By ADSCIENTIS
The acidity and the basicity constants of a solid can be determined by injecting polar probes whose electron acceptor (AN) and donor number (DN) are known. These numbers belong to the semi-empirical scale of Gutmann (1978), corrected by Mukhopadhyay P., Schreiber (1995). The ISP of a probe is related to its constants DN and AN and to the acid KA and Basic KB, of the solid by equation: ISP = DN.KA + AN.KB Where DN and AN are respectively the electron donor and acceptor numbers and KA and KB, those of the solid surface. In fact, this relation describes the interaction between two amphoteric partners. The values of constants KA and KB are finally determined by transforming the previous equation as following: ISP/AN = (DN/AN).KA + KB An example of application of this method to the determination of the acid-base character of a cellulose is depicted on the figure below. We can notice that a satisfactory linear relation is obtained. The slope gives the acid surface constant (KA) whereas the ordinate gives the basic surface constant (KB). The Ka and KB values are suitable to compare the acid-base character of different solids as displayed on the last figure. We can see that the carbon blacks are mainly, but weakly basic. The pyrogenic silica presents a major acid character attributed to the presence of surface silanol groups. The precipitated silica, which is more hydroxylated, exhibits higher acid-base constants. Finally the aluminium oxide that carries important basic and acid sites, has clearly a more pronounced amphoteric character.
