explain kjeldahl'method

Asked by  | 18th Sep, 2011, 05:46: PM

Expert Answer:

The method consists of heating a substance with sulphuric acid, which decomposes the organic substance by oxidation to liberate the reduced nitrogen as ammonium sulphate.In this step potassium sulphate  is added to increase the boiling point of the medium (from 337°F to 373°F / 169°C to 189°C). Chemical decomposition of the sample is complete when the medium has become clear and colorless (initially very dark).

The solution is then distilled with sodium hydroxide (added in small quantities) which converts the ammonium salt to ammonia.The amount of ammonia present (hence the amount of nitrogen present in the sample) is determined be back titration. The end of the condenser  is dipped into a solution of  boric acid.The ammonia reacts with the acid and the remainder of the acid is then titrated with a  sodium carbonate solution with methyl orange.

Degradation: Protein + H2SO4 ? (NH4)2SO4(aq) + CO2(g) + SO2(g) + H2O(g)

Liberation of ammonia: (NH4)2SO4(aq) + 2NaOH ? Na2SO4(aq) + 2H2O(l) + 2NH3(g)

Capture of ammonia: B(OH)3 + H2O + NH3 ? NH4+ + B(OH)4–

Back-titration: B(OH)3 + H2O + Na2CO3 ? NaHCO3(aq) + NaB(OH)4(aq) + CO2(g) + H2O

Nowadays, the Kjeldahl method is largely automated and makes use of specific catalysts (mercury oxide or copper oxide to speed up the decomposition).

Answered by  | 19th Sep, 2011, 10:10: AM

Queries asked on Sunday & after 7pm from Monday to Saturday will be answered after 12pm the next working day.