Catalyst used in Kjeldahl's method for the estimation of nitrogen is :

Dumas' approach takes longer than Kjeldahl's method. The only organic compounds that can be transformed quantitatively to ammonium sulphate using this approach are those that are heated vigorously with concentrated sulfuric acid.

Hg or ${\mathrm{CuSO}}_4$ are used as the catalyst.

For organic molecules with nitrogen in the ring, such as pyridine, quinoline, etc., Kjeldahl's technique cannot be utilised.
containing diazo $(-\mathrm{N} = \mathrm{N}-)$ and nitro $\left({\mathrm{NO}}_2\right)$ groups.