Synthesis of phenolphthalein

The name phenolphthalein indicates that the substance is made from phenol and phthalic acid.

Equipment

Test tube, beaker (400 ml), pipettes, filter

Chemicals

Phenol (C), phthalic anhydride (Xn), concentrated sulfuric acid (C), sodium hydroxide solution (c=1 mol/l; C), hydrochloric acid (c=1mol/l; Xi).

How-to

The same amounts of phenol and phthalic anhydride are mixed in a test tube and a few drops of concentrated sulfuric acid are added to the mixture. The test tube is then carefully heated until a thick, deep red liquid forms. This is poured into a beaker filled with plenty of water (500 ml). It solidifies into almost white flakes. If you’ve heated the mixture too long, they can also turn light yellow. If the melt is allowed to cool in the test tube, a red solid forms.

A sample of the solution including some flakes is placed in a fresh test tube. When adding caustic soda you can see the pink color of phenolphthalein. The solution can be decolorized with hydrochloric acid.

Note: You have to add a surprisingly large amount of caustic soda for the first time. This is because the solution is strongly acidic due to the sulfuric acid.

The phenolphthalein is then filtered off and rinsed with distilled water until it reacts neutrally. It is then dried and stored. The powder can also be recrystallized for further purification.

What happened?

Sulfuric acid serves as a catalyst. In addition, the sulfuric acid binds any water and thus shifts the equilibrium. (This condensation reaction can be compared to the classic Fischer-esterification) The sulfuric acid also ensures that the melt is initially red in color, which is strongly acidic.

If we pour the red melt into a lot of water, it becomes less acidic and therefore colorless. But: You only have to add caustic soda, and the solution quickly turns purple. The color therefore depends on the pH environment of the solution. That is why phenolphthalein is called an acid/base indicator. Because of its distinctive color change, it is probably the most famous of all indicators.

The reaction equation for the synthesis of phenolphthalein
The reaction equation for the synthesis of phenolphthalein

Substances that change color with the pH value show the phenomenon of halochromism.

In this experiment we have learned that a large dye molecule can be built up from small, colorless molecules.