Standard Enthalpy of Ionization (∆H) Specific to the Color-Transitions of Phenolphthalein Indicator in Different Medium

Abstract

Phenolphthalein is a widely used acid-base indicator whose visible color response is associated with pH-dependent structural transformation. This manuscript discusses the thermodynamic basis of the color transition of phenolphthalein, with emphasis on the relationship between ionization equilibrium, molecular rearrangement, and standard enthalpy change. In acidic and neutral media, phenolphthalein predominantly exists in a colorless lactone or benzenoid form. Under alkaline conditions, deprotonation promotes ring opening and formation of a conjugated quinonoid species, producing the characteristic pink coloration within the practical transition range of approximately pH 8.2–10.0. At very high alkalinity, further structural change may lead to fading or decolorization through formation of a colorless carbinol-associated form. The theoretical discussion is supported by basic thermodynamic relations, including the Van’t Hoff equation and the Gibbs free energy expression, to explain how temperature, equilibrium constant, enthalpy, and entropy are connected during ionization. The manuscript also summarizes selected pH transition ranges of common acid-base indicators to place phenolphthalein in a comparative context. Overall, the work presents phenolphthalein color change as a coupled acid-base, structural, and thermodynamic process. The discussion highlights the need for controlled pH, temperature, concentration, and observation conditions when interpreting color transitions and related thermodynamic parameters.