The dissociation of tetrachloroaluminate anions in room temperature AICI~-I-n butylpyridinium chloride mixtures has been investigated as far as both reproducibility and reliability are concerned. The determination of the dissociation constants is based on the aluminum electrode potential variation measured around the equimolar composition. Experimental conditions have been determined which avoid the reduction of the pyridinium cation by A1 in slightly basic melts and where the potentials of the A1 electrode are stable. The average values of the dissociation constants are, respectively, (1.2 _ 0.2)10 -1~, (4.5 • 0.4)10 -13, and (7.5 • 0.8)10 -13 at 40 ~ 49 ~ and 55~ and are systematically lower than previously published values. The corresponding average values of hH and AS are, respectively, equal to 20.3 kcal mo1-1 and 6.5 eu. It has also been shown that the dissociation equilibrium of A1C14-into A12C17-and C1-is the only one required even in very acidic melts (up to 66.6 m/o of A1CI~), provided a correction is made for the junction potential.For the past few years, several groups of scientists have been interested in aluminum chloride containing melts. Most of these investigations deal with A1C18-alkali halide mixtures because of their particular properties. In particular, the chloroacidity (pC1) can be varied to a large extent, especially about the equimolar composition range (1-4). It has also been shown that the acidic nature of such melts stabilizes low oxidation state ions and organic radical cations (5-12). However, as pointed out by Koch et al., the use of aluminum chloride-inorganic chloride molten salts for organic reaction investigation is limited, essentially because of temperature problems (13).Recently, a new molten salt, first reported by Hurley and Wier (14), namely a 67/33 mol percent (m/o) aluminum chloride-ethylpyridinium bromide mixture, has been proposed as solvent for organic studies (15). It is liquid at room temperature, apparently very acidic (in a chloroacidity sense), and exhibits a large electrochemical window. However, although it has been used successfully to investigate different organic or inorganic systems (13,15,16), no quantitative data exist, either about the acidic strength of such melt or about the dependence of the acidic properties with respect to the melt composition. The main reason is that the mixture ethylpyridinium bromide-A1C13 is liquid at room temperature only in a limited range of composition, close to 2:1 and no investigation has been made outside this range.As a consequence, investigations have been directed to mixtures of A1C18 with other pyridinium salts, mainly n-methyl and n-butylpyridinium chlorides. The latter appears to be the most interesting; it is liquid at temperatures lower than 40~ across a large composition range (66-43 m/o of A1C13) whereas the former mixture is liquid at room temperature only in the range of 2:1 composition. Physical and spectroscopic properties of these melts (pure or mixed with benzene) have been investigated by Osteryoung et al...
