Activity product constant of cryolite at 25°C and one atmosphere using selective-ion electrodes to estimate sodium and fluoride activities

The activity product constant of cryolite (Na₃AlF₆) at 25°C and 1 atm total pressure was calculated from data for solutions from which synthetic cryolite or mixtures of cryolite and a solid apparently related to ralstonite had precipitated. The activities of fluoride and of sodium were estimated using specific ion electrodes. The activity of Al³⁺ was calculated from aluminum concentrations using equations which involve aluminum species existing in water solutions containing fluoride and hydroxide ions.

For the reaction Na₃AlF₆(c) = 3Na_(aq)⁺ + Al³⁺_(aq) + 6E^-_(aq) the activity product constant K_so is 10^−33.84, and the standard free energy of formation at 25°C is -745.4±1.0 kcal mole⁻¹. This latter is identical to a value from the literature which is based on calorimetric data.

Although cryolite and related minerals are easily precipitated in the laboratory, their relative scarcity in nature suggests that the effect of influences, perhaps solutes other than those occurring in the structure, may inhibit or prevent the formation of cryolite.