Question

Differentiate between the principle of estimation of nitrogen in an organic compound by (i) Dumas method and (ii) Kjeldahl’s method.

Answer 1

In Dumas's method, a known quantity of nitrogen-containing organic compound is heated strongly with an excess of copper oxide in an atmosphere of carbon dioxide to produce free nitrogen in addition to carbon dioxide and water. The chemical equation involved in the process can be represented as
 
The traces of nitrogen oxides can also be produced in the reaction, which can be reduced to dinitrogen by passing the gaseous mixture over a heated copper gauge. The dinitrogen produced is collected over an aqueous solution of potassium hydroxide. The volume of nitrogen produced is then measured at room temperature and atmospheric pressure.
On the other hand, 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 sulfate. It is then distilled with an 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
Organic compound \(\xrightarrow[]{Conc.\,H_2SO_4}\) (NH₄)₂SO₄
(NH₄)₂SO₄+2NaOH \(\xrightarrow[]{}\) Na₂SO₄+2NH₃+2H₂O
2NH₃+H₂SO₄ \(\xrightarrow[]{}\) (NH₄)₂SO₄
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 is also not applicable to compounds containing nitro and azo groups.