How this calculator works
For NaOH back titration, the blank consumes more base than the sample. The difference measures recovered nitrogen, which is converted to crude soluble protein. Enter the original sample mass represented by the digested filtrate aliquot, including extraction and dilution corrections. Dividing soluble protein by total protein gives solubility. The default factor 6.25 is conventional, not universal. Direct acid titration has the opposite titre subtraction and is outside this mode.
Formula
Soluble protein mass = (Vblank - Vsample) cNaOH MN f. Soluble mass fraction = soluble protein mass/meffective. Protein solubility = soluble mass fraction/total protein mass fraction. MN = 14.007 g/mol (IUPAC).
Worked example
A 10 mL blank-minus-sample difference at 0.1 mol/L contains 0.014007 g nitrogen. With factor 6.25 and effective mass 1 g, soluble protein is 8.754375% of sample mass. If total protein is 50%, solubility is 17.50875%.
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Frequently asked questions
Is normality a mass concentration?
No. NaOH normality is equivalents per litre and equals mol/L for this one-equivalent titration.
Why enter total protein separately?
Protein content in the filtrate is not itself the fraction of all original protein that dissolved.
What does effective mass mean?
It is the original dry or as-received sample mass represented by the aliquot, using the same basis as total protein.