DETERMINATION OF ORGANIC-ACIDS IN DILUTE HYDROCARBON SOLUTION USING POLY(ETHYLENIMINE)-COATED PIEZOELECTRIC-CRYSTALS

Some of the factors that influence the resonant frequency of oscillating 10-MHz AT-cut quartz crystals in liquids are investigated. The relationship between the measured frequency shifts and the physical properties of the liquids are examined, and reasons for departure from ideal behavior are discussed. The sorption characteristics of crystals coated with apolymer film (poly(ethylenimlne)) containing basic functional groups are reported. These coated crystals are shown to behave as selective detectors for a range of organic acids in dilute hydrocarbon solution. The lower limit for reliable measurement using a crystal coated with a 40-kHz load of poly(ethylen-Imlne) is estimated to be approximately 25 ppm of phenol in n-heptane. Fick's law appears to govern the rate of uptake of mass by the polymer film; however some strong acids show Irreversible sorption. The crystals function reliably undersolvent flow conditions, where the response time is determined by the thickness of the coating and the flow cell volume. Sensors of this type may find application in areas such as high-performance liquid chromatography detection, Industrial process stream analysis, and fuel-quality monitoring. © 1990, American Chemical Society. All rights reserved.