Henderson-Hasselbalch Calculator

Calculate buffer pH using the Henderson-Hasselbalch equation. Enter pKa and conjugate base/acid concentrations. Includes buffer capacity, effective range, and common buffer systems.

mol/L
mol/L
Buffer pH
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[A⁻]/[HA] Ratio —
Effective Range Min —
Effective Range Max —
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mol/L
mol/L
Buffer pH
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[A⁻]/[HA] Ratio —
Professional Full parameters & maximum detail ▾
mol/L
mol/L
mol/L
mol/L
°C
Buffer pH
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pH (ionic-strength corrected) —
Buffer Capacity β —
Effective Range Min pH —
Effective Range Max pH —
Approx pKa at Temperature —

How to Use This Calculator

  1. Enter the pKa of your weak acid.
  2. Enter the conjugate base [A⁻] and acid [HA] concentrations in mol/L.
  3. The calculator returns pH and effective buffer range. Use the tabs for ratio-finding or common buffers.

Formula

pH = pKa + log₁₀([A⁻]/[HA])

Buffer capacity: β = 2.303 × C × ([H⁺]Ka)/([H⁺]+Ka)²

Example

Example: Acetate buffer, pKa = 4.76, [A⁻] = 0.1 mol/L, [HA] = 0.1 mol/L → pH = 4.76 + log(1) = 4.76. Effective range: 3.76–5.76.

Frequently Asked Questions

  • pH = pKa + log₁₀([A⁻]/[HA]), where [A⁻] is the conjugate base concentration and [HA] is the weak acid concentration. It describes the pH of a buffer solution.
  • A buffer works best within pKa ± 1 pH unit. Outside this range the buffer capacity drops sharply and the solution loses its ability to resist pH changes.
  • Buffer capacity β = 2.303 × C × ([H⁺]Ka)/([H⁺]+Ka)². It represents how many moles of acid or base 1 litre of buffer can absorb before the pH changes by 1 unit. Maximum capacity occurs at pH = pKa.
  • Acetate buffer (pKa 4.76) is used in pH 3.6–5.6 range. Phosphate (pKa 7.21) is common in biochemistry. Tris (pKa 8.06) is standard in molecular biology. Bicarbonate (pKa 6.35) is the main blood buffer.
  • High ionic strength shifts the effective pKa. The Davies equation corrects activity coefficients: log(γ) = −0.509z²(√I/(1+√I) − 0.3I), which adjusts the apparent pKa at ionic strength I.

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Sources & References (5) ▾
  1. Buffer Solutions — ACS Chemistry for Life — American Chemical Society
  2. IUPAC Compendium of Chemical Terminology (Gold Book) — Buffer — IUPAC
  3. NIST Standard Reference Database — Buffers — NIST
  4. OpenStax Chemistry 2e, Chapter 14 — Acid-Base Equilibria — OpenStax
  5. LibreTexts Chemistry — Henderson-Hasselbalch Approximation — LibreTexts