Buffer solutions resist changes in pH upon the addition of acids or bases. They are necessary in laboratories where a solution of constant pH is required. There are two main types of buffer solutions: acidic buffers, made from a weak acid and its salt with a strong base, and alkaline buffers, made from a weak base and its salt with a strong acid. The buffering action occurs when the addition of H+ or OH- ions causes the equilibrium of the weak acid/base and its conjugate salt to partially offset the change.