Henderson-Hasselbalch (pH tampão)

Henderson-Hasselbalch equation

The Henderson-Hasselbalch equation gives the pH of a buffer from the ratio of conjugate base to weak acid: pH = pKa + log₁₀([A⁻]/[HA]). The buffer works best at pH = pKa, where [A⁻] = [HA]. Blood bicarbonate example: pKa = 6.1; with [HCO₃⁻] = 24 mEq/L and dissolved CO₂ ≈ 0.03·PaCO₂ ≈ 1.2 mmol/L, the ratio comes out to 20, so pH = 6.1 + log(20) ≈ 7.4. The effective range spans pKa ± 1 unit of pH; step outside that window and the buffer loses capacity. One caveat: the equation assumes a weak (partially dissociated) acid or base. For strong acids and bases, work out the pH directly from concentration instead.

Applications

Foundational equation for arterial blood gas (ABG) interpretation in ICU and anesthesia; analytical-lab buffer formulation (phosphate, Tris, HEPES at a target pH); pharmaceutical formulation (IV drugs, eye drops, injectables require pH compatibility with blood/tissue); and physiology teaching on acid-base balance, urinary pH manipulation in salicylate poisoning, and weak-acid/weak-base drug ionization.

FAQ

Why is pH = pKa the optimal point? When [A⁻] = [HA] the log term is zero and the buffer absorbs added acid or base symmetrically with minimum pH change.

How wide is the effective buffering range? Roughly pKa ± 1. Outside this window one side dominates and capacity falls off quickly.

Does the equation apply to strong acids? No — it assumes partial dissociation. For HCl, H₂SO₄, NaOH, compute pH directly from concentration.