Explain Kjeldal's method in brief.

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jeew-m eNotes educator| Certified Educator

Kjedal's method or Kjedal's digestion is a method of determining the amount of Nitrogen which is a component in organic substance. In other words this method is a quantitative analysis of N in a chemical substance.

The method consist of 4 major steps.

  • Degradation of sample 

This involves heating of a substance with Sulphuric acid. After this process the we get a clear medium which was initially dark colour. Here because of sulphate ions in sulphuric we get ammonium sulphate {`(NH_4)_2SO_4` }. 


  • Next we will free the N in ammonium sulphate by adding NaOH to it. Here we will get ammonia (`NH_3` )


  •  Then we capture the freed ammonia using known amount of Boric acid. {`B(OH)_3` }
  • Then the remaining boric acid is titrated with sodium carbonate using methyl orange as the PH indicator.

The reactions involve with the above 4 steps is as follows.

sample`+H_2SO_4 rarr (NH_4)_2SO_4 `  

`(NH_4)_2SO_4+NaOH rarr 2NH_3`

`NH_3+B(OH)_3 rarr NH_4^+ + B(OH)_4^-`

`B(OH)_3+Na_2CO_3 rarr NaHCO_3+Na(BO)_4`

Note that all the components of the reactants or results are not shown here. Only the essential parts.

Using back calculation you will get the amount of Nitrogen in the initial chemical compound.