of Nitrogen in Kjeldahl’s Method 

Percentage of N = 1.4VN​/W

Here, 

• V= Volume of acid that is used
• N= Normality of acid 
• W= Weight of the involved substance

Kjeldahl’s Method and its Limitations

The Kjeldahl method is an internationally recognised method due to its high accuracy, universality, and good reproducibility. In spite of these benefits, the Kjeldahl method suffers from certain disadvantages. These disadvantages are as follows:

• It doesn’t provide a true protein measure because all nitrogen is not in the protein form.
• Different types of proteins require different correction factors in the Kjeldahl method. This is due to the different sequences of amino acids.
• Using concentrated sulfuric acid at high temperatures can be a matter of significant hazard. 
• The Kjeldahl method is a lengthy process and takes a lot of time to be completed.
• The Kjeldahl method cannot be applied to nitrogen-containing compounds pertaining to nitro and azo groups.

Conclusion

Experts use the Kjeldahl method for determining the nitrogen content in samples. This leads to efficient pharmaceutical analysis. This assists in the determination of nitrogen amount in mixtures of substances. The Kjeldahl method in pharmaceutical analysis has received global acceptance as the standard for measuring the protein content in a wide category of materials. Such materials are foods, fossil fuels, wastewater, fertiliser, etc. The Kjeldahl Method involves three sequential steps- digestion, distillation, and titration. There are certain limitations of Kjeldahl’s method.