1. Acid-Base Theories, Buffer Action & Henderson-Hasselbalch Equation

An Acid-Base Buffer is an aqueous solution consisting of a mixture of a weak acid and its conjugate base (e.g., $CH_3COOH + CH_3COONa$) or a weak base and its conjugate acid (e.g., $NH_4OH + NH_4Cl$) that possesses the property of resisting significant changes in pH upon the addition of small amounts of strong acid ($H^+$) or strong base ($OH^-$). Buffer Capacity ($\beta$, Van Slyke) is the ratio of the increment of strong acid/base added to the small resulting change in pH: $\beta = \frac{\Delta B}{\Delta \text{pH}}$. Buffer capacity is maximal when $\text{pH} = \text{pK}_a$.