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Kjeldahl Method Questions

The Kjeldahl method first came into existence in 1883 and was developed by a Danish chemist named Johan Kjeldahl. This method was specifically developed for determining the nitrogen contents in organic and inorganic substances.

In today’s world, Kjeldahl nitrogen determinations are used on several samples like wastewater, soil, fertilizers, meat, feed, grain, and many other substances. The method is also used for estimating the protein content in foods.

Definition: The method entails converting all nitrogen in a weighed sample to ammonium sulphate via sulfuric acid digestion, alkalizing the solution, and determining the resulting ammonia via distillation into a measured volume of standard acid, the excess of which is determined via titration.

Kjeldahl Method Chemistry Questions with Solutions

Q1. Ammonia evolved from 0.75g of sample neutralized 10ml. of 1M H2SO4 in Kjedahl’s method for estimating nitrogen present in soil samples. The nitrogen percentage in the soil is:

a.) 37.33

b.) 445.33

c.) 35.33

d.) 43.33

Correct Answer- (a.) 37.33

Q2. Copper sulphate acts as ____ in Kjeldahl’s nitrogen estimation method.

a.) Oxidizing agent

b.) Silver spiral

c.) Catalytic agent

d.) Hydrolyzing agent

Correct Answer- (c.) Catalytic agent

Q3. Kjeldahl’s method is used to estimate nitrogen. For the complete neutralization of NH3 gas, 2.8 gm of an organic compound required 20 millimoles of H2SO4. The nitrogen percentage in the sample is:

a.) 20%

b.) 10%

c.) 40%

d.) 30%

Correct Answer- (a.) 20%

Q4. Kjeldahl’s method cannot be used for:

a.) PhNO2

b.) Azobenzene

c.) Pyridine

d.) All of the above

Correct Answer- (d.) All of the above

Explanation- Kjeldahl’s method cannot be used to determine nitrogen in compounds with nitro groups, azo groups, or nitrogen present in rings because the nitrogen in these compounds cannot be converted to ammonium sulphate under the conditions of this method.

Q5. Which of the following raises the boiling point of H2SO4 while CuSO4 catalyzes the reaction.

a.) K2SO4

b.) Na2SO4

c.) BaSO4

d.) All of the above

Correct Answer- (a.) K2SO4

Q6. What is the principle of the Kjeldahl method?

Answer. Johann Kjeldahl, a brewer, invented the Kjeldahl method in 1883. The protocol is based on the idea that strong acid aids in food digestion by releasing nitrogen, which can be measured using a suitable titration technique.

Q7. Which indicator is used in the Kjeldahl method?

Answer. The Tashiro indicator, which is added to the boric acid solution, is one of the most commonly used indicator solutions. The endpoint is represented by a grey / pink colour. The analyst manually adds the acid titrant solution to the boric acid containing distilled ammonia using a glass burette.

Q8. Why boric acid is used in the Kjeldahl method?

Answer. The ammonia gas is captured by the boric acid, resulting in the formation of an ammonium-borate complex. The colour of the receiving solutions changes as the ammonia accumulates. The boric acid method has the advantage of requiring only one standard solution for the determination and having a long shelf life.

Q9. What are the limitations of the Kjeldahl method?

Answer. Only nitrogen bound to organic components (proteins, amino acids, nucleic acids) and ammonium are measured in this method. This method is incompatible with compounds containing nitrogen in azo and nitro groups or rings (quinoline, pyridine, nitrate, nitrite, etc.).

Q10. What are the steps of the Kjeldahl method?

Answer. The Kjeldahl method consists of three major steps: digestion, distillation, and titration.

Q11. Why is the Kjeldahl method important?

Answer. For its universality, precision, and reproducibility, the Kjeldahl method has become the internationally recognized method for estimating the protein content in foods, and it serves as the standard against which all other methods are judged. It is also used in the analysis of soils, wastewaters, fertilizers, and other materials.

Q12. 29.5 mg of an organic compound containing nitrogen was digested according to Kjeldahl’s method and the evolved ammonia was absorbed in 20 mL of 0.1M HCl solution. The excess of the acid required 15 mL of 0.1 M NaOH solution for complete neutralization. What will be the percentage of nitrogen in the compound?

Answer. Moles of HCl reacting with ammonia = (moles of HCl absorbed) – (moles of NaOH solution required)

= (20 x 0.1 x 10–3) – (15 x 0.1 x 10–3)

= Moles of NH3 evolved

= Moles of nitrogen in organic compound

Hence,

Weight of nitrogen in organic compound = 0.5 x 10–3 x 14

We get,

= 7 x 10–3 g

% of nitrogen = (7 x 10–3)/(29.5 x 10–3) x 100

We get,

= 23.7%

Therefore, the percentage of nitrogen in the compound is 23.7%

Q13. How does the Kjeldahl Method work?

Answer. Johann Kjeldahl, a brewer, invented the Kjeldahl technique in 1883. The procedure is based on the theory that strong acid aids in food digestion by releasing nitrogen that can be measured using an accurate titration technique.

In the traditional Kjeldahl method, the proteins are ‘digested’ in sulfuric acid with a catalyst (selenium, mercury, or copper salts). An acid deposit of ammonium sulphate is formed, which is then dissolved in water. The excess acid is back-titrated with normal NaOH to determine the total ammonia and, thus, nitrogen.

Q14. Explain the very first step of this method.

Answer. In this method, a certain substance or sample is heated in the presence of sulphuric acid. The acid breaks down the organic substance via oxidation, and reduced nitrogen in the form of ammonium sulphate is liberated. Potassium sulphate is usually added to increase the boiling point of the medium. Catalysts like mercury, selenium, copper, or ions of mercury or copper are also used in the digestion process. The sample is fully decomposed when we obtain a clear and colorless solution.

Organic compound + H2SO4 → [digest]Cu2+(NH4)2SO4

Q15. What is the point of including H2SO4 in the first step of the Kjeldahl method?

Answer. This method is entirely dependent on a high boiling point. The addition of salt and catalyst greatly accelerates digestion. Furthermore, adding potassium sulphate raises the boiling point of sulfuric acid even more. Finally, the catalyst’s action increases the speed and efficiency of the digestion procedure, thus improving the analytical method.

Practise Questions on the Kjeldahl Method

Q1. Kjeldahl’s method produced 0.17 g of NH3 from 0.28 g of a nitrogenous compound during the estimation of nitrogen present in an organic compound. What is the nitrogen content of the organic compound?

a.) 5

b.) 30

c.) 50

d.) 80

Q2. Kjeldahl’s method employs the following acid:

a.) conc. HNO3

b.) conc. H2SO4

c.) dil.HNO3

d.) dil. H2SO4

Q3. State True or False.

Kjeldahl’s method for the estimation of nitrogen is applicable to all types of nitrogen-containing organic compounds.

Q4. Explain the distillation process in Kjeldahl’s method.

Q5. Explain the three-step procedure of Kjeldahl’s method.

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