Henderson-Hasselbalch equation

What is the Henderson-Hasselbalch equation used for?

The Henderson-Hasselbalch equation is a fundamental tool in chemistry and biochemistry for predicting the pH of buffer solutions. It is derived from the acid dissociation constant (Ka) and is expressed as: pH = pKa + log10([A⁻]/[HA]), where [A⁻] is the concentration of the conjugate base and [HA] is the concentration of the undissociated acid. This equation is particularly useful in pharmaceutical formulation, enzyme kinetics, and analytical chemistry, where maintaining a stable pH is critical.

How does the equation apply to biological systems?

In biological systems, the equation helps understand the ionisation state of molecules such as amino acids, proteins, and drugs at physiological pH. For example, it can predict whether a drug molecule will be in its ionised or non-ionised form, which affects its solubility, membrane permeability, and bioavailability. The equation is also used in the design of buffer systems like HEPES or Tris, which are essential in cell culture, PCR, and protein purification workflows.

What are the limitations of the equation?

The equation assumes ideal behaviour and is most accurate when the concentrations of acid and base are significantly higher than the concentration of H⁺ or OH⁻ ions. It becomes less reliable at very low or high pH values, or when the acid is strong or highly concentrated. Additionally, it does not account for ionic strength effects or activity coefficients, which can be significant in complex biological matrices.

Related concepts

The equation is closely linked to buffer capacity, pKa determination, and the use of pH indicators. It is often applied alongside techniques such as HPLC, NMR, and titration to characterise compounds and validate formulations.