Time-resolved electrochemical impedance spectroscopy study of calcium fluoride formation and ion transport dynamics in highly concentrated CaSO₄–NaF systems

This work investigates the reaction kinetics in a highly concentrated (×50) mixture of calcium sulfate (CaSO₄) and sodium fluoride (NaF) using real-time, in-situ Electrochemical Impedance Spectroscopy (EIS) over 48 hours. The primary objective was to establish the electrochemical profile and monitor ion transport dynamics under aggressive, non-equilibrium conditions designed to accelerate precipitation kinetics. Analysis of the complex conductivity spectra (σ′ and σ′′) and Nyquist plots, modeled with an equivalent electrical circuit R₁(R₂//CPE), revealed two distinct kinetic regimes. The temporal evolution of the dc conductivity (σ₀) showed an initial decrease, followed by a rise, culminating in stabilization after approximately 19 hours. This stabilization is interpreted as the establishment of a dynamic equilibrium state. Concurrent trends in the high-frequency conductivity (σ∞), CPE exponent (p), and relaxation time (τ) suggest significant microstructural evolution within the system. While the electrochemical data are consistent with the expected precipitation of CaF₂ and provide a kinetic profile suggesting a potentially faster route to equilibrium than traditional methods, this study focuses on establishing EIS as a monitoring tool. Direct analytical confirmation of the solid phase and quantitative yield analysis are recognized as essential next steps and are the focus of immediate future work.