KjelMaster System K-375 / K-376
Nitrogen and protein determination in food and feed are
described in numerous standard procedures [1-3]. Therein,
the titration specifications vary between boric acid and back
titration.
Boric acid titration is the direct determination of the amount
of distilled ammonia. Ammonia and boric acid form a
complex which is directly titrated with sulfuric acid. An
excess of boric acid guarantees that ammonia is bound in a
complex.
Back titration is an indirect determination of the amount of
distilled ammonia. An excess of sulfuric acid standard
solution reacts with the NH3 in the distillate. The residual
sulfuric acid is titrated with sodium hydroxide standard
solution. Consumption of sodium hydroxide solution
indirectly correlates with the amount of ammonia.
1. Introduction
The aim of the following study was to determine the
reproducibility and to compare the two different titration
methods, boric acid and back titration. Therefore, the
nitrogen content of the dry reference substance ammonium
dihydrogen phosphate monobasic was determined by
distillation applying the BUCHI KjelMaster System K-375 /
K-376.
2. Experimental
3. Results
Boric acid titration yielded a higher recovery rate (99.2 %)
than back titration (98.6 %). Nevertheless, both titration
methods fulfilled the recovery requirements of ± 2 % (98 %
- 102 %) [4].
4. Conclusion
Depending the standard followed, boric acid or back titration
methods are required. Both methods have their advantages
and disadvantages.
Boric acid titration
+ Direct titration.
+ No exact volume dosage for receiver solution required →
quick method and fewer measurement errors.
+ Depending on the regulation or method, either
potentiometric or colorimetric titration can be chosen.
+ Time saving: Distillation and titration can be synchronized
by means of the «Online-Titration mode».
- Point of inflection is weaker leading to a potential loss in
case of large amounts of NH3. In this case choose a larger
volume boric acid solution and/or reduce the original sample
weight.
- The pH of the boric acid must be adjusted to 4.65.
First, the nitrogen content of the blanks was determined.
Subsequently, the nitrogen recovery rate of the reference
sample (ammonium dihydrogen phosphate monobasic) was
measured, either using boric acid titration or back tiration
(n = 5).
Back titration
+ Back titration can be used when boric acid should be
avoided.
At least 3 blank determinations were performed before
sample distillation.
+ The acid immediately neutralizes the ammonia. The
distillate receiver remains acidic, thus there are no losses
even for large amounts of ammonia.
The distillation and titration parameters are listed in Table 1
and Table 2.
+ No pH adjustment of the receiving solution necessary.
- Indirect titration.
Table1: Distillation parameters for boric acid titration.
H2O volume
20 mL
Titration type
Boric acid
NaOH (32 %) volume
40 mL
Receiving solution vol.
50 mL (Boric acid)
Reaction time
5s
Titration solution
H2SO4 0.1 mol/L
Distillation mode
Fixed time
Titration mode
Standard
Distillation time
240 s
Endpoint pH
4.65
Steam output
100 %
Titration algorithm
Optimal
- No colorimetric titration possible (potentiometric titration
only).
- The sulfuric acid concentration and volume must be
precisely measured and is a potential source of errors →
second burette is required.
In this study, both titration methods fulfilled the recovery
requirements ± 2 % (98 % - 102 %) using the KjelMaster
K-375 with KjelSampler K-376.
Table 2: Distillation parameters for back titration.
H2O volume
20 mL
5. References
Titration type
Back titration
NaOH (32%) volume 40 mL
Receiving solution
H2SO4 0.1 mol/L
Reaction time
5s
Receiving solution vol. 20 mL
[2] AOAC 930.25-1930, protein in macaroni products.
Distillation mode
Fixed time
Titration solution
NaOH 0.5 mol/L
[3] AOAC 935.39-1935, baked products.
Distillation time
240 s
Titration mode
Standard
Steam output
100 %
Endpoint pH
7.0
Titration algorithm
Optimal
[1] AOAC 928.08, nitrogen in meat. Kjeldahl method.
[4] AOAC 2001.11 Protein (crude) in animal feed, forage
(plant tissue), grain, and oilseeds.
For more detailed information and safety considerations
please refer to the Application Note No. 242/2016.
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