Question

Kjeldahl’s method cannot be used to estimate nitrogen for which of the following compounds.
A. ${C_6}{H_5}N{H_2}$
B. $N{H_2} - CO - N{H_2}$
C. ${C_6}{H_5}N{O_2}$
D. $C{H_3}C{H_2} - C \equiv N$

Answer 1

The Kjeldahl method 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. Other forms of inorganic nitrogen, for instance nitrate, are not included in this measurement.

Complete Step By Step Answer:
The Kjeldahl method is used for estimation of nitrogen atoms. The Kjeldahl method consists of three steps:
1. The sample is first digested in strong sulfuric acid in the presence of a catalyst, which helps in the conversion of the amine nitrogen to ammonium ions,
2. The ammonium ions are then converted into ammonia gas, heated and distilled. The ammonia gas is led into a trapping solution where it dissolves and becomes an ammonium ion once again,
3. Finally the amount of the ammonia that has been trapped is determined by titration with a standard solution, and a calculation made.
The kjeldahl method is not applicable to nitro and diazo groups present in the ring, as nitrogen atoms cannot be converted into ammonia sulphate under the reaction condition.
So, the correct answer is (C) ${C_6}{H_5}N{O_2}$ .

Note:
The Kjeldahl method's precision and reproducibility have made it the internationally recognized method for estimating the protein content in foods and it is the standard method against which all other methods are judged. It is also used to assay soils, waste waters, fertilizers and other materials. It does not, however, give a measure of true protein content, as it measures non-protein nitrogen in addition to the nitrogen in proteins.