GIFSQ Tools · Module 1 — Food Composition & Proximate Analysis · Tool 5 of 500
Kjeldahl digestion turns the sample's nitrogen into ammonia, which is distilled and titrated. Enter the titration volumes and the acid strength to get nitrogen %, and the crude protein for your food matrix.
Try sample values — fills the form with the worked example below.
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Nitrogen (%)
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Nitrogen (mg)
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Crude protein (%)
Equation: Nitrogen % = (Vsample − Vblank) × N × 1.4007 ÷ msample
Nitrogen (mg) = (Vsample − Vblank) × N × 14.007 · Crude protein % = Nitrogen % × factor
The constant 1.4007 = 14.007 (atomic weight of nitrogen) ÷ 10. Volumes in mL, normality in mol/L, sample in g.
Method: Kjeldahl digestion, distillation and titration, AOAC 990.03 / AOAC 984.13.
Assumptions: all sample nitrogen is recovered as ammonia; the blank corrects for nitrogen in the reagents; the titration endpoint is exact.
Limitations: Kjeldahl measures total nitrogen, not protein — non-protein nitrogen (urea, free amino acids, nucleic acids) inflates crude protein; nitrate and nitrite nitrogen is not fully recovered without a modification; the protein factor is a convention for the matrix, not a measurement.
Rounding: nitrogen % and nitrogen mg to 3 decimal places; crude protein % to 2 decimal places; intermediate values are not rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Nitrogen (mg) = (Vsample − Vblank) × N × 14.007 · Crude protein % = Nitrogen % × factor
The constant 1.4007 = 14.007 (atomic weight of nitrogen) ÷ 10. Volumes in mL, normality in mol/L, sample in g.
Method: Kjeldahl digestion, distillation and titration, AOAC 990.03 / AOAC 984.13.
Assumptions: all sample nitrogen is recovered as ammonia; the blank corrects for nitrogen in the reagents; the titration endpoint is exact.
Limitations: Kjeldahl measures total nitrogen, not protein — non-protein nitrogen (urea, free amino acids, nucleic acids) inflates crude protein; nitrate and nitrite nitrogen is not fully recovered without a modification; the protein factor is a convention for the matrix, not a measurement.
Rounding: nitrogen % and nitrogen mg to 3 decimal places; crude protein % to 2 decimal places; intermediate values are not rounded.
This is a calculation aid, not a laboratory report — confirm against your accredited method before citing a value.
Worked example: 1.000 g wheat flour, sample titration 12.85 mL, blank 0.50 mL, 0.1 mol/L HCl. Net volume = 12.85 − 0.50 = 12.35 mL. Nitrogen = 12.35 × 0.1 × 1.4007 ÷ 1.000 = 1.730 % (17.299 mg). Crude protein with the wheat factor 5.70 = 1.730 × 5.70 = 9.86 %.
Enter values above to see results.
A 0.1 mL titration error moves the result by about 0.01–0.09 % nitrogen, depending on sample size — weigh and titrate carefully. Use the factor for your food: the general 6.25 overstates protein in wheat and understates it in milk. Tool 5 of 500 — GIFSQ
