What is pKa?
pKa is the negative base-10 logarithm of the acid dissociation constant (Ka). It measures the strength of an acid — the lower the pKa, the stronger the acid.
pKa = −log₁₀(Ka)
Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA])
pKa scale reference
- pKa < 0: Very strong acids (HCl, H₂SO₄)
- pKa 0–5: Moderate acids (acetic acid = 4.76)
- pKa 5–15: Weak acids (carbonic acid = 6.35)
- pKa > 15: Very weak acids / essentially non-acidic