Acid-base interactions measurement strength distribution

The catalyst surface may contain acid and base sites that can interact together. On a certain surfaces the acid or base behaviour may prevail even if both sites are always present. In a catalyst characterisation, it is very useful to define the nature (Lewis or Bronsted, see par. 4.3) of these sites, their density, location, distribution and strength. Generally, an acid site is defined as a site that can react with a base and, on the contrary, a base site is one that can react with an acid. The above information (acid-base sites density) can be obtained by performing a chemisorption measurement using an acid (such as CO2, SO2) or base gas (such as dry ammonia) while their strength could be measured by temperature programmed techniques (desorption). 

The solvent triangle classification method of Snyder is the most enduring approach to solvent characterization used by chromatographers, but in several respects is not entirely satisfactory [568,575,576]. Snyder classified solvents based on their interactions with three prototypical solutes determined by their gas-liquid distribution constants corrected for differences in solvent size, polarizability and dispersion interactions (assumed identical to the interactions of a hypothetical n-alkane with the same molar volume). Each value was then corrected empirically to give a value of zero for the polar distribution constant for saturated hydrocarbon solvents. Snyder chose the solutes nitromethane, ethanol and dioxane as probes for a solvent s capacity for dipole-type, hydrogen-bond base and hydrogen-bond acid interactions, respectively. The sum of the three polar distribution constants provides a measure of the solvent strength P ) and the ratio of the individual polar distribution constants to their sum a measure of selectivity (xn, and Xd). 

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