The Phthalein-Dye Test: Step by Step
The phthalein-dye test is a test to determine the presence of the phenol functional group. The test can be split into two parts, making a phenolphthalein solution then observing a color change.
Making the phenolphthalein:
• Measure 0.1g of both the phenol powder and phthalic anhydride powder into a test tube.
• Dissolve the solids in sulfuric acid (a solvent and a catalyst).
• Heat the solution in a water bath (to provide energy for the reaction).
• The product of this reaction is the phenolphthalein solution.
Observing the color change:
•Measure 15ml of NaOH into a beaker.
•Use a pipette to add a few drops of the phenolphthalein solution to the beaker.
A positive result will be indicated with a color change. The color the solution turns and the pH at which it does so depend on the phenol originally used. For example, alpha-Naphthol will create Naphtholphthalein which turns from pink to green around pH = 9.
Figure 1 - The reactions in the phthalein-dye test. Top: Phenol reacts with phthalic anhydride in a heated acidic environment to make phenolphthalein. Bottom: Phenolphthalein changes in structure when base is added, causing the color change.