Dissoziation des β-Cyclodextrin-Phenolphthalein-Komplexes

1. Experiment is contained in the WAKER's Experimental Kit for Schools	yes
2. Experimental procedure has been modified	no
3. Video clip available	no
4. Flash animation available	no

8. Dissociation of the β-Cyclodextrin-Phenolphthalein Complex

1 Materials, Chemicals, Time Needed

Pipette
Test tube with bung or snap-cap vial
(Test tube stands)

β-Cyclodextrin-phenolphthalein-solution from Experiment 6
Sodium hydroxide solution conc = 5 mol•L^-1 C

This experiment can be performed in 2 minutes if you have already done the experiment to form the host-guest complex from β-cyclodextrin and phenolphthalein.

2 Procedure and Observations

To the β-cyclodextrin-phenolphthalein complex solution add sodium hydroxide solution, conc. = 5 mol•L^-1 drop by drop. The solution turns pink after a few drops have been added.

3 Discussion of Results

In this experiment, too, the existing equilibrium can be altered by modifying the concentration of hydroxide ions. The modified conditions compel some of the encapsulated phenolphthalein dianions to leave the host cyclodextrin. Discoloration of the solution can also be explained by looking at the equilibrium between the phenolphthalein dianion and the phenolphthalein lactone form.

Fig. 2.18: Molecular structures of phenolphthalein at pH < 8.5 and pH > 9

If the equilibrium at pH > 9 is not completely on the side of the dianion, the addition of hydroxide ions causes the equilibrium to shift to the right. If all the cyclodextrin molecules are already occupied by guest molecules, then the dianionic form is present in the solution, which causes the red-violet color observed in the solution.