Henderson-Hasselbalch (Buffer pH)

pH = pKa + log([A⁻]/[HA]). Predict buffer pH from acid + conjugate base concentrations.

Inputs

Acetic acid: 4.76. Carbonic: 6.35.

Result

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How to use this calculator

  • Enter pKa of weak acid.
  • Enter [A⁻] and [HA].
  • Read pH.

About this calculator

Henderson-Hasselbalch: pH = pKa + log([A⁻]/[HA]). When [A⁻] = [HA], pH = pKa exactly. A buffer's "optimal" range is pH = pKa ± 1 (ratio 1:10 to 10:1). Below or above, buffering capacity drops dramatically. Used in biology (blood ~7.4, buffered by carbonate), pharmaceuticals (drug formulation), brewing, and pool chemistry.

Frequently asked

When does formula apply?+
Weak acid + conjugate base buffer. Strong acids/bases need ICE tables. Approximation valid when both [A⁻] and [HA] >> [H⁺] (works for >0.001 M typically).
Optimal buffer range?+
pH = pKa ± 1. Outside this, ratio >10:1 means weak buffering capacity in one direction.
Common buffer pKa?+
Acetate 4.76. Carbonate 6.35 / 10.33. Phosphate 2.15 / 7.20 / 12.35. Tris 8.07. HEPES 7.55.
Blood pH buffering?+
Carbonate (CO₂/HCO₃⁻) is primary blood buffer. pH stays 7.35-7.45 despite metabolic acid load.
Why log?+
pH and pKa are log scales. Ratio in linear form = sum/difference in log form.

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