Study of the dynamic equilibrium in the CaF2/aqueous solution system using 45Ca2+ as radiotracer

Abstract

The transport of Ca²⁺ ions at the interface between CaF₂ crystals and an aqueous solution of CaF₂ has been studied under equilibrium conditions with the aid of ⁴⁵Ca²⁺ as a radioactive tracer. The kinetics of the isotopic exchange between CaF₂ and its saturated solution appears to consist of a fast and slow component with rate constants of ca. 2 × 10^–5 and 1 × 10^–6 s^–1, respectively. Total fluxes of Ca²⁺ from the solution to individual crystal faces of the order of 10^–10 mol m^–2 s^–1 were found. The exchange was retarded by the presence of some foreign ions in the solution, but eventually the same number of Ca²⁺ ions were transported as for pure solutions. The crystallographic orientation ({100} or {111}) of the faces did not have a large influence on the exchange rate, but the relative amount of transported Ca²⁺ ions (compared with the amount present in the first crystal layer) was lower for the {100} faces than for the {111} faces. Also, it has been demonstrated that some 30% of the ⁴⁵Ca²⁺ remains inside the crystal when re-exchange with a saturated CaF₂ solution which initially does not contain tracer ions is attempted, probably by diffusion into the crystal.