The effect of additives on activities in cryolite melts

Research paper by Ernest W. Dewing

Indexed on: 01 Oct '89Published on: 01 Oct '89Published in: Metallurgical and Materials Transactions B


This paper assembles literature data, mainly vapor pressures and concentration-cell electromotive forces (emf's), from which the variation of activity of NaF and A1F3 when a third substance is added to Na3AlF6 can be deduced. The necessary theory is derived. Where there is more than one source of information on a given substance, the agreement is poor, showing imprecision in the experimental data. Substances are characterized by ∂ In aAlF3/∂N3, whereN3 is the molar fraction of the additive, and a value of -1 is neutral, since it corresponds to a dilution effect. Positive values show acids; A1F3 itself has a value of +27.9, while NaF has a value of -27.9. BeF2 and MgF2 are strong acids with values of +11, CaF2 is a weak acid with a value of +2, and LiF, SrF2, BaF2, and 1/3(A12O3) are weak bases with values of -2 to -3.