** **The **Henderson Hasselbalch equation** is** **an approximate equation that shows the relationship between the pH or pOH of a solution and the pK_{a} or pK_{b} and the ratio of the concentrations of the dissociated chemical species. In order to use the equation, the acid dissociation constant must be known.

## Equation

There are multiple ways to write the equation. Two of the most common are:

pH = pK_{a} + log ([conjugate base]/[weak acid])

pOH = pK_{a} + log ([conjugate acid]/[weak base])

## History

An equation to calculate the pH of a buffer solution was derived by Lawrence Joseph Henderson in 1908. Karl Albert Hasselbalch rewrote this formula in logarithmic terms in 1917.

## Sources

- Hasselbalch, K. A. (1917). "Die Berechnung der Wasserstoffzahl des Blutes aus der freien und gebundenen Kohlensäure desselben, und die Sauerstoffbindung des Blutes als Funktion der Wasserstoffzahl."
*Biochemische Zeitschrift*. 78: 112–144. - Henderson, Lawrence J. (1908). "Concerning the relationship between the strength of acids and their capacity to preserve neutrality."
*Am. J. Physiol*. 21: 173–179.