Thymolphthalein acid-base reaction. Footage of an acid being poured into an alkali, with an acid-base (pH) indicator in the acid being used to track the reaction. The indicator used here is thymolphthalein, which changes colour as the pH changes. Below a pH of around 10, it is colourless. Above that, it is blue. The pH of the acid is below 10, so the thymolphthalein is colourless. As the acid is added to the alkali, the thymolphthalein being added turns blue because the combined pH is above 10. In this clip, the acid is added slowly up to the point where the volumes are equal (75 millilitres each). At this point, the final volume of acid is added rapidly, the pH falls below 10, and the indicator turns colourless again.