Buffer Capacity Calculator
Calculate the buffer capacity β — how much strong acid or base a buffer can absorb per pH unit change.
Maximum capacity occurs at pH = pKa.
Buffer capacity (β) measures how resistant a buffer is to pH change. It is defined as the number of moles of strong acid or base needed to change 1 liter of buffer by 1 pH unit.
Formula (Van Slyke equation):
β = 2.303 × C × Ka × [H⁺] / (Ka + [H⁺])²
Where:
- C = total buffer concentration = [HA] + [A⁻] (mol/L)
- Ka = acid dissociation constant
- [H⁺] = hydrogen ion concentration = 10^(-pH)
Maximum buffer capacity:
β is maximum when pH = pKa (when [HA] = [A⁻]).
At maximum: β_max = 0.576 × C
Estimating amount of acid/base to add:
Moles of strong acid/base = β × ΔpH × Volume (L)
Effective buffer range: Buffers work well within pH = pKa ± 1. Outside this range, buffer capacity drops dramatically.
Common buffer systems:
| Buffer | pKa | Useful pH Range |
|---|---|---|
| Acetic acid / Acetate | 4.76 | 3.76–5.76 |
| Phosphate (H₂PO₄⁻/HPO₄²⁻) | 7.21 | 6.21–8.21 |
| Bicarbonate / CO₂ (blood) | 6.35 | 5.35–7.35 |
| Tris buffer | 8.06 | 7.06–9.06 |
| Ammonia / Ammonium | 9.25 | 8.25–10.25 |
Biological importance: Blood is buffered primarily by the bicarbonate/CO₂ system at pH 7.35–7.45. A pH drop below 7.35 (acidosis) or above 7.45 (alkalosis) is a medical emergency. The kidneys and lungs work together to maintain blood pH within this narrow range.