Effect of the pH in the Color of an Alcoholic Extract of Red Hibiscus and p K h Determination

The spectrophotometric study in the visible region of a methanolic extract of red hibiscus petals (Hibiscus rose quinensis) establishes the acid–base equilibrium of an anthocyanin. A three-step didactic strategy was followed in this exercise. Starting with a clearly defined problem, the students learn the chemical principles involved, the design and the execution of experiments to obtain reliable data, and data interpretation. The absorption spectra of cayena methanolic extract shows a maximum at 530 nm and remains unperturbed when an aluminum chloride solution is added to the extract, which indicates that the molecule responsible for the color lacks hydroxyl groups at ortho positions; thus, it cannot produce chelates between carbonyl C-4 and free OH at the C-3 or C-5 positions. The extract of the red flower changes its color from red to green when the pH increases from 2 to 4. This was associated with the flavilium-ion dissociation according to the equation HAn ↔ H+ + An−, which is usual in many monobasic acid–base indicators. The pKh value obtained was 3.27.