Demonstration of phenolphthalein

Chemicals needed

distilled water

acetone  F   R 11    S (2)-9-16-23-33

sodium hydroxide solution 7,5%  C    R 35     S (1/2)-26-37/39-45

phenol C T     R 24/25-34    S (1/2)-28-45

phthalic anhydride Xn   R 22-37/38-41-42/43    S (2)-23-24/25-26-37/39-46

sulphuric acid 96%  C    R 35   S (1/2)-26-30-45

Equipment needed

2 test tubes
alcohol burner

Test procedure

• Safety goggles! Ventilation or exhaust hood!
• One adds into a test tube a small spatula-tipful phthalic anhydride as well as a spatula-tipful phenol (about twice as much) and then adds a drop of concentrated sulphuric acid.
• Now one carefully melts the mixture with the burner, whereby it turns blood-red and carefully heats it further until the mixture is such a deep red that it almost appears black. One should pay attention so that if at all possible the mixture does not boil.
• One lets cool and then adds about 3 ml acetone and sways or shakes vigorously so that the reaction mixture largely dissolves.
• In the second test tube one adds about 5 ml water and then adds a couple of drops of the acetone solution, whereby a lightly yellow color results. Now one further adds some drops of diluted sodium hydroxide solution. The typical pink-violet color of phenolphthalein in the alkaline range becomes visible.
  

Disposal

• The acetone solution can be disposed of as halogene-free organic waste.
• The watery solution can be discharged into the drainage.

Elucidation

Out of one molecule of phthalic anhydride and two molecules of phenol phenolphthalein together with water discharge results.

Phenolphthalein

Photos

The reaction mixture at the beginning(left) and at the end(right) of heating.

The typical coloration with phenolphthalein

Literature

Derived from experiment 20511 S, "Phenolphthalein", p. 335, [2],
where instead of concentrated sulphuric acid anhydrous zinc chloride is used.