Abstract
The well-known cryoscopy equation for a binary system was extended to the ternary system NaF-AlF3-B, where B is a substance present in small amounts. It was derived that the depression in the liquidus temperature for a given molar fraction of B depends on how B influences the activity of NaF, as well as on the NaF/AlF3 molar ratio. This implies that it is difficult to estimate the effect on the liquidus temperature of contaminants in the Hall-Héroult bath, and the three-component cryoscopy equation can be regarded as a theoretical justification of the cross-terms used in most empirical equations for calculating the liquidus temperature. New cryoscopic data are given for the system Na3AlF6-CaF2 as well as for some substances that are normally present in only small amounts, and empirical equations containing cross-terms between these substances and aluminium fluoride are suggested.