Kjeldahl’s method depends upon the fact that most of the organic compounds containing nitrogen are quantitatively decomposed to give $(\mathrm{N}{\mathrm{H}}_4{)}_2\mathrm{S}{\mathrm{O}}_4$ when heated strongly with conc. ${\mathrm{H}}_2\mathrm{S}{\mathrm{O}}_4$ . In this method $\mathrm{C}\mathrm{u}\mathrm{S}{\mathrm{O}}_4$ acts as catalytic agent.