This Henderson-Hasselbalch calculator finds a buffer’s pH from pKa and the conjugate base/acid concentrations — and runs backwards: the ratio a target pH demands.
pH = pKa + log([A⁻] ÷ [HA]) — The Buffer Equation
It’s the acid-dissociation equilibrium rearranged: when both members of a conjugate pair are present in comparable amounts, the log-ratio term stays small and pH hovers near pKa, resisting added acid or base by shifting the ratio slightly. Blood runs on it (H₂CO₃/HCO₃⁻ at pKa 6.35 holding pH 7.4 — a ratio of 11:1, outside the “ideal” window yet managed by open-system CO₂ exchange — physiology cheating elegantly). General pH problems: our pH Calculator. Part of our Chemistry Calculators collection.
Worked Examples (Verified)
- Acetic buffer, equal 0.1 M pair (defaults): log(1) = 0 → pH = pKa = 4.76 — maximum capacity point
- Double the base ([A⁻] 0.2, [HA] 0.1): pH = 4.76 + log 2 = 5.06 — one doubling buys 0.30 pH; ten-fold buys exactly 1.00
- Phosphate buffer for cell culture (pKa 7.20) at physiological 7.40: ratio = 10^0.20 = 1.58 : 1 — the ratio mode’s daily bread
- Ammonium buffer (pKa 9.25) at pH 9.25 = equal NH₃/NH₄⁺ — base buffers are the same equation read from the conjugate acid’s side
Using It Wisely
- Capacity needs concentration, not just ratio: 0.001 M buffer at the perfect ratio still collapses under a drop of strong acid.
- The equation assumes equilibrium with both species present in non-trivial amounts — it degrades when one side is ~100× the other (pH beyond pKa ± 1.5).
- Temperature and ionic strength nudge pKa values (tables list 25 °C, ideal); precision work calibrates the meter, not the algebra (our pH Calculator covers measurement conversions).
- Polyprotic acids: use the pKa whose step brackets your target pH (phosphate’s 7.20, not its 2.15 or 12.35).
Related Tools
- pH Calculator — pH from [H⁺] and back
- Titration Calculator — walking through the buffer region
- Chemistry Calculators — the full collection
Frequently Asked Questions
What is the Henderson-Hasselbalch equation?
pH = pKa + log₁₀([A⁻]/[HA]) — derived directly from the Ka expression by taking logarithms. Equal conjugate concentrations give pH = pKa; each tenfold ratio change moves pH by exactly 1. This calculator evaluates it in both directions (pH from ratio, ratio for a target pH).
When does Henderson-Hasselbalch fail?
When the buffer assumption fails: very dilute solutions, ratios far from 1 (pH more than ~1.5 from pKa), or strong acids/bases where the “pair” doesn’t coexist. It also presumes equilibrium and neglects water autoionization — exact charge-balance solutions take over at the extremes.
How do I choose a buffer for a target pH?
Pick an acid with pKa within ±1 of the target (closer = more capacity), set the ratio via the target-mode here, and use the highest total concentration your system tolerates. Phosphate (7.20) for near-neutral biology, acetate (4.76) for acidic work, Tris (8.07) and ammonium (9.25) for alkaline ranges.
How does this apply to blood buffering?
Blood’s bicarbonate system (pKa 6.35) holds pH 7.40 at a base:acid ratio of ~11:1 — outside the classic window, yet effective because it’s an open system: lungs exhale the CO₂ (acid side) continuously and kidneys regenerate bicarbonate. Henderson-Hasselbalch describes the snapshot; physiology manages the flows.
Can I use it for bases directly?
Use the conjugate acid’s pKa (NH₄⁺ 9.25 for ammonia buffers) with the pair [base]/[conjugate acid] — identical equation. Or the pOH twin: pOH = pKb + log([BH⁺]/[B]), then pH = 14 − pOH. The calculator’s pKa field accepts conjugate-acid pKa values for exactly this.
Is this Henderson-Hasselbalch calculator free?
Yes — free, instant, no sign-up.