Phenolphthalein is a weak acid used in titration to provide visual indication of the equilibrium point between the concentration of moles of acid and moles of base. Phenolphthalein is colorless when in an acidic solution and pink when in a basic solution.
When performing a titration, a specific volume of a solution with unknown concentration is volumetrically pipetted into a flask. A few drops of phenolphthalein are added to determine if the solution is acidic or basic. If the solution turns pink, it is basic. If it remains clear, it is acidic. A stir bar and plate are employed throughout the titration to maintain a homogenous solution. For titration of an acidic solution, a basic solution with known concentration is added to a volumetric burette. The starting volume is noted. Slowly, the burette is opened and the solution is observed. When pale pink streaks appear, the flow rate is decreased. The entire solution will turn pink before flashing back to clear. The burette should be closed at this point. Subsequent volume additions are done one drop at a time and the burette tip is rinsed with water between drops to ensure that all moles of the base have been added. When the stirring solution turns and remains pink, the final volume is noted.
The starting volume is subtracted from the final volume to determine the total volume delivered to the flask. The unknown concentration (M1) is determined by using the dilution equation: (M1) (V1) = (M2) (V2), where V1 is the initial pipetted volume, M2 is the known molarity of the basic solution, and V2 is the known total volume used in the titration.