How this instrument works
Protein solubility testing, run by the Kjeldahl method, measures how much of a sample's nitrogen is present in a soluble, extractable form by titrating it against a standardized base. A sample is prepared, digested, and titrated with sodium hydroxide (NaOH) of known normality; a separate blank (run identically but without sample) is titrated the same way. The difference between the blank titer and the sample titer, scaled by the titrant's normality and the sample's effective weight, gives the percent of the sample that is soluble protein.
This calculation is built around a fixed set of constants specific to this method's protocol: 1.401 is ten times nitrogen's atomic weight (14.007 g/mol) divided by 100, converting a titration's chemical equivalents into a nitrogen mass percentage; 6.25 is the standard Jones' factor, based on the common assumption that protein is about 16% nitrogen by mass (1/0.16 = 6.25); and the 5 is this specific titration protocol's aliquot ratio (how the total digested volume relates to the portion actually titrated). Different protein sources use different, more accurate nitrogen-to-protein factors (milk uses 6.38, for instance), but 6.25 is the standard default absent a more specific published factor for the material being tested.
This is a distinct titration-parameter layout from a general Kjeldahl crude-protein calculation: where a general crude-protein tool exposes separate acid factor and dilution factor fields for a fully customizable AOAC-style titration, this one is built around a fixed protocol structure (blank titer, sample titer, NaOH normality, effective sample weight) matching a specific soluble-protein test method, with the aliquot-ratio constant baked directly into the formula rather than left as an adjustable input.
- Enter the titrant volume used for the blank run into Blank titer, b (mL).
- Enter the titrant volume used for the actual sample run into Sample titer, t (mL) — this should be less than the blank titer.
- Enter the concentration of the NaOH titrant into Normality of NaOH, n.
- Enter the sample's effective weight (the portion actually represented by the titrated aliquot) into Effective sample weight, m (g).
- Read Protein (%) below the inputs — it recalculates instantly as any field changes.
Worked example — a 0.5 g feed sample
Enter 20 into Blank titer, b (mL), 15 into Sample titer, t (mL), 0.1 into Normality of NaOH, n, and 0.5 into Effective sample weight, m (g) — a 5 mL titer difference against a 0.5 gram effective sample. Protein (%) reads 43.78125%: 1.401 x 6.25 x 5 x (20-15) x (0.1/0.5) = 1.401 x 6.25 x 5 x 5 x 0.2.
That 43.78% result reflects how much of this sample's weight is estimated to be soluble protein, based on the nitrogen this titration detected and the standard assumption that protein is about 16% nitrogen by mass. A soy-protein-isolate-range sample — a genuinely protein-rich material — would plausibly land in this same 40%+ range, which is a useful sanity check that the titer difference and sample weight you entered are in a realistic combination.
Questions
Why is there a blank titration as well as a sample titration?
The blank run accounts for anything in the reagents and procedure itself — not the sample — that would otherwise consume titrant and inflate the apparent nitrogen content. Subtracting the sample's titer from the blank's titer (b - t) isolates just the titrant volume actually consumed by the sample's own nitrogen, which is why the formula uses that difference rather than the sample titer alone.
Where does the constant 1.401 come from?
It's derived from nitrogen's atomic weight, 14.007 g/mol, divided by 100 and multiplied by 10 to convert the titration's chemical-equivalents math into a mass-percentage figure for nitrogen specifically. It's a fixed conversion constant baked into this particular protocol, not a value you adjust — it appears identically across Kjeldahl-based nitrogen determinations that follow this same titration-based unit convention.
Why 6.25 specifically for the protein conversion factor?
Because it's the reciprocal of 0.16 — the common assumption that protein, on average, is about 16% nitrogen by mass, so total protein is roughly 6.25 times the measured nitrogen. It's a general-purpose default, not a universal constant: specific proteins have measurably different nitrogen content, which is why some materials use a different published factor (milk protein commonly uses 6.38, for example) when a more accurate figure is available.
What is 'effective sample weight,' and how is it different from the total sample weight?
It's the portion of the original sample actually represented by the titrated aliquot — accounting for any dilution or subsampling that happened between weighing the original sample and titrating the specific portion tested. If the entire prepared sample were titrated directly with no aliquoting step, effective sample weight and total sample weight would be the same number; where the protocol calls for titrating only a fraction of the digested sample, effective weight scales down accordingly.
Can the sample titer be larger than the blank titer?
No — that would mean the sample somehow consumed more titrant than a run with no sample at all, which isn't physically meaningful for this method. A correctly run titration should always show the sample consuming less titrant than the blank (since some of what the blank's acid would otherwise neutralize has already reacted with the sample's nitrogen), so the sample titer should be strictly less than the blank titer.