Kjeldahl Method

Introduction

The determination of nitrogen content or determining protein content in samples such as food was a major issue some years ago. The method that already existed some years ago was not so accurate. That is a proper method was not discovered at that time. Later the Danish chemist Johan Gustav Christoffer Thorsager Kjeldahl developed a laboratory method for the determination of Nitrogen because the existing method at that time was very much inaccurate. The method he introduced was given the name Kjeldahl method after him. The method he first introduced contains two steps that are distillation and back titration.

What is the Kjeldahl Method?

The Kjeldahl method is a quantitative method used in the determination of the amount of Nitrogen present in particular substances. Kjeldahl’s digestion is also its other name. This name is given because of the reason that the procedure contains a digestion step. The use of a catalyst will increase the rate of digestion. The catalyst used was mercuric oxide. It has high efficiency but it is a toxic chemical. Later cupric sulfate catalysts were introduced but the efficiency was low. Now Titanium dioxide is used as a catalyst. This method measures the nitrogen content present inorganic and organic matters. It can be used for the determination of protein content too.

Roshan220195, Kjeldahl digestion, CC BY-SA 3.0

The Procedure involved in the Kjeldahl Method

The Kjeldahl method is a large procedure for the determination of protein content. The important steps involved in the Kjeldahl method are discussed below.

• Digestion − The first step that is involved in the Kjeldahl method is a digestion process in which the sample is heated at a high temperature of 360–410°C in the presence of concentrated sulfuric acid. Organic samples become destroyed or digested in the process with the help of acid. And which further results in the formation of ammonium sulfate.

• Distillation − The ammonium sulfate formed after the digestion process gets converted to ammonia, NH₃ by boiling followed by condensation. This process is initiated by the addition of an excess of a base such as sodium hydroxide. Bubbles of ammonia are formed as a result of this process.

Roshan220195, Kjeldahl's distillation, CC BY-SA 3.0

• Titration − The ammonia concentration can be estimated and thereby the amount of nitrogen content can be obtained. And is done by using titration. Acid-base titration can be used if the boric acid is used in the process of digestion. Back titration is usually employed if the acid is used as hydrogen chloric acid or sulphuric acid. A strong base such as sodium hydroxide is used for neutralizing the solution. From the difference between the amount of acid and sodium hydroxide, the amount of ammonia present in the sample can be easily obtained. And thereby the amount of Nitrogen. Tashiro's indicator is used in this process.

Applications of the Kjeldahl Method

The Kjeldahl method is an important method with many applications in different sectors. And is mainly because of its accuracy and precision. The quantity of Nitrogen obtained was very accurate and the result has high reproducibility too. Some of the applications of the Kjeldahl method are discussed below.

• The method is used for the chemical analysis of sewage treatment plants. Since the Kjeldahl method easily captures data related to soil and water.

• It is widely used in the food industry for analyzing the proteins present in food samples.

• It is also helpful in the pharmaceutical industry for determining nitrogen content.

• Organically bound nitrogen content can be easily estimated with the help of this method.

• It is also useful for the estimation of nitrogen in animal feed and dairy products.

• The samples with high weight can be estimated easily with the help of this method.

Kjeldahl Method Reaction

The first step is the digestion or the decomposition of an organic sample with the help of sulphuric acid. The reaction involved here is the oxidation of carbon,

$$\mathrm{C + 2H_2 SO_4 → CO_2 + 2 SO_2 + 2 H_2 O}$$

$$\mathrm{S + 2H_2 SO_4→ 3SO_2 + 2H_2 O}$$

The next step involves the distillation process in which the conversion of the ammonium salt to ammonia is taking place.

$$\mathrm{(NH_4 )_2 SO_4 + 2NaOH → Na_2 SO_4 + 2H_2 O + 2NH_3}$$

In the titration process, ammonia will participate in the reaction. If hydrochloric acid is used the reaction will be,

$$\mathrm{NH_3 + HCl → NH₄Cl}$$

And if boric acid is used the reaction becomes,

$$\mathrm{B(OH)_2 +H_2 O + Na_2 CO_3 → NaHCO_3 + CO_2 + H_2 O }$$

So these are the reactions taking place at the Kjeldahl method.

Kjeldahl Method Formula

The formula from which the amount of Nitrogen can be calculated is given below.

Estimation of Nitrogen using the Kjeldahl method= 1.4V × NW

N=normality of standard acid.

W=weight of sample

V=volume of acid used in the titration process.

Limitations of the Kjeldahl Method

Even though it is an efficient method for the estimation of nitrogen present in an organic and inorganic compound. The method also has certain limitations. Some of the limitations are,

• The method is not suitable for the determination of Nitrogen present in compounds such as pyridine, pyrrole, etc. This is because they are aromatic compounds so the conversion of nitrogen present in them to ammonium sulfate is not possible.

• Estimation of nitrogen is only possible for compounds such as proteins, nucleic acids, etc.

Conclusion

The Kjeldahl method is an important method that can be used for the estimation of nitrogen content in organic and inorganic compounds. It is an efficient method that has been developed by overcoming other ineffective methods. There exist mainly three steps in the determination of Nitrogen content. They are digestion, distillation, and titration. The evolution of ammonia after this process is the main technique that can then be used for the determination of nitrogen. This method is common in many areas such as food, pharmaceutical, sewage industry, etc. The method is very efficient because it is very accurate, precise, and has high reproducibility.

FAQs

1. What is total Kjeldahl nitrogen?

This term is associated with the Kjeldahl method, which determines the amount of nitrogen that is present in the organically bound nitrogen and ammonium present in samples. This is mainly useful in sewage treatment.

2. What is total nitrogen?

Total nitrogen is a different term from total Kjeldahl nitrogen, which includes nitrite and nitrate count in addition to organically bound nitrogen and ammonium in samples.

3. What is the use of potassium sulfate in the Kjeldahl method?

Potassium sulfate is added during the Kjeldahl method to raise the boiling point of sulfuric acid used.

4. What is Tashiro's indicator?

Tashiro's indicator is commonly called the mixed indicator since it is a mixture of two indicators, methyl red, and methylene blue. And is used in the titration step of the Kjeldahl method.

5. Which reagent is used in detecting protein?

Biuret reagent is used for the detection of proteins. It will cause a change in colour from pink to purple.