🧪 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. Since the equation depends on the log of the base-to-acid ratio, a tenfold excess of base raises the pH one unit above the pKa, and a tenfold excess of acid lowers it one unit below.
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.
What ratio of base to acid gives a target pH?
Rearrange to [A⁻]/[HA] = 10^(pH − pKa). To buffer at pH 5.76 with an acid of pKa 4.76, you need a ratio of 10¹ = 10 parts conjugate base to 1 part acid.
How do I choose an acid for a buffer?
Pick one whose pKa is within about one unit of your target pH, so the base:acid ratio stays between roughly 1:10 and 10:1. A phosphate buffer (pKa₂ ≈ 7.2) suits physiological pH 7.4, for instance.
What is buffer capacity?
The amount of acid or base a buffer can absorb before its pH shifts significantly. It is greatest when [A⁻] = [HA] (pH = pKa) and rises with the total concentration of the buffer components.
Does the Henderson, Hasselbalch equation account for strong acids or bases?
No. It assumes a weak acid and its conjugate base at equilibrium. For a strong acid or base, which ionises completely, calculate the pH directly from the concentration instead of using this equation.