A buffer is a solution that can resist pH change upon the addition of an acidic or basic components. It is able to neutralize small amounts of added acid or base, thus maintaining the pH of the solution relatively stable. This is important for processes and/or reactions which require specific and stable pH ranges. Buffer solutions have a working pH range and capacity which dictate how much acid/base can be neutralized before pH changes, and the amount by which it will change.
Thumbnail: Simulated titration of an acidified solution of a weak acid (pKa = 4.7) with alkali. Image used with permission (Public Domain; Lasse Havelund).