The principle of estimation of nitrogen by Dumas method and that by Kjeldahl’s method may be summarized as :
(i) Dumas method : The weighed amount of organic compound is heated with cupric oxide in the atmosphere of CO2 to get free nitrogen along with CO2 and H2O. Some oxides of nitrogen are also formed which are reduced to free nitrogen by passing over a hot reduced copper gauze.
N + CuO → N2 + small amount of oxides of nitrogen
Oxides of nitrogen + Cu → CuO + N2
Nitrogen gas thus formed is collected over conc. KOH solution which absorbs all other gases except N2. Volume of nitrogen collected can be noted and thus percentage of nitrogen can be calculated.
Percentage of nitrogen =
Volume of N2 collected at room temperature must be converted to volume at NTP.
(ii) Kjeldahl’s method : In this method the weighed compound is heated with conc. H2SO4 in presence of K2SO4 and CuSO4 (catalyst), so that nitrogen is quantitatively converted into ammonium sulphate. Ammonium sulphate thus obtained is boiled with excess NaOH solution to liberate ammonia gas, which is absorbed in a known excess of a standard acid like H2SO4 or HCl. The amount of ammonia produced is determined by estimating the amount of sulphuric acid consumed in the reaction. It is done by estimating unreacted sulphuric acid left after the absorption of ammonia by titrating it with standard alkali solution. The difference between the initial amount of acid taken and that left after the reaction gives the amount of acid reacted with ammonia.
% of nitrogen =
% of nitrogen =
This is a simple and convenient method and used in estimation of nitrogen in food stuff, fertilizers, drugs, etc. But this method fails to the compounds containing nitrogen in the ring (e.g., pyridine) and compounds containing nitrogen directly linked to oxygen atom (e.g., NO2, NO, etc.) or another nitrogen atom like –N = N– (azo), azoxy, diazo, hydrazones, oximes, etc