Question

In Kjeldahl's method, nitrogen present is estimated as:
A) ${N_2}$
B) $N{H_3}$
C) $N{O_2}$
D) None of these

Answer 1

Hint:‌ To solve this question, we must first understand the whole concept of Kjeldahl’s Method of estimation of Nitrogen. Then we need to assess the mechanism and reaction involved in the method and then only we can conclude the correct answer.

Complete step by step answer:
Before we move forward with the solution of this given question, let us first understand some basic concepts:
The Kjeldahl method or Kjeldahl digestion in analytical chemistry is a method for the quantitative determination of nitrogen contained in organic substances plus the nitrogen contained in the inorganic compounds ammonia and ammonium.
Overall, the Kjeldahl method is divided into three main steps. The method has to be carried out in proper sequence. The steps include digestion, distillation, and titration.
1. Digestion: 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.
\[Organic{\text{ }}compound + {H_2}S{O_4} \to \left[ {digest} \right]C{u_2} + {\left( {N{H_4}} \right)_2}S{O_4}\]
2. Distillation: The distillation of the solution now takes place and a small quantity of sodium hydroxide is added to convert the ammonium salt to ammonia.
\[{\left( {N{H_4}} \right)_2}S{O_4} + 2NaOH \to \Delta N{a_2}S{O_4} + 2{H_2}O + 2N{H_3}{\text{ }}N{H_3} + HCl \to N{H_4}Cl\]
3. Titration: The amount of ammonia or the amount of nitrogen present in the sample is then determined by back titration. As the ammonia dissolves in the acid trapping solution some HCl is neutralized. The acid that is left behind can be back titrated with a standard solution of a base such as NaOH or other bases.
\[B{\left( {OH} \right)_2} + {H_2}O + N{a_2}C{O_3} \to NaHC{O_3} + C{O_2} + {H_2}O\]
The Kjeldahl's method is based on the fact that nitrogen of an organic compound is quantitatively converted to \[{\left( {N{H_4}} \right)_2}S{O_4}\] on heating with ${H_2}S{O_4}\,\,(conc.)$ . The \[{\left( {N{H_4}} \right)_2}S{O_4}\] is then treated with \[KOH\] to liberate \[N{H_3}\] , which is absorbed in ${H_2}S{O_4}\,\,(conc.)$ to obtain \[\% {\text{ }}of{\text{ }}N\] .
So, Thus in Kjeldahl's method, nitrogen present is estimated as \[N{H_3}\] .
So, clearly we can conclude that the correct answer is Option B.

Note:While the Kjeldahl method of nitrogen analysis has become the worldwide standard, this method is not suitable for compounds containing nitrogen in azo and nitro groups or in rings (quinoline, pyridine, etc.). In these cases, the nitrogen cannot be converted to ammonium sulphate by following the Kjeldahl method.