HendersonHasselbalchlikheten

Henderson-Hasselbalch equation is a mathematical expression used to calculate the pH of a buffer solution. A buffer solution is a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid, which resists changes in pH when small amounts of acid or base are added. The Henderson-Hasselbalch equation is derived from the definition of pH and the equilibrium expression for a weak acid dissociation.

The equation is expressed as:

pH = pKa + log([A-]/[HA])

where pH is the negative logarithm of the hydrogen ion concentration, pKa is the negative logarithm of the acid dissociation constant, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the weak acid.

The Henderson-Hasselbalch equation is particularly useful in biochemistry and medicine, where it is used to determine the pH of blood and other bodily fluids, as well as to design buffer systems for various applications. It is also used to calculate the pKa of a weak acid or base from the pH of a buffer solution and the ratio of the concentrations of the conjugate base and the weak acid.

The equation assumes that the concentrations of the weak acid and its conjugate base are much greater than the concentrations of other species in the solution, and that the activity coefficients are constant. These assumptions are generally valid for dilute solutions, but may not hold for concentrated solutions or solutions with high ionic strength. In such cases, more complex equations may be required to accurately calculate the pH of the solution.