The compound containing nitrogen is heated with concentrated sulphuric acid. Nitrogen in the compound gets converted to ammonium sulphate. The resulting acid mixture is then heated with excess of sodium hydroxide. The liberated ammonia gas is absorbed in an excess of standard solution of sulphuric acid.
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.
Let the mass of organic compound taken = m g
Volume of H2SO4 of molarity, M, taken = V mL
Volume of NaOH of molarity, M, used for titration of excess of H2SO4 = V1 mL
V1 mL of NaOH of molarity M = V1/2 mL of H2SO4 of molarity M
(V-V1/2) mL of H2SO4 of molarity M = 2(V-V1/2) mL of NH3 solution of molarity M
1000 mL of 1 M NH3 solution contains 17g NH3 or 14 g of N
2(V-V1/2) mL of NH3 solution of molarity M contains:
14 x M x 2(V-V1/2)/1000 g of N
Kjeldahl method is not applicable to compounds containing nitrogen in nitro and azo groups and nitrogen present in the ring as nitrogen of these compounds does not change to ammonium sulphate under these conditions.