CHLORO COMPLEXES IN MOLTEN-SALTS .6. POTENTIOMETRIC AND VAPOR-PRESSURE STUDY OF THE SYSTEM KCL-ALCL3 IN THE TEMPERATURE-RANGE 275-350 DEGREES-C

By potentiometric measurements with chlorine/chloride electrodes (and aluminum electrodes) the solubility of KCl in KCl-AICI3 melts was examined in the temperature range 275-350 °C. A plot of -pKs (where KS is the solubility product of K+ and CI- in molar concentrations) vs the inverse absolute temperature in the above temperature range gave an almost linear plot. From this plot ΔS° and ΔH° (for dissolution of KCl in KAlCl4) were calculated to be 46.8 (11) J/(deg mol) and 19.7 (6) kJ/(mol), respectively. It was furthermore found that for up to a mole fraction of AlCl3 of 0.524 62 in the temperature range from 275 to 350 °C, the potentiometric results were best explained by a combination of two reactions: 2AICI4-⇆Al2Cl7- +Cl- and Al2Cl7-⇆Al2Cl6+Cl- (model 1) or 2AlCl4-⇆Al2C17- +CI- and 3Al2Cl7-⇆2Al3Cl10-+Cl (model 2). However, combined potentiometric and vapor pressure measurements could better be explained by three reactions (model 3): (i) 2A1Cl4- ⇆Al2Cl7- +Cl-, (ii) 3Al2Cl7- ⇆2Al3Cl10- +Cl-, and (iii) 2Al3Cl10- ⇆3Al2Cl6+2C1-. The pK values (based on molar concentrations) for reaction (i) were found to be 8.15 (1), 7.80 (2), 7.47 (2), and 7.15 (3) at 275, 300, 325, and 350 °C, respectively. Possible values for reaction (ii) were found to be 7.6 (4), 7.3 (6), 7.1 (7), and 7.1 (11) and for reaction (iii) 15.4, no value found, 14.0, and 12.1 at 275, 300, 325, and 350 °C, respectively. From the temperature dependence of reaction (i) it was found that in the temperature range 275-350 °C the values of ΔS° and ΔH° were 0.9 (22) J/(deg mol) and 86.0 (12) kJ/mol, respectively. © 1979, American Chemical Society. All rights reserved.