1. What is the Henderson-Hasselbalch Equation?
In chemistry and biological sciences, the Henderson-Hasselbalch equation provides a direct quantitative relationship between the pH of an aqueous solution, the acid dissociation constant (pKa) of a weak acid, and the relative equilibrium concentrations of the conjugate base and weak acid pair:
pH = pKa + log₁₀([Conjugate Base] / [Weak Acid])
Originally formulated by Lawrence Joseph Henderson in 1908 and re-expressed logarithmically by Karl Albert Hasselbalch in 1917 to evaluate carbonic acid in blood, it remains the standard model for designing physiological buffers in biochemistry and molecular biology.
2. How Do Buffer Solutions Work?
A buffer solution is an aqueous mixture of a weak acid (HA) and its conjugate base (A⁻) that resists drastic changes in hydrogen ion concentration ([H⁺]) upon the addition of small amounts of strong acid or base:
- When strong acid (H⁺) is added: The conjugate base neutralizes it: A⁻ + H⁺ → HA.
- When strong base (OH⁻) is added: The weak acid neutralizes it: HA + OH⁻ → A⁻ + H₂O.
Because the added protons or hydroxide ions are converted into weak, poorly dissociating species, the pH of the solution shifts only minimally.
3. Common Biological Buffers & Their Optimal Ranges
Standard buffers used in cell biology and recombinant protein workflows:
- Phosphate Buffered Saline (PBS, pKa₂ = 7.20): Matches physiological osmolarity and pH (7.2–7.4). Isotonic to mammalian cells.
- Tris Buffer (pKa = 8.06): Standard for nucleic acids, plasmid purification, and TAE/TBE electrophoresis gels.
- HEPES (pKa = 7.48): A zwitterionic Good's buffer that maintains physiological pH in tissue culture without requiring specialized CO₂ atmosphere.
- MES (pKa = 6.15): Popular acidic buffer for enzymatic assays and plant tissue media.
4. Buffer Capacity & The pKa ± 1.0 Rule
Buffer capacity (β) refers to the amount of strong acid or base required to change the pH of 1 Liter of buffer by exactly 1.0 pH unit.
Buffer capacity reaches its maximum when [Base] = [Acid] (pH = pKa). As a practical rule of thumb, a buffer possesses useful resisting capacity only within:
Useful Buffering Window = pKa ± 1.0 pH unit
Outside this window, the ratio of components exceeds 10:1 or drops below 1:10, meaning more than 90% of the buffer exists as a single species with negligible reserve to neutralize counter-additions.
