Buffer solutions have constant acidity and alkalinity because:

A buffer solution resists change in pH because of its ability to neutralize small amounts of added acid or base.If ${\text{H}}^{+}$ ions are added, they react with the conjugate base of the buffer to form unionized weak acid.If ${\text{OH}}^{-}$ ions are added, they react with the weak acid of the buffer to form unionized water and the conjugate base.This keeps the concentrations of free ${\text{H}}^{+}$ or ${\text{OH}}^{-}$ ions relatively constant.