Buffer Capacity Calculator

Formula & explanation

Definition: β = Δn / ΔpH where Δn is moles (per litre) of strong acid or base added to change the pH by ΔpH. This calculator simulates adding a small amount Δ (mol·L⁻¹) of strong base (or acid):

Initial pH ≈ pKa + log10([A⁻]/[HA])
After adding Δ mol·L⁻¹ base: [HA]' = [HA] - Δ, [A⁻]' = [A⁻] + Δ
New pH' = pKa + log10([A⁻]'/[HA]')
Buffer capacity ≈ Δ / (pH' - pH)
        

Useful approximation: maximum buffer capacity occurs near pH = pKa. If [A⁻] = [HA] and pH = pKa then β_max ≈ 0.576 × C_total, where C_total = [HA] + [A⁻] (mol·L⁻¹).

Example

If [HA] = 0.1 M and [A⁻] = 0.1 M (so C_total = 0.2 M) and pH = pKa, then β_max ≈ 0.576 × 0.2 = 0.1152 mol·L⁻¹·pH⁻1. This calculator will compute the simulated Δ additions and show both acid and base results.