Kjeldahl's Method: Principle, Reaction, Formula and Limitations

Since being discovered in 1883 by Danish chemist Johan Kjeldahl, the Kjeldahl method has stood the test of time in staying fundamental in analytic techniques in chemistry. The principal applications include nitrogen determination in organic compounds, which has large applications in the industries of agriculture, food science, and environmental monitoring. Nitrogen measurement is so important because it forms part of amino acids, proteins, and nucleic acids; hence, it is essential to life itself.

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This Story also Contains

1. Vcd of the Kjeldahl Method
2. Dimensions of the Kjeldahl Method:
3. Applications in Real-Life and Relevance
4. Some Solved Examples
5. Summary

The Kjeldahl method is an indispensable tool in life, more so in food: safety and quality control. Food manufacturers apply it in measuring protein levels, which is quite a critical application for nutrition labeling and health compliance. It is used in agriculture to estimate the nitrogen content in fertilizers. The method will be explained in this article from its principles and procedures. Then, we would be able to point out the many different aspects of nitrogenous compounds that can be analyzed through this process.

Vcd of the Kjeldahl Method

The working principle involved in the Kjeldahl method includes digestion, whereby organically bound nitrogen is converted to ammonium sulfate using concentrated sulfuric acid. This simply refers to heating the sample in the presence of sulfuric acid. The organic matter is broken down with the release of nitrogen in the form of ammonium sulfate. After the solution thus formed has been made alkaline, liberated ammonia is distilled and collected in a standard acid solution. The amount of ammonia is then ascertained by titration, whereby the original nitrogen content can be calculated in the given sample.

Dimensions of the Kjeldahl Method:

The Kjeldahl method can be used for the determination of nitrogen in substances ranging from simple organic and inorganic compounds containing ammonium/ammonia to complex matrices like soil, wastewater, and animal feed. The reason for its extended use in the estimation of protein content of food is the direct relationship between the content of nitrogen and protein. The method, however, has its drawbacks, since the Kjeldahl process cannot be applied to compounds containing nitrogen in azo and nitro groups or rings because such forms of nitrogen are not convertible into ammonium sulfate.

The compound containing nitrogen is heated with concentrated sulphuric acid. Nitrogen in the compound gets converted to ammonium sulfate. The resulting acid mixture is then heated with an excess of sodium hydroxide. The liberated ammonia gas is absorbed in an excess of a standard solution of sulphuric acid. The amount of ammonia produced is determined by estimating the amount of sulphuric acid consumed in the reaction. It is done by estimating unreacted sulphuric acid left after the absorption of ammonia by titrating it with a standard alkali solution. The difference between the initial amount of acid taken and that left after the reaction gives the amount of acid reacted with ammonia.

Organic compound $+\mathrm{H}_2 \mathrm{SO}_4 \longrightarrow\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4 \xrightarrow{2 \mathrm{NaOH}} \mathrm{Na}_2 \mathrm{SO}_4+2 \mathrm{NH}_3+2 \mathrm{H}_2 \mathrm{O}$

Kjeldahl method is not applicable to compounds containing nitrogen in nitro and azo groups and nitrogen present in the ring (e.g. pyridine) as nitrogen of these compounds does not change to ammonium sulfate under these conditions.

Applications in Real-Life and Relevance

The applications of Kjeldahl analysis include agriculture, food science, environment monitoring, and clinical analysis. In agriculture, it is used in determining the nitrogen content in fertilizers with respect to the required standards. In food science, the Kjeldahl method evaluates protein content so that proper nutritional labeling and quality control in foodstuffs can be done. Environmental monitoring agencies implement the Kjeldahl technique for total Kjeldahl nitrogen determination in wastewater and soil samples, an important parameter for both water quality and soil fertility assessments. In clinical analysis, the Kjeldahl method is used in the determination of nitrogen content in biological samples such as urine and blood samples for diagnosis.

Recommended topic video on (Kjeldahl's Method )

Some Solved Examples

Question;1 Which of the following compounds will be suitable for Kjeldahl’s method for nitrogen estimation?

1) Benzene

2) Nitrobenzene

3) Aniline (correct)

4) Pyridine

Solution:
Nitrogen present in aromatic rings, nitro compounds, and azo compounds are not estimated by Kjeldahl’s method because these nitrogens are not converted into ammonium ions under the reaction conditions. Thus, only aniline will respond to the test among the given options. Hence, the correct answer is Option (3).

Question:2 Kjeldahl's method can be used for:

1) Nitrobenzene $\left(\mathrm{PhNO}_2\right)$
2)Phenylamine $\left(\mathrm{PhNH}_2\right)$ (correct)

3) Azobenzene

4) Pyridine

Solution:

Kjeldahl's Method -

Kjeldahl's method is a method for the quantitative determination of nitrogen contained in organic substances.

However, Kjeldahl's method cannot be used for nitrogen determination of compounds having nitro-group or azo-group or nitrogen present in rings as the nitrogen of these compounds cannot be converted to ammonium sulfate $\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4$ under the conditions of this method.

Hence, the answer is the option (2).

Question 3: Kjeldahl's method cannot be used to estimate nitrogen for which of the following compounds?

1) Nitrobenzene $\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{NO}_2\right)$ (correct)
2) Ethylamine
3) Propionitrile $\left(\mathrm{CH}_3 \mathrm{CH}_2-\mathrm{C} \equiv \mathrm{N}\right)$
4) Aniline $\left(\mathrm{C}_6 \mathrm{H}_5 \mathrm{NH}_2\right)$

Solution:
Kjeldahl's method is not applicable to compounds containing nitrogen in nitro and azo compounds and nitrogen present in rings (e.g., pyridine). Hence, Option (1) is correct.

Question:4 Nitrogen can be estimated by Kjeldahl's method for which of the following compounds?

1) Benzene

2) Phenylamine (correct)

3) Azobenzene

4) Pyridine

Solution:
The Kjeldahl method is not applicable to compounds containing nitrogen in the nitro group, azo groups, and nitrogen present in the ring (e.g., pyridine) as the nitrogen of these compounds does not change to ammonium sulfate under these conditions. Therefore, the correct option is (2).

Summary

It is such a versatile technique that it has revolutionized the way nitrogen content, whether in food, fertilizers, or biological samples, can be measured. Only by understanding its principles and applications will a deep understanding of its importance to the scientific community and its impact on our lives be reached. As noted earlier, the Kjeldahl method is not only a laboratory process but has far-reaching implications for farming practices, food safety regulations, and environmental risk assessments.