Hydrogen ion on cation-exchange

When studying the surface acid-base properties of montmorillonite, it is essential to understand that hydrogen ions and cations of the support electrolyte can also participate in cation-exchange processes. The processes on the internal and external surfaces have to be taken into consideration simultaneously, and they both have to be included into the equilibrium thermodynamical models. 

Anions, most commonly simple carboxylic acids (e.g., tartaric, malic, citric, lactic, acetic, succinic, formic, propionic, butyric, etc.), are separated on cation-exchange resins in acidic form. Salts of weak acids can also be analyzed, as they are converted to the corresponding acid by the hydrogen ions in the exchanger. Cations (weak bases and their salts) are separated on anion-exchange resins in the hydroxide form. 

The alkaline error can be satisfactorily explained by assuming an exchange equilibrium between the hydrogen ions on the glass surface and the cations in solution. This process is simply the reverse of that shown in Equation 21-6 ... 

Hammet and collaborators140, 141 studied in more detail the hydrolysis of aliphatic esters with a cation-exchange resins as catalyst. They found that replacement of 70% of the hydrogen ions in a crosslinked polystyrenesulfonic add by cetyl-trimethylammonium ions had a specifically favorable effect on the effectiveness of the remaining hydrogen ions for the hydrolysis of ethyl-n-hexanoate. From these findings, the important contributions of the hydrophobic forces, in addition to the electrostatic forces, is clearly demonstrated. 

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