Chemistry

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Isoelectric Point Calculator.

Estimate pI as the mean of two explicit bracketing pKa values.

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Isoelectric point (pI): 5.95

Isoelectric point (pI)

5.950000

Educational arithmetic only. Keep units consistent and use measured or explicitly supplied chemistry inputs; this tool does not infer reaction pathways, property tables, or safety guidance.

Use Cases

Determine pI for amino acids

Quickly estimate the isoelectric point of an amino acid by entering its two pKa values (carboxyl and amino groups). Useful for biochemistry labs and understanding protein behavior.

Example: For glycine: pKa1=2.34, pKa2=9.60, pI=(2.34+9.60)/2=5.97

Check pI for small peptides

For dipeptides or simple molecules with two ionizable groups, this calculator provides a fast estimate of pI, aiding in electrophoresis and purification experiments.

Example: For a dipeptide with pKa1=3.1 and pKa2=8.2, pI=5.65

Frequently Asked Questions

What is the isoelectric point (pI)?
The isoelectric point is the pH at which a molecule carries no net electrical charge. It is calculated as the average of the two pKa values that bracket the zwitterionic form, using the formula pI = (pKa1 + pKa2) / 2.
How do I choose the lower and upper pKa values?
Select the pKa values that correspond to the two ionizable groups closest to the neutral form. For amino acids, these are typically the carboxyl group (pKa ~2) and the amino group (pKa ~9). For other molecules, identify the two relevant acidic/basic groups.
Can this calculator be used for peptides or proteins?
This calculator is designed for simple molecules with two ionizable groups. For peptides or proteins with multiple ionizable groups, a more complex calculation considering all pKa values is needed.

Tips & Common Mistakes

Tips

  • Ensure you use the correct pKa values for the specific molecule and conditions (temperature, ionic strength).
  • For molecules with more than two ionizable groups, this simple average may not be accurate; consider using specialized software.
  • Double-check that the lower pKa is indeed lower than the upper pKa; otherwise, the result will be incorrect.
  • Use this calculator as a quick reference, but verify with experimental data when precision is critical.

Common Mistakes to Avoid

  • Using pKa values from different sources without checking consistency in conditions.
  • Entering pKa values in the wrong order, leading to an incorrect average.
  • Assuming this method works for polyprotic molecules with more than two ionizable groups.

Last updated: August 13, 2026