In the Kjeldal’s estimation of nitrogen in the form of ${\mathrm{NH}}_3$  small amounts of ${\mathrm{K}}_2{\mathrm{SO}}_4$ is added to the system.

Kjeldahl's method is used for the estimation of nitrogen. The organic compound is heated with conc. H₂​SO₄​ in presence of K₂​SO₄​ and a little CuSO₄​ to convert all the nitrogen into (NH₄​)₂​SO₄​⋅K₂​SO₄​ raises the boiling point of H₂​SO₄​ while CuSO₄​ catalyzes the reaction.