# How do you calculate the pH of a buffer solution?

$p H = p {K}_{a} + {\log}_{10} \left(\frac{\left[{A}^{-}\right]}{\left[H A\right]}\right) .$
I am not going to derive this equation here (it is rather simple to do and worth doing), but it tells us that the $p H$ of a given buffer should remain tolerably close to the $p {K}_{a}$ of the acid that comprises the buffer. Added base or acid simply protonates/deprotonates the given base/acid, and should not grossly change solution $p H$.