Solubility of FeCl₂in Molten NaCl–AlCl₃

Measurements of the solubility of FeCl2 in molten NaCl-AlCl3 are determined by an aliquot sampling technique at 175 and 255°C for acidic and basic compositions. There is a pronounced compositional dependence of the FeCl2 solubilities with a minimum in solubility at approximately the equimolar solvent composition. The solubilities (in mol fraction FeCl2) at the minimum are determined to be 3.79 × 10−5 and 1.59 × 10−4 at 175 and 255°C, respectively. The minima in solubilities are described by physical solution theories and concepts (i.e. coordination cluster theory (CCT)). The solubilities in these highly ordered NaCl–AlCl3 solutions are also described in chemical terms. The true solubility product for FeCl2, K sptrue, is calculated to be 9.18 × 10−17 and 2.58 × 10−15 at 175 and 255°C, respectively. The magnitudes of the solubility products are primarily related to the fact that the standard free energy of formation of 2NaAlCl4 from 2NaCl and 2AlCl3 is considerably more negative than the corresponding free energy for Fe(AlCl4)2. In basic and in somewhat acidic melts, solubilities are largely related to the formation of the associated ionic species FeCl+, FeCl2, FeCl3−, and FeCl42−. From our measurements, we calculate the formation constant (in ionic fractions) of FeCl+ to be K 11 = 8.63 × 106 at 255°C. Furthermore, the specific bond free energy for this associated complex, ∆A, −14.50±0.50 kcal mol−1 (−60.67 ±2.09 kJ mol−1), is independent of temperature. The formation constant for the associated species FeCl22− from Fe2+ and 4Cl− ions is estimated to be of the order of 1014 at 255°C.