The indicator thymol blue has $\mathrm{pH}=2.0$ when half of the indicator is present in the ionized form. The  $\mathrm{p}{K}_{\text{Indicator }}^0$ is

$\mathrm{pH}=\mathrm{p}{K}_{\mathrm{HIn}}^{\circ }+\log \frac{\left[{\mathrm{In}}^{-}\right]}{\left[\mathrm{HIn}\right]},$

Since $\left[{\mathrm{In}}^{-}\right]=\left[\mathrm{HIn}\right],\mathrm{p}{K}_{\mathrm{HIn}}^{\circ }=\mathrm{pH}$

A chemical substance that changes color in the presence of an acid or base is an indicator. The majority of indicators come from plant pigments and have a moderately acidic or basic composition.