Henderson-Hasselbalch equation
The Henderson-Hasselbalch equation calculates the pH of a buffer solution from the pKa of the weak acid and the concentrations of the acid/conjugate base pair:
pH = pKa + log([A⁻] / [HA])
- When [A⁻] = [HA]: pH = pKa (half-equivalence point)
- Effective buffer range: pH = pKa ± 1
Common buffer systems
- Acetate buffer: pKa = 4.76 (pH 3.76–5.76)
- Phosphate buffer: pKa = 7.21 (pH 6.21–8.21)
- Bicarbonate buffer: pKa = 6.35 (blood plasma, pH ~7.4)
- Tris-HCl: pKa = 8.06 (pH 7.06–9.06)