Molecular Recognition: Detection of Colorless Compounds Based On Color Change

A laboratory experiment is described in which students measure the amount of cetirizine in allergy-treatment tablets based on molecular recognition. The basis of recognition is competition of cetirizine with phenolphthalein to form an inclusion complex with β-cyclodextrin. Phenolphthalein is pinkish under basic condition, whereas it’s complex form with β-cyclodextrin is colorless. Addition of the cetirizine leads to release of phenolphthalein from the phenolphthalein-β-cyclodextrin inclusion complex that alters the solution color under basic conditions. The intensity of the color change is proportional to the cetirizine concentration. This simple experiment provides an opportunity for students to become familiar with the concepts of inclusion complexes and molecular recognition. Students also gain hands-on experience with a spectrophotometer and engage in plotting a calibration curve based on absorbance or color changes and determination of cetirizine.