Estimation of Nitrogen

Dumas Method:

• A known mass w gram of the organic compound is heated with cupric oxide in an atmosphere of carbon dioxide.
• Carbon and hydrogen are oxidised to CO₂ and H₂O.
• Sulphur oxidised to SO₂.
• Nitrogen is set free — any oxide of N is reduced to free N₂ by heated copper gauze.
• Gaseous mixture (CO₂, H₂O, N₂) collected over KOH solution → all gases except N₂ are absorbed.
• Volume of N₂ measured (V mL at STP).

\[\%N=\frac{28\times V\times100}{22400\times w}=\frac{0.125\times V}{w}\]

Nitrogen is estimated as gaseous nitrogen using Dumas' method.

Kjeldahl's Method:

Faster than Dumas' method.

Used only for organic compounds that are converted quantitatively to ammonium sulphate on heating strongly with concentrated sulphuric acid.

Cannot be used for compounds containing:

• Nitro (–NO₂) and diazo (–N=N⁻) groups
• Nitrogen in the ring (e.g., pyridine, quinoline)

Step 1 — Digestion: Organic compound digested with conc. H₂SO₄ in the presence of a small quantity of potassium sulphate and copper sulphate (in Kjeldahl's flask).

\[\text{Organic compound}+\mathrm{H}_{2}\mathrm{SO}_{4}\xrightarrow{CuSO_{4}+K_{2}SO_{4}}(\mathrm{NH}_{4})_{2}\mathrm{SO}_{4}\]

Step 2 — Distillation: (NH₄)₂SO₄ decomposed with excess NaOH → NH₃ liberated → absorbed in standard acid (say H₂SO₄) of known normality:

• V_1​ mL of N normality H₂SO₄ taken
• V₂​ mL of N normality NaOH used for back titration
• Volume of H₂SO₄ left used = V₂​ mL of N normality

\[\%N=\frac{1.4\times\text{Normality of acid}\times\text{Volume of acid used to neutralise }NH_3}{\text{Mass of the substance (w)}}\]