Abstract: We developed a molecular simulation experiment focusing on the color-changing mechanism of phenolphthalein. The project incorporates molecular model construction, structural optimization, frontier orbital analysis, and spectral calculations, allowing students to explore the electronic-level correlation between pH-induced molecular structural changes and corresponding color variations. Serving as an extension to conventional acid-base titration experiments, this approach employs molecular visualization to demonstrate phenolphthalein's color transition, thereby facilitating students’ fundamental understanding of acid-base indicator mechanisms.

Key words: Quantum chemical calculation, Inorganic and analytical chemistry experiment, Color-changing mechanism of phenolphthalein, Acid-base titration