Surface acidity of bentonite was investigated by using three methods. The first method was inverse gas chromatography; the acid-base character of bentonite surface was estimated by the retention time with different nonpolar and polar probes at infinite dilution region. The adsorption specific free energy ( G SP ), specific enthalpy ( H SP ), and specific entropy ( S SP ) of polar probes on bentonite were determined. By correlating H SP with donor and acceptor numbers of the probes, the acidic (K A ) and the basic (K D ) parameters of the sample were calculated. The second method was recording the spectrums of pyridine by Fourier transform infrared spectroscopy (FTIR); the spectrum of pyridine bonded by Bronsted and Lewis acid sites was obtained. The last method was modified amine titration by using Hammett indicators.
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