Abstract: Provided is an ionic liquid, represented by the formula K+A?, having a very low content of impurities, especially a low content of halogen ions and/or a low content of alkali metal, wherein K+is a quaternary ammonium ion, for example an imidazolium ion. Also provided is a quaternarization process for making the ionic liquid in reacted with an acid ester such as a sulfate or phosphate.

Abstract: An ionic liquid with an extremely low content of an impurity such as halogen ions which can be obtained easily at a relatively reduced cost, the ionic liquid comprising a pair of a cation K+ and an anion A? represented by the general formula (1) in which the content of an alkali metal as an impurity is 5 ppm or less and the content of a halogen ion is 1 ppm or less, the cation being preferably selected from the group represented by the following general formula (2) obtained by quarternizing a tertiary amine compound or a tertiary phosphine compound by using an acid ester represented by the following general formula (3) and then conducting salt exchange: in which R1 to R4 in the formula (2) each independently represents a linear or branched alkyl group of 1 to 8 carbon atoms, X represents a hetero atom and R in the formula (3) represents a linear or branched alkyl group of 1 to 8 carbon atoms.