Labconco Open Combination (Macro) Kjeldahl Systems 21237 Series - Manual (Page 26)

Appendix C: Total Kjeldahl Nitrogen Method
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7. Titration
7.1. Depending on the samples expected nitrogen level and the sensitivity
of the titration desired, select the appropriate standardized H
2
SO
4
solution (0.02 or 0.10 normal). High or low nitrogen concentrations
will require standardized H
2
SO
4
solutions with stronger or weaker
normalitys. Titrations should require at least 15 ml of titrant to be
accurate. Fill a class A. buret to the zero line with the standardized
H
2
SO
4
solution. Titrate the H
3
BO
3
receiving solution with
standardized H
2
SO
4
solution to first trace of the original purple
color. A white stirring plate will aid color visualization of end
point. Record ml H
2
SO
4
titrated to the at least nearest 0.05 ml. Match
the endpoint against a sample blank containing the same volume of
distilled water and H
3
BO
3
solution.
8. Calculation
For Dry Samples
% Nitrogen =
% Nitrogen, = (ml H
2
SO
4
, sample - ml H
2
SO
4
, blank ) x Normality H
2
SO
4
x
1.4007/ weight of sample in grams. (1.4007 = a single factor that takes into
account the molecular weight of Nitrogen, the conversion of milli-equivalent results of
V*N, and the conversion to %)
% Nitrogen, = (ml H
2
SO
4
, sample - ml H
2
SO
4
, blank) x normality H
2
SO
4
x
1400.7 x 100/ weight of sample in milligrams.
For Liquid Samples: calculate Total Kjeldahl Nitrogen, in mg/1, in the
original sample as follows:
Milligrams Total Nitrogen per Liter = (ml H
2
SO
4
, sample - ml H
2
SO
4
,
blank) x normality H
2
SO
4
x 14.007x 1000/ volume of sample in milliliters.
If desired to determine % protein instead of % nitrogen, the calculated % N is
multiplied by a factor, the magnitude of the factor depending on the sample
matrix.
Common Protein Factors
5.7 – bread, wheat and wheat flour
6.25 – other grains
6.38 – milk
6.25 – unknown source
Total Kjeldahl Nitrogen Method Overview
Digestion is accomplished by boiling a homogeneous sample in concentrated
sulfuric acid. The end result is an ammonium sulfate solution. The general
equation for the digestion of an organic sample is shown below:
Organic N + H
2
SO
4
→
(NH
4
)SO
4
+ H
2
O + CO
4
+ other sample matrix byproducts