Ion-specific control of chlorine hydrolysis in concentrated NaCl and NaClO4 solutions

Thermodynamic descriptions of the chlor-alkali process are sensitive to the treatment of chlorine hydrolysis under concentrated electrolyte conditions. Reported equilibrium constants for this reaction exhibit meaningful variation when extrapolated from dilute solutions, complicating comparison between studies and introducing uncertainty into process-relevant speciation calculations. In this work, UV–Vis spectroscopy is combined with a Pitzer-based thermodynamic framework to examine chlorine hydrolysis in aqueous sodium perchlorate and sodium chloride electrolytes across industrially relevant ionic strengths 0.028 ≤ I m ≤ 2.1 mol k g − 1 at 298.15 K. A thermodynamically consistent reference value for the chlorine hydrolysis is adopted and used to quantify electrolyte-dependent non-ideal effects through explicit treatment of ionic activity coefficients and water activity. The analysis separates the competing contributions of electrostatic stabilisation and solvent activity suppression, yielding a non-monotonic dependence of the equilibrium position on ionic strength. Comparison of chloride and perchlorate media at matched ionic strengths demonstrates that equilibrium shifts cannot be described by ionic strength alone but depend on electrolyte identity through ion-solvent and ion-ion interactions. The experimental measurements are consistent with the framework in both electrolytes, providing a structured basis for interpreting chlorine hydrolysis in concentrated aqueous systems and for reconciling historical equilibrium data under non-ideal conditions. • Separates intrinsic chlorine hydrolysis thermodynamics from electrolyte effects. • Demonstrates transferability of a single activity-based reference across brines. • Reveals ion-specific equilibrium shifts beyond ionic strength alone. • Provides a methodology for comparing equilibrium constants across electrolytes.