Henderson-Hasselbalch Buffer pH

Henderson-Hasselbalch: pH of weak-acid buffers

The Henderson-Hasselbalch equation relates the pH of a buffer to the ratio of conjugate base to weak acid: pH = pKa + log₁₀([A⁻]/[HA]). Maximum buffering capacity occurs at pH = pKa, where [A⁻] = [HA]. Blood bicarbonate buffer example: with pKa = 6.1 and the ratio [HCO₃⁻]/(0.03·PaCO₂), the equation predicts arterial pH (normal 7.35–7.45). Acidemia when pH <7.35; alkalemia when pH >7.45. Compensation is respiratory (fast, minutes — by altering minute ventilation and PaCO₂) and renal (slow, hours to days — by adjusting HCO₃⁻ reabsorption and H⁺ secretion). Example: [HCO₃⁻] = 24 mEq/L, PaCO₂ = 40 mmHg → pH = 6.1 + log(24/(0.03·40)) = 6.1 + log(20) ≈ 7.40.

Clinical context

Foundational equation for arterial blood gas interpretation in ICU, anesthesia, and pulmonology; for pharmaceutical formulation (IV drug buffers, ophthalmic and injectable preparations require pH compatibility with blood/tissue); for analytical chemistry (preparing biological buffers such as phosphate, Tris, HEPES at a target pH); and for physiology teaching on acid-base balance. The same logic governs urinary pH manipulation in salicylate or phenobarbital poisoning, and predicts ionization of weak-acid/weak-base drugs (informing absorption, renal excretion).

FAQ

Why the 0.03 factor for CO₂? It converts PaCO₂ (mmHg) to dissolved [CO₂] (mmol/L) using Henry's law solubility at 37 °C. Dissolved CO₂ behaves as the weak acid arm of the bicarbonate buffer.

How wide is the effective buffering range? Roughly pKa ± 1 pH unit. Outside that window the buffer loses capacity quickly.

Does the equation apply to strong acids? No — it assumes the weak acid is only partially dissociated. For strong acids (HCl, H₂SO₄) compute pH directly from concentration.