Acid-base titration. Demonstration of an acid-base titration using the indicator phenolphthalein. A 1M (Molar) solution of hydrochloric acid is in the flask, with a few drops of the indicator added. 1M sodium hydroxide solution is slowly added. As the mixture in the flask approaches its end point (when the acid and base are exactly neutralised) adding a further drop of base just pushes the solution into being alkaline. This causes the indicator to show as purple.