How this calculator works
For a direct Kjeldahl titration, subtract blank volume, calculate reactive titrant equivalents and assign one nitrogen atom per mole ammonia. Correct for digest aliquoting and divide nitrogen mass by original sample mass. Multiply nitrogen percentage by a stated protein factor. The general 6.25 factor assumes protein contains 16% nitrogen; food-specific factors and non-protein nitrogen change the interpretation.
Formula
mN(g) = (Vsample - Vblank)(L) x c(mol/L) x acid factor x dilution x 14.007. Protein mass fraction = nitrogen mass fraction x protein factor.
Worked example
10 mL blank-corrected titrant at 0.1 mol/L with acid factor 1 corresponds to 0.014007 g N. In a 1 g sample this is 1.4007% N and 8.754375% crude protein at factor 6.25.
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Frequently asked questions
Does this measure true protein directly?
No. Crude protein estimates protein from total nitrogen and can include non-protein nitrogen.
Can I use a back titration?
Convert the back-titration result to ammonia equivalents first. The volume subtraction here is for direct titration.
What is the acid factor?
It is the number of reactive equivalents per mole titrant at the stated endpoint, such as one for HCl.