Acidity of neutral buffered 1-ethyl-3-methylimidazolium chloride AlCl3 ambient-temperature molten salts

A series of studies on the acidity of AlCl3-1-ethyl-3-methylimidazolium chloride (EMIC) melts buffered with alkali metal chlorides were carried out. The solubility of HCl, a strong Bronsted acid in these melts, was measured in melts buffered with LiCl, NaCl, and KCl. The solubility of HCl in all three melts is 450-475 mM under 1 atm of HCl, approximately the same as that in the acidic (AlCl3-rich) melts. The relative solubility products of LICl, NaCl, and KCl were measured, and it was found that K-sp(NaCl)/K-sp(LiCl) = 72 +/- 6 and K-sp(KCl)/K-sp(NaCl) = 1000 +/- 400. It is likely that the differences in the acidity of HCl in the various melts are due to the differences in the solubility product of the relevant alkali metal chlorides. These ratios are consistent with the results of previous studies on the acidity of HCl in the melts. The concentrations of the strongly Lewis acidic Al2Cl7- ion in melts buffered with LiCl were measured using an aluminum electrode. The results of the potentiometric work indicate that a melt containing 1 M Li+ (approximately nAlCl(3)/n(EMIC) = R = 1.25:1 prior to buffering) would contain about 200 mu M Al2Cl7-. This corresponds to a solubility product of about (1.5 +/- 0.5) x 10(-12) M-2. The liquid junction potentials between unbuffered and buffered melts were found to be about 49 mV x ([Li+]/M). These results are related to previous work on the acidity of HCl in these melts, and it is shown that it is possible to explain many of the acidity and latent acidity results on the basis of the solubility products of the alkali metal chlorides.