Principle: When a nitrogenous organic compound is heated with cone. H2SO4 using (CuSO4K2SO4) as a catalyst, the nitrogen from the compound is quantitatively converted to ammonium sulphate. This ammonium sulphate is decomposed by heating with excess of alkali and the ammonia evolved is absorbed in known excess of a standard solution of H2SO4. Part of acid is neutralized by ammonia. The excess of acid left behind after neutralization with ammonia is estimated by back titration with standard alkali. From this, the amount of acid actually consumed by ammonia can be obtained which can be used to determine the percentage of nitrogen in the compound.
Procedure: A known exact mass of organic compound (about 0.5g) is mixed with 10g K2SO4, 1g CuSO4 and 25 ml of cone. H2SO4. The mixture is heated strongly in a Kjeldahl’s flask. Till the contents become clear. This step is known as digestion.
The Kjeldahl’s flask is now cooled and the liquid is heated in a round-bottomed flask with excess of caustic soda solution The ammonia evolved is absorbed in a known volume of a standard acid.
The amount of unreacted acid is determined by titrating it against a standard alkali.
(NH4)2SO4 + 2NaOH → 2NH3↑ + Na2SO4 + 2H2O
Observation:
1. Mass of organic compound taken = Weight
2. Normality of standard acid = N
3. Volume of standard acid taken = V1 ml
4. Volume of alkali (Normality = N) required for back titration = V2 ml
Calculation: Volume of acid used up by ammonia = Volume of ammonia produced = (V2 – V1) = V ml of normality N.
Now,
1000 ml of 1 normal NH3
= 17g NH3
= 14g N2
∴ Vml of N-normal ammonia will contain \(\frac{14\times N\times V}{1000}\)gN2
This amount of nitrogen was present in w g of the compound
Where, N and V are the normality and volume respectively of the acid used up by ammonia.
