Differentiate between the principle of estimation of nitrogen in an organic compound by (i) Dumas method and (ii) Kjeldahl’s method.
(i) Dumas method: The nitrogen-containing organic compound is heated with copper oxide in an atmosphere of carbon dioxide to yield free nitrogen gas, along with carbon dioxide and water. Traces of nitrogen oxides are reduced to nitrogen over heated copper gauze. The mixture is collected over an aqueous potassium hydroxide solution, which absorbs carbon dioxide, allowing the volume of nitrogen gas to be determined.
(ii) Kjeldahl’s method: The compound is heated with concentrated sulphuric acid, converting nitrogen into ammonium sulphate. The mixture is then heated with excess sodium hydroxide to liberate ammonia gas. This ammonia is absorbed in an excess of standard sulphuric acid solution. The amount of nitrogen is determined by estimating the unreacted sulphuric acid left, found by titrating it with a standard alkali solution.
Explanation
The Dumas method relies on the volumetric estimation of free nitrogen gas evolved when the organic compound is oxidized with copper oxide. In contrast, Kjeldahl's method is a titrimetric method where nitrogen is first converted to ammonium sulphate using concentrated sulphuric acid, then released as ammonia, and finally measured by acid-base titration.