Question

Differentiate between the principle of estimation of nitrogen in an organic compound by

(i) Dumas method and
(ii) Kjeldahls method

Answer 1

Dumas method:
A known quantity of compound containing nitrogen is heated with excess copper oxide in carbon dioxide atmosphere to form free nitrogen in
addition to carbon dioxide and water.
$C_x{H}_y{N}_z+(2x+y/2)CuO\to xC{O}_2+y/2H_{2}O+z/2N_2+(2x+y/2)Cu$
Nitrogen is collected over an aqueous solution of KOH. Volume of nitrogen produced is measured at room temperature and atmospheric pressure.
Kjeldahl's method: A known amount of compound containing nitrogen is heated with concentric sulphuric. The nitrogen present in the compound is quantitatively converted into ammonium sulphate and distilled with excess NaOH. Ammonia evolved is passed into known volume of sulphuric acid. Unreacted acid is estimated by volumetric analysis and the amount of ammonia produced is determined. The percentage of nitrogen in the compound is estimated. This method is not applicable to compounds containing nitro and azo groups and compounds in which nitrogen is part of ring structure.