Hydrochloric Acid → 1-Butyl-3-methylimidazolium tetrafluoroborate

116.5 g (0.667 mol) of 1-butyl-3-methylimidazolium chloride are initially introduced at 70° C. in the liquid state and dissolved in 120.6 g of approx. 50% aqueous HBF4 (approx. 3% excess). No warming and only slight HCl gas formation is observed in the process. 250 ml of 1,4-dioxane are subsequently added, and 250 ml of HCl-containing water/dioxane azeotrope are distilled off at atmospheric pressure (85-90° C.). A further 200 ml of dioxane are then added, and 250 ml of HCl-containing water/dioxane mixture are distilled off at atmospheric pressure (85-101° C.). The residue in the distillation flask has a low content of chloride ions (silver nitrate test). After a further azeotropic distillation with 100 ml of dioxane, the chloride ion content in the residue is too low to be detected using the silver nitrate test. Drying of the distillation residue under reduced pressure at 1.3 Pa and 80° C. gives 149.2 g of 1-butyl-3-methylimidazolium tetrafluoroborate as liquid. The yield of 1-butyl-3-methylimidazolium tetrafluoroborate is approximately quantitative. The chloride content in the ionic liquid is 11 ppm, measured by microtitration using a silver nitrate solution in non-aqueous medium. The end point is determined potentiometrically using a Cl-selective electrode.