Solution of a weak acid and its anion (that is, its conjugate base) or of a base and its common cation are buffered. When we add a small amount of acid or base to any one of them, the $pH$  of solution changes very little, $pH$  of buffer solution can be computed as

for  $\text{acidic\hspace{0.17em}buffer\hspace{0.17em}}:pH=p{K}_a+\log \frac{\left[\text{Conjugate\hspace{0.17em}\hspace{0.17em}base}\right]}{\left[\text{Acid}\right]}$
for  $\text{basic buffer\hspace{0.17em}}:pOH=p{K}_b+\log \frac{\left[\text{Conjugate\hspace{0.17em}\hspace{0.17em}acid}\right]}{\left[\text{Base}\right]}$
It is generally accepted that a solution has useful buffer capacity ( $pH$ change resisting power) provided that the value of [salt or conjugate base]/[acid] for acidic buffer lies within the range of 1 : 10 to 10 : 1. Buffer capacity is maximum when [conjugate base] = [acid]