Transport numbers in the molten system NaF-KF-AlF3-Al2O3

Transport numbers in the molten system NaF-KF-AlF3 (Al2O3, CaF2) were investigated by the Hittorf method. It was confirmed that in molten cryolite, Na3AlF6, 1,010 A degrees C, the current is transported almost exclusively by the Na+ cations (t(Na+) = 0.99). When AlF3 was added to a Na3AlF6 melt, the transport number of sodium cations decreased to 0.74 at the composition corresponding to NaAlF4. In molten K3AlF6, the transport number of K+ cations equals 0.836 at 1,005 A degrees C. In melts containing both Na+ and K+, the cations contribute to the charge transport approximately in the ratio of the squares of their ionic radii. When 5 mass % of CaF2 was added to the molten NaF-KF-AlF3 system, it remarkably influenced the transport numbers of potassium and fluoride anions.