lotus

previous page: 16.4 What does the Dean and Stark distillation measure?
  
page up: Chemistry FAQ
  
next page: 16.6 What does a Soxhlet extractor do?

16.5 What does Kjeldahl nitrogen measure?




Description

This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.

16.5 What does Kjeldahl nitrogen measure?

The Kjeldahl procedure is routinely used to measure the protein nitrogen
content of organic compounds, especially natural foodstuffs. Contrary to
popular belief, the procedure does not determine total nitrogen on all
organic compounds, as it is not applicable to materials containing N-O or
N-N linkages without modifications to the method. This discrepancy is
becoming of more significance as automated nitrogen analysers using other
techniques are producing different results because they measure the total
nitrogen present.

The method usually involves high temperature ( 390C ) digestion of the
sample using concentrated sulfuric acid, a catalyst ( Cu, Hg, or Se ),
and a salt to elevate the acid boiling point. In some cases 30% hydrogen
peroxide is also used, making the digestion effectively a high-temperature
piranha solution attack on the organic matter. After digestion, the sample
is made strongly alkaline and the ammonia is steam distilled into a boric
acid solution, and aliquots are titrated against a standard acid using an
indicator solution endpoint.

Some organics compounds require aggressive digestion conditions to make
all the organic nitrogen available, consequently Kjeldahl procedures should
not normally be used on samples that may have N-O or N-N bonds. Details of
procedures for foods are in the AOAC handbooks, and general Kjeldahl
procedures are detailed in the ASTM volumes.

 

Continue to:













TOP
previous page: 16.4 What does the Dean and Stark distillation measure?
  
page up: Chemistry FAQ
  
next page: 16.6 What does a Soxhlet extractor do?