What is the Henderson-Hasselbalch equation?

pH = pKa + log([A⁻]/[HA]). It gives the pH of a buffer made from a weak acid HA and its conjugate base A⁻.

When does pH equal pKa?

When [A⁻] = [HA]. This is the centre of the buffer's effective range, where its capacity is highest.

A buffer works well within ±1 pH unit of the pKa. Outside that range, one component is depleted and the buffer can't resist further pH change.

Yes. Use the pKa of the conjugate acid of the weak base. For example, an NH₃/NH₄⁺ buffer uses the pKa of NH₄⁺ (about 9.25).

NSW · HSCChemistry

Henderson-Hasselbalch in one line. Enter pKa, [HA], [A⁻], get pH.

Weak acidpKa[HA] (acid) (mol/L)[A⁻] (conjugate base) (mol/L)

Result

Buffer pH

4.745

[A⁻]/[HA] ratio

1.000

Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]). When the ratio is 1, pH = pKa.

pH = pKa + log₁₀([A⁻]/[HA]). The calculator picks a sensible pKa from your acid preset (or you can enter your own) and applies the formula.

What is the Henderson-Hasselbalch equation?: pH = pKa + log([A⁻]/[HA]). It gives the pH of a buffer made from a weak acid HA and its conjugate base A⁻.
When does pH equal pKa?: When [A⁻] = [HA]. This is the centre of the buffer's effective range, where its capacity is highest.
What is the buffer range?: A buffer works well within ±1 pH unit of the pKa. Outside that range, one component is depleted and the buffer can't resist further pH change.
Does the formula work for basic buffers?: Yes. Use the pKa of the conjugate acid of the weak base. For example, an NH₃/NH₄⁺ buffer uses the pKa of NH₄⁺ (about 9.25).