HendersonHasselbalch

The Henderson–Hasselbalch equation is a formula used in chemistry and biochemistry to relate the pH of a buffer solution to the pKa of a weak acid and the ratio of its conjugate base to the acid. For a weak acid HA that partially dissociates to H+ and A−, the equation is pH = pKa + log10([A−]/[HA]). It can also be applied in the form pH = pKa + log10([base]/[acid]) when the species are present in the appropriate conjugate forms. For bases, a related expression for pOH exists: pOH = pKb + log10([BH+]/[B]), and pH can be obtained from pH = 14 − pOH at 25 °C.

Purpose and use: the equation provides a practical way to estimate the pH of buffer solutions and to design buffers with a desired pH by adjusting the relative amounts of the weak acid and its conjugate base. It is widely used in analytical chemistry, biochemistry, pharmacology, and teaching labs, as well as in understanding physiological buffering in biological systems.

Assumptions and limitations: the Henderson–Hasselbalch equation assumes a relatively simple, single-buffer system with weak acid/base and conjugate base/acid, and that activities can be approximated by concentrations. It is most accurate at moderate concentrations and near the pKa value. It is less reliable for very strong acids or bases, polyprotic systems with widely separated pKa values, or solutions with high ionic strength where activity coefficients deviate significantly.

History: named after Lawrence Joseph Henderson, who described the relation in 1908, and Karl Hasselbalch, who refined and popularized it in 1916.