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.

Phenolphthalein is used as a pH indicator with a color change from clear to light pink around pH = 8. We can use this to distinguish a positive result.

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.