$HIn \rightleftharpoons H ^{+}+\operatorname{In}^{\ominus}$

$\therefore E _{\text {Equilibrium Constant }}, K _{ In }=\frac{\left[ H ^{+}\right]\left[ In ^{-}\right]}{[ HIn ]}$

$\therefore pK _{\text {In }}= pH -\log \frac{\left[ In ^{\ominus}\right]}{[ HIn ]}$

$\Rightarrow \log \frac{\left[\operatorname{In}^{\ominus}\right]}{[ HIn ]}= pH - pK _{ In }$