Acid-base titration. In this sequence titration technique is used to find volume of an acid needed to neutralize a basic solution. Erlenmeyer flask contains 50 mL of sodium hydroxide (NaOH) solution of unknown concentration. Burette above it contains 1M solution of hydrochloric acid (HCl). First, a few drops of phenolphthalein is added to Erlenmeyer flask, turning the solution dark pink/fuchsia. Hydrochloric acid is then added gradually and the solution is stirred, until it becomes colorless. At this point, the solution in Erlenmeyer flask is considered neutral. The volume of the acid used is recorded and the initial concentration of the basic solution is calculated.