Acid-base titration. Demonstration of an acid-base titration using the indicator phenolphthalein. An alkaline solution is in the flask, with a few drops of the indicator added to give a purple colour. Acid is slowly added from the burette above. When the acid has completely neutralised the alkali in the solution, the indicator turns colourless. This technique is used to gather data for many investigations, such as determining the concentration of the acid or alkali.