Labconco Two and Six-Place (Macro) Kjeldahl Distillation Systems 2127603 - Manual (Page 21)

Appendix C: Total Kjeldahl Nitrogen Method
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Distillation: Excess base is added to the digestion product to convert NH
4
to
NH
3
as indicated in the following equation. The NH
3
is recovered by
distilling the reaction product.
ammonium
sulfate
heat
ammonia
gas
(NH
4
)
2
SO
4
+ 2NaOH → 2NH
3
+ Na
2
SO
4
+ 2H
2
O
Titration quantifies the amount of ammonia in the receiving solution. The
amount of nitrogen in a sample can be calculated from the quantified amount
of ammonia ion in the receiving solution.
There are two types of titration—back titration and direct titration. Both
methods indicate the ammonia present in the distillate with a color change.
In the direct titration, boric acid is used as the receiving solution instead of a
standardized mineral acid, the chemical reaction is:
ammonia
gas boric
acid ammonium-
borate complex excess
boric acid
NH
3
+ H
3
BO
3
→ NH
4
+ H
2
BO -
3
+ H
3
BO
3
(color change occurs)
The boric acid captures the ammonia gas, forming an ammonium-borate
complex. As the ammonia collects, the color of the receiving solutions
changes.
ammonium-
borate
complex
sulfuric
acid ammonium
sulfate boric
acid
2NH
4
+ H
2
BO -
3
+ H
2
SO
4
(NH
4
)
2
SO
4
+ 2H
3
BO
3
(color change occurs in reverse)
The boric acid method has the advantages that only one standard solution is
necessary for the determination and that the solution has a long shelf life.