🧪 Henderson–Hasselbalch Calculator
Find a buffer’s pH from its pKa and the conjugate base/acid ratio — or solve for the pKa or a concentration.
pH
4.76
Henderson–Hasselbalch: pH = pKa + log₁₀([A⁻] / [HA]) — buffer pH from the conjugate base/acid ratio. 🔒 Computed in your browser.
How the henderson–hasselbalch calculator works
The Henderson–Hasselbalch equation is pH = pKa + log₁₀([A⁻]/[HA]), where [A⁻] is the conjugate base and [HA] the weak acid. Enter the pKa and the two concentrations to get pH; or solve for pKa, [A⁻] or [HA]. Because only the ratio matters, you can use concentrations or moles.
The go-to buffer equation. When [A⁻] = [HA], pH = pKa — the buffer is at its most effective. Valid for weak acid/conjugate-base buffers within about one pH unit of the pKa.
Frequently asked questions
What is the Henderson–Hasselbalch equation?
pH = pKa + log₁₀([A⁻]/[HA]) — it gives the pH of a buffer from the acid’s pKa and the ratio of conjugate base [A⁻] to weak acid [HA].
How do I calculate buffer pH?
Enter the pKa and the concentrations of the conjugate base and weak acid; the tool computes pH = pKa + log([A⁻]/[HA]). For 0.1 M each with pKa 4.76, pH = 4.76.
Why does pH equal pKa when the ratio is 1?
Because log(1) = 0, so pH = pKa. Equal acid and base concentrations give a buffer centred on the pKa, where it resists pH change best.
Can I use moles instead of concentration?
Yes — since [A⁻] and [HA] are in the same volume, their ratio equals the mole ratio, so moles work too.
When is the equation valid?
For weak-acid buffers, and most reliably within roughly ±1 pH unit of the pKa, where the approximation that added base/acid barely changes the ratio holds well.