Question

How does the Kjeldahl Method Work?

Answer 1

Working of Kjeldahl method

1. Kjeldahl method: This method is used in analytical chemistry. The quantitative analysis of nitrogen in organic as well as inorganic substances. It has great application in the field of foodstuff, fertilizers, soil and other substances containing nitrogen. It is mainly used to check the amount of protein present in food.
2. The steps involve in the Kjeldahl method: There are three steps which are involved in Kjeldahl method

Step 1 -Digestion- The concentrated acid is added to the sample, The acid solution is boiled at a very high temperature for the digestion of the sample. Due to this reaction, $\mathrm{ammonium}\mathrm{sulfate}$ solution is produced.

$\mathrm{Organic}\mathrm{sample}(\mathrm{C},\mathrm{H},\mathrm{N})+{\mathrm{H}}_2{\mathrm{SO}}_4\to \mathrm{Digested}\mathrm{sample}+{\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4$

Step 2 -Distillation- To this step excess of base is added to convert ammonium sulfate solution to ${\mathrm{NH}}_3$ gas. This process has a combination of boiling and condensation.

${\left({\mathrm{NH}}_4\right)}_2{\mathrm{SO}}_4+2\mathrm{NaOH}\to {\mathrm{Na}}_2{\mathrm{SO}}_4+2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{NH}}_3$

Step 3-Titration- Ammonia gas is produced from distillation which is absorbed by using boric acid. For the quantification of nitrogen, the titration is done by using strong acid

$2{\mathrm{NH}}_3+4{\mathrm{H}}_3{\mathrm{BO}}_3\to {\left({\mathrm{NH}}_4\right)}_2{\mathrm{B}}_4{\mathrm{O}}_7+5{\mathrm{H}}_2\mathrm{O}$

${\left({\mathrm{NH}}_4\right)}_2{\mathrm{B}}_4{\mathrm{O}}_7+5{\mathrm{H}}_2\mathrm{O}+2\mathrm{HCl}\to 2{\mathrm{NH}}_4\mathrm{Cl}+4{\mathrm{H}}_3{\mathrm{BO}}_3$

Thus, Kjeldahl method is used for measuring the amount of nitrogen present in given sample by titration.