Conductometric study of lithium halides and thiocyanate in the mixed solvents of tetrahydrofuran and 2-ethyl-1-hexanol

Abstract

Ion association constants of lithium chloride, bromide and thiocyanate were evaluated by means of conductometry in the mixed solvents of THF (εr=7.58) and 2-ethyl-1-hexanol (εr=7.58) at 25°C. The mixed solvents were assumed to have a nearly constant permittivity but different solvation abilities. Lithium bromide gave a minimum on the molar conductivity Λ (S cm2 mol−1)−c1/2 curve at a rather low salt concentration (cmin=4.9×10−3 mol dm−3) in THF; however, the cmin increased suddenly to 1.2×10−2 mol dm−3 by the addition of 1.0 vol.% alkanol, and increased gradually by the further addition of the alkanol. The cmin for LiSCN increased gradually with increasing content, up to 30 vol.% of the alkanol. The conductivities of LiBr and LiSCN were analyzed by our method to obtain the formation constants of the ion pair (K1), the triple ions (2Li++X−⇌Li2X+, Li++2X−⇌LiX2−; K2=K3) and the quadrupole (K41) in THF, whereas quadrupole formation constants were not needed for explaining the conductivities in the mixed solvents. In the higher concentration range of LiBr and LiSCN in THF, the ion pair and the quadrupole were found to be the predominant species over the single ions or the triple ions. The increase in cmin with increasing content of 2-ethyl-1-hexanol was in good accordance with the decrease in the formation constants of the higher aggregations. A linear relation (the slope of −1.0) was given between log(K2/K1) and log cmin for LiBr and LiSCN in the binary solvent systems. Lithium chloride gave no minimum, not only in THF but also in the mixed solvents, at least ≤10 vol.% of 2-ethyl-1-hexanol, because of a strong trend to form the quadrupole.