Calculate buffer pH from pKa and the amounts of weak acid and conjugate base.
| pH | – |
| [A−]/[HA] | – |
Use this calculator when making an acetate or Tris buffer: you know pKa and the A⁻/HA amounts and want the expected pH before you put in the probe.
Henderson–Hasselbalch: pH = pKa + log₁₀([A⁻]/[HA]). Amounts can be moles or molarity as long as both species use the same unit. The acid amount must be greater than zero. This ignores activity coefficients and CO₂ pickup from air.
pKa: pKa of the acid form. Acetic acid is about 4.76; Tris is about 8.1.
[HA]: Amount of weak acid, moles or concentration, same unit as the base field.
[A−]: Amount of conjugate base on that same scale. Equal acid and base gives pH = pKa.
pH: Predicted buffer pH.
[A−]/[HA]: Base-to-acid ratio used in the logarithm.
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