When can the Henderson-Hasselbalch equation be used?

The equation is applicable under the following conditions: – The solution contains a weak acid and its conjugate base, or a weak base and its conjugate acid. – The concentrations of the acid and base are known, and the acid dissociation constant (Ka) or base dissociation constant (Kb) is available. – The solution behaves ideally, meaning activity coefficients are close to unity, which is generally valid for dilute solutions.