Acid-Base Indicators

An acid-base indicator is itself an acid-base pair, but one in which the acid form absorbs light in one part of the visible spectrum and the base form in another. The central point of transition is the pH at which the acid and base forms occur at equal concentrations. That pH determines the pK of the acid form, or the pKa. The full color change occurs over an approximate pH range of 2, representing a change of [base form]/[acid form] from 1:10 to 10:1. The figure below shows the transition pH of nineteen common acid-base indicators.