pH indicators. pH indicators change their color depending on the pH of the solution. Here methyl orange indicator (left bottle) and phenolphthalein indicator (right bottle) have been added to an acidic solution and an alkaline solution (on left and right of the corresponding indicator, respectively). The acidic solution is 0.1 M sulfuric acid (H2SO4) and the alkaline solution is 0.1 M potassium hydroxide (KOH).