# How do chemical buffer systems resist changes in pH?

A buffer consists of a weak acid and its conjugate base mixed together in APPRECIABLE concentrations. Added ${H}_{3} {O}^{+}$ or $H {O}^{-}$ tends to be sopped up by the acid or base already in solution, and $p H$ change is only marginal. In the absence of a buffer, $p H$ might change from 3-4 units.