Protein Solubility Calculator

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Volume of NaOH solution required to back-titrate the blank digest (run without sample). Subtract this from the true reading to cancel out reagent background nitrogen.
mL
Volume of NaOH solution required to back-titrate the sample digest. Must be less than the blank titer for a positive nitrogen reading.
mL
Concentration of the sodium hydroxide back-titration solution in equivalents per litre. Typical lab concentrations range from 0.01 N to 1.0 N.
N
Dry mass of the sample (supernatant fraction) digested for analysis. Weigh accurately; errors here propagate directly into the final percentage.
g
Dry mass of the original (uncentrifuged) protein sample used as the 100% reference. The solubility percentage is the ratio of supernatant protein to total protein.
g
Blank titer used for the total (uncentrifuged) digest run. Often the same value as the supernatant blank titer if the same reagent batch is used.
mL
Volume of NaOH required to back-titrate the total (uncentrifuged) sample digest. Used to calculate total protein for the denominator.
mL
Protein contains about 16% nitrogen on average (factor 6.25), but the exact value varies with amino-acid composition. Choose the factor that matches your protein source.
Protein SolubilityModerately soluble
75%

Fraction of total protein that remained soluble after centrifugation

Nitrogen (supernatant)1.765%
Protein (supernatant)11.031%
Nitrogen (total sample)0.588%
Protein (total sample)3.677%
75% %
Insoluble<20%Poorly soluble20%-50%Moderately soluble50%-80%Highly soluble80%+

Protein solubility is 75.0%

  • Nitrogen in the supernatant fraction: 1.765%, equivalent to 11.031% protein using the 6.25 conversion factor.
  • Solubility between 50% and 80% is moderate. Consider adjusting pH further from the isoelectric point, reducing ionic strength, or adding a small amount of a compatible co-solvent such as glycerol.
  • Solubility measured by the Kjeldahl method captures total nitrogen, not just native protein. If the sample contains non-protein nitrogen (urea, free amino acids), the reading will be slightly elevated.

Next stepFor a complete protein quality profile, pair solubility with a nitrogen-to-protein conversion factor specific to your protein source, and cross-check against a Bradford or BCA assay for total protein concentration.

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