In Kjeldahl's method, nitrogen presentin estimated as:

The Kjeldahl's approach is based on the observation that when an organic molecule is heated with ${\mathrm{H}}_2{\mathrm{SO}}_4$, its nitrogen is quantitatively transformed to ${\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$ (conc.). After treating the (NH4)2SO4 with KOH to release NH3, which is subsequently absorbed in ${\mathrm{H}}_2{\mathrm{SO}}_4$ to get% of N.