Chemistry
Isoelectric Point Calculator
Find the pH at which an amino acid carries no net charge from its pKa values.
Your inputs
Change any value to update the result instantly.
Isoelectric point pI
5.97 pH
Method used
Average of the carboxyl and amino pKa values
Below the pI
The molecule carries a net positive charge and moves towards the cathode
Above the pI
The molecule carries a net negative charge and moves towards the anode
Solubility note
Solubility is lowest at the pI, which is why proteins precipitate there
The two-pKa average works for a single amino acid. A peptide or protein has many ionisable groups, so its pI must be found by solving for zero net charge across all of them.
What this calculator does
Finds the pH of zero net charge for a single amino acid from the two pKa values bracketing the zwitterion.
What your result means
At this pH the molecule does not move in an electric field and is least soluble.
Formula and working
Neutral side chain: pI = (pKa₁ + pKa₂)/2; ionisable side chain: average the two pKa values bracketing the zwitterion
- 1
Collect the inputs
pKa of the carboxyl group = 2.34; pKa of the amino group = 9.6; Side chain = 1; pKa of the side chain = 4
- 2
Apply the formula
Neutral side chain: pI = (pKa₁ + pKa₂)/2; ionisable side chain: average the two pKa values bracketing the zwitterion
- 3
Result
Isoelectric point pI = 5.97 pH
Frequently asked questions
Symbols and assumptions
- pI
- Isoelectric point
- pKa1, pKa2
- Carboxyl and amino pKa values
- pKaR
- Side-chain pKa
- • A single amino acid with at most one ionisable side chain.
- • pKa values are for the conditions of interest.
Sources and review
Reviewed 2026-09-22 · Next review 2027-09-22
- • Peptides and proteins have many ionisable groups and need a full net-charge calculation, not this two-pKa average.
Important limitation
This is an educational estimate based only on the values and formula shown. Verify current rates, rules, units, and professional standards before relying on the result. It is not personalized financial or investment advice.
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