Abstract: The determination of the acidity of ionic liquids is of crucial significance to the utilization of ionic liquids. By using pyridine and acetonitrile as IR spectroscopic probes, the acidity of common room temperature ionic liquids was determined. For the probe pyridine, the IR band near 1450, 1540 cm^－1 of pyridine is indicative the Lewis/Brønsted acidity of ionic liquids respectively. For another probe acetonitrile, the increase of the C≡N stretching wave number near 2253 cm^－1 with a new band appear at the higher wave number is indicative of the Lewis acidity. The influence of the structure of ionic liquids on the Lewis/Brønsted acidity by the IR spectroscopy was investigated. The Lewis acid strength of different ionic liquids has been compared through the shift of the 1450 cm^－1 of pyridine. A more obvious comparison of the Lewis acid intensity of different ionic liquids has been made by acetonitrile probe.

Key words: Ionic liquids, Lewis acidity, Brø, nsted acidity, Acidity determination, IR spectroscopy