Strength and Amount of Solid Acid

When measuring the strength of a solid acid or base, it should be recognized that activity coefficients for species on the solid are unknown. Therefore, acidity and basicity functions for the solid are not properly defined thermodynamically. Nevertheless, the acidity and basicity functions are clearly valuable in a relative sense, while the absolute values are also useful provided the above limitations are recognized and numerical accuracy is not overstated. 

The acid strength of a solid is defined as the ability of the surface to convert an adsorbed neutral base into its conjugate acid. If the reaction proceeds by means of proton transfer from the surface to the adsorbate, the acid strength is expressed by the Hammett acidity function 

The amount of acid on a solid is usually expressed as the number or mmol of acid sites per unit weight or per unit surface area of the solid, and is obtained by measuring the amount of a base which reacts with the solid acid. This is also sometimes loosely called acidity . 

For the determination of strength and amount of a solid acid, there are two main methods an amine titration method using indicators and a gaseous base adsorption method. 

The experimental details of the acid strength determination are described in earlier publications. The acid strength of a solid superacid which is very sensitive to moisture can be determined by observing the color change of an indicator whose vapor 

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