Question

Describe Dumas and Kjeldahl's method for the estimation of Nitrogen.

Answer 1

Dumas method: In Dumas method, a known mass of an organic compound is heated strongly with coarse cupric oxide in an atmosphere of carbon dioxide.

Carbon and hydrogen get oxidised to carbon dioxide and water, while the nitrogen present in the organic compound is converted into free nitrogen.

C + 2CuO → CO2 + 2Cu

H2 + CuO → H2O +Cu

Nitrogen present in organic compound +CuO → N2 + Some oxides of N2

A part of the nitrogen may get oxidised into oxides of nitrogen. When passed over a heated copper gauge, these oxides of nitrogen are reduced into free nitrogen again.

Oxides of nitrogen + Cu → N2 +CuO

The mixture produced, which contains carbon dioxide, water and nitrogen, is collected over an aqueous solution of potassium hydroxide.

This solution absorbs the carbon dioxide gas from the mixture, leaving nitrogen behind.

This nitrogen is collected in the upper part of the graduated tube.

From the volume of this nitrogen collected, the percentage of nitrogen in the compound is calculated.

Kjeldahl method:

The method can be divided into two major stages, namely, digestion and distillation.

During the digestion stage, the nitrogen present in the organic compound is converted into ammonium sulphate. This is achieved by heating the compound in Kjeldahl flask with concentrated sulphuric acid in the presence of small amount of copper sulphate.

C, H, S + Conc H2SO4 → CO2 ,H2O, SO2

N + Conc H2SO4 → (NH4)2SO4

Now, the contents of the flask are cooled and transferred into a distillation flask and heated with concentrated sodium hydroxide to liberate ammonia gas. This is the distillation stage.

Answer 2

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$\boxed{\sf\pink{Kjeldahl's\:method}}$

✔ In this method, the compound

heated with concentrated sulphuric acid in the presence of small amount of ${CuSo_4}$to convert nitrogen into Ammonium Sulphate

✔The contents of the flask are transferred to another flask and heated with excess of sodium hydroxide solution to liberate ammonia gas

✔ Ammonia gas so liberated is passed and absorbed known as the volume concentrated sulphuric acid

✔Organic compound +${H_2SO_4{\rightarrow}(NH_4)_2SO_4}$

${(NH_4)_2SO4+2NaOH{\rightarrow}Na_2SO_4+2H_2O+72NH_3}$

✔${2NH_3+H_2SO_4{\rightarrow}(NH_4)_2SO_4}$