A buffered solution is one that resists changes in its pH when small amounts of acid or base are added, or when the solution is diluted. Buffer solutions contain an acid to react with added OH and a base to react with added H+. These can be any weak acid–weak base pair, but are usually a conjugate acid–conjugate base pair. The pH of the buffer depends on the pKa of the buffering substance and on the relative concentrations of conjugate acid and base, and can be calculated using the Henderson–Hasselbalch equation.

Acidic buffer solutions (pH < 7) are commonly made from a weak acid and one of its salts—often a sodium salt. An example is a solution of acetic acid (pKa=4.75) and sodium acetate. If the solution contains equimolar concentrations of the acid and salt, it will have a pH of 4.75. An alkaline buffer solution (pH > 7) is commonly made from a weak base and one of its salts. An example is a solution of ammonia (pKa =9.25) and ammonium chloride. If these are mixed in equimolar proportions, the solution has a pH of 9.25.