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Isoelectric Point from Two Bracketing pKa Values

Estimate pI from the two acid-dissociation pKa values surrounding a neutral species, or recover a missing pKa using the same two-step charge model.

Formula & worked example
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Calculator inputs

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How to use this calculator

Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

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Formula and method

For acid-dissociation pKa values of 2 and 10 on either side of the neutral species, the simplified isoelectric point is pI = 6.

pI = (pKa₁ + pKa₂)/2; pKa₁ = 2pI − pKa₂; pKa₂ = 2pI − pKa₁.

Worked example

Enter these known values and leave the other values blank.

pKa₁ (positive to neutral): 2
pKa₂ (neutral to negative): 10

Isoelectric point (pI): 6

Assumptions and limitations

Both inputs are acid-dissociation pKa values: one for the positive-to-neutral transition and one for the neutral-to-negative transition. Do not enter a true base-dissociation pKb as the second input.
The arithmetic mean follows from balancing positive and negative species in a simplified +1/0/−1 two-dissociation equilibrium. Choose the two dissociations adjacent to the neutral species when additional groups can ionize.
For an amino acid with an ionizable side chain, identify the appropriate bracketing pair instead of averaging every pKa. This tool does not calculate a whole protein’s pI from its sequence.
Use compatible dissociation constants at the same solvent, temperature and activity convention. If only a conjugate-base pKb is known, convert it using the appropriate pKw − pKb under those conditions; pKw is not universally 14.
Finite negative values and values above 14 are supported without clipping. The computed mean by itself does not establish that a real molecule or solution follows this model. No measured amino-acid dataset is embedded.

Common questions

Does the second input mean pKb?

No. It is the acid-dissociation pKa of the neutral-to-negative transition. Averaging a true pKb with pKa generally gives an incorrect pI.

Which pKa values should I use for an amino acid with a side chain?

Use the two acid-dissociation pKa values surrounding its electrically neutral form. The correct pair depends on the charge sequence.

Can I recover an unknown pKa from pI?

Yes, within this two-step model. Enter pI and the other pKa; the missing value is twice pI minus the known pKa.

References

Retain the captured three-variant arithmetic graph, but correct the second input’s meaning to acid-dissociation pKa₂. Its captured internal key pKb is preserved for capture comparison only; it is never presented as a base-dissociation pKb.

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