All calculatorsHenderson–Hasselbalch Calculator (Acid & Base Buffers)
Compute buffer pH using Henderson–Hasselbalch. Supports acid buffers (pH = pKa + log([A⁻]/[HA])) and base buffers via pOH (pOH = pKb + log([BH⁺]/[B]); pH = 14 − pOH). You can also solve for the required ratio or for pK.
Background
Henderson–Hasselbalch estimates buffer pH from the acid/base dissociation constant and the base-to-acid ratio in solution. Best accuracy when 0.1 ≤ ratio ≤ 10. For base buffers we compute pOH first, then pH. We assume pH + pOH = 14.00 at 25 °C.
How to use this calculator
- Acid buffer: pH = pKa + log([A⁻]/[HA])
- Base buffer: pOH = pKb + log([BH⁺]/[B]); pH = 14 − pOH
- Solve for ratio: Acid: ratio = 10^(pH − pKa); Base: ratio = 10^((14 − pH) − pKb)
- Solve for pK: Acid: pKa = pH − log(ratio); Base: pKb = (14 − pH) − log(ratio)
Accurate when 0.1 ≤ ratio ≤ 10. We use pH + pOH = 14.00 at 25 °C.
Example Problems & Step-by-Step Solutions
Example 1 (Acid buffer pH)
pKa = 4.76, [A⁻]/[HA] = 1.0 ⇒ pH = 4.76.
Example 2 (Base buffer pH)
pKb = 4.75, [BH⁺]/[B] = 1.0 ⇒ pOH = 4.75 ⇒ pH = 9.25.
Example 3 (Solve ratio, acid)
Target pH = 7.40, pKa = 7.20 ⇒ ratio = 10^(0.20) ≈ 1.58.
Frequently Asked Questions
Q: What’s the difference between acid and base buffers here?
Acid buffers use pH = pKa + log([A⁻]/[HA]). Base buffers use pOH = pKb + log([BH⁺]/[B]), then pH = 14 − pOH (at 25 °C).
Q: When is Henderson–Hasselbalch accurate?
When 0.1 ≤ ratio ≤ 10 and both species are present at reasonable concentrations. At high ionic strength, activity effects can introduce error.
Q: Can I enter concentrations instead of a ratio?
Yes. Divide the base form concentration by the acid form concentration to get the ratio: [A⁻]/[HA] for acid buffers, [BH⁺]/[B] for base buffers.
Q: Does temperature affect results?
Yes—pKa and pKb are temperature-dependent, and pH + pOH = 14.00 is exact at 25 °C. Use values appropriate for your working temperature.
