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Question
Differentiate between the principle of estimation of nitrogen in an organic compound by Kjeldahl’s method.
Solution
In Kjeldahl’s method, a known quantity of nitrogen containing organic compound is heated with concentrated sulphuric acid. The nitrogen present in the compound is quantitatively converted into ammonium sulphate. It is then distilled with excess of sodium hydroxide. The ammonia evolved during this process is passed into a known volume of H2SO4. The chemical equations involved in the process are
\[\ce{Organic compound ->[Conc. H2SO4] (NH4)2 SO4}\]
\[\ce{(NH_4)_2SO_4 + 2NaOH -> Na_2SO_4 + 2NH_3 + 2H_2O}\]
\[\ce{2NH_3 + H_2SO_4 -> (NH_4)_2 SO_4}\]
The acid that is left unused is estimated by volumetric analysis (titrating it against a standard alkali) and the amount of ammonia produced can be determined. Thus, the percentage of nitrogen in the compound can be estimated. This method cannot be applied to the compounds, in which nitrogen is present in a ring structure, and also not applicable to compounds containing nitro and azo groups.
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