The Kjeldahl method is a technique for determining the nitrogen level in a sample and has 3 phases. This does not allow the determination of nitrates and nitrites.
The first step is the mineralization of organic nitrogen to obtain its ammonium salt form. The acid pH allows the ammonium salt to be in its protonated form NH4+. The nitrogenous organic matter degrades in the presence of a catalyst (copper sulphate and potassium sulphate) and by attack of concentrated sulfuric acid at high temperature. Carbon is eliminated in the form of carbon dioxide (CO2), hydrogen in the form of water (H2O) and nitrogen remains in the form of ammonium ion NH4+.
After digestion in concentrated sulfuric acid comes the second step which is the distillation of ammonium. Adding excess sodium hydroxide alkalizes the medium, making it possible to pass from ammonium sulfate to ammonia gas. This is entrained by the distillation water steam and is condensed on contact with a condenser.
In the last step, the titration, the ammonia collected in a solution of boric acid H3BO3 (ammonia trap) forms ammonium borate which is titrated with standardized sulfuric acid in the presence of a colored indicator. This titration is said to be indirect.
The Kjeldahl method is a universal, precise and reproducible technique.