CaCl2 and MgCl2 as buffering agents for room-temperature chloroaluminate ionic liquids

We have found that it is possible to buffer acidic 1-ethyl-3-methylimidazolium chloride (EMIC)/AlCl3 melts to neutrality using CaCl2 as a buffering agent, while MgCl2 will partially buffer acidic melts. The buffering reaction is MCl2(s) + 2Al(2)Cl(7)(-) --> M2+ + 4AlCl(4)(-) where M is Ca or Mg. From studies using the Ag/AgCl electrode as a chloride-sensitive electrode and from measurements of the relative solubility products of LiCl and CaCl2, it appears that the residual concentration of the acidic Al2Cl7- ion in the CaCl2- buffered melts is significantly higher than in any of the other neutral buffered melts studied so far, making this melt more acidic. These melts show the phenomenon known as "latent acidity," forming an AlCl3 complex with the weak Lewis base acetylferrocene. When MgCl2 is used as a buffering agent, the reaction above does not go to completion; this is explained in terms of the solubility product of MgCl2. (C) 1999 The Electrochemical Society. S0013-4651(98)12-094-3. All rights reserved.