For acids and bases that do not completely dissociate, the chemical systems exist as acid-base equilibria in which the forward (dissociation) reaction occurs at the same rate as the reverse (recombination) reaction. The cations of many salts, when dissolved in water, also affect the pH and produce an equilibrium between water and hydronium (H3O+) or hydroxide (OH–) ions. In addition, acids that can donate more than one proton (polyprotic acids) exist as several species in equilibrium. Adding more acid, base, or salt to a solution affects the pH, a phenomenon that can be minimized by the use of buffers. Buffers usually consist of a weak acid and its conjugate base. They are able to absorb added particles to a degree and therefore limit the change in pH. The concentration of ions at equilibrium can be determined by titration, a process of adding either acid or base until the base or acid in solution has been neutralized, often performed using an acid-base indicator.