Solutions that resist changes in their pH, even when small amounts of acid or base are added.
A common solution for an acidic buffer is ethanoic acid (CH3COOH) and sodium ethanoate (NaCH3COO). The ethanoic acid acts as the weak acid (CH3COO-) and the sodium ethanoate forms the conjugate base (Na+ CH3COO-).
A common solution for an alkaline buffer is ammonium chloride (NH4Cl) and ammonia (NH3). The ammonium chloride acts as source of the weak acid NH4+ and the ammonia acts as the conjugate base.