Henderson–Hasselbalch Calculator
Find the pH of a buffer from its pKa and the ratio of conjugate base to weak acid — or solve for the ratio needed to hit a target pH.
Quick answer: pH = pKa + log₁₀([A⁻]/[HA]). For an acetate buffer (pKa 4.76) with equal amounts of acetate and acetic acid, log(1) = 0, so pH = 4.76. Doubling the base raises pH by log(2) ≈ 0.30.
The Henderson–Hasselbalch equation predicts the pH of a buffer solution from the acid's pKa and the relative amounts of the conjugate base and the weak acid. It's the everyday tool for designing buffers in biochemistry and analytical chemistry, and it explains why a buffer resists pH change near its pKa.
The equation
pH = pKa + log₁₀([A⁻] ÷ [HA])
[A⁻]/[HA] = 10^(pH − pKa)
[A⁻]/[HA] = 10^(pH − pKa)
Worked example
An acetate buffer (pKa 4.76) contains 0.20 M acetate and 0.10 M acetic acid. pH = 4.76 + log(0.20/0.10) = 4.76 + log(2) = 4.76 + 0.30 = 5.06.
Frequently asked questions
What is the Henderson–Hasselbalch equation?
pH = pKa + log([A⁻]/[HA]) — it links buffer pH to the acid's pKa and the base-to-acid ratio.
When does a buffer work best?
Near its pKa, where the base and acid amounts are comparable. Effective range is roughly pKa ± 1.
What if base and acid are equal?
The log term is log(1) = 0, so pH equals the pKa exactly.
Can I use moles instead of concentrations?
Yes — the two share the same volume, so the ratio is the same whether you use moles or molarity.
Related calculators
For general and educational use. Assumes ideal weak-acid behaviour near the pKa.