Some acid—base and complexation properties of silica-bound ligands

Abstract

Several aspects of the acid-base properties of aminopropyl, aminoethylaminopropyl-, and 8-quinolinol silica gel (APSG, EDSG, and QSG, respectively) are considered. Acid wash of APSG and EDSG reduces the metal-uptake capacity to nearly zero. Following a base wash, most of this capacity is restored. Effective basicity of each silica gel is found to be significantly less than that of the free base analog for APSG and EDSG, but only slightly less for QSG. Hydrogen bonding of the γ-amine group to residual silanols is postulated as at least one cause of lower base-strengths, although dependence of p K b on the degree of neutralization is also a possibility. Solution equilibrium is reached very slowly in potentiometric pH titrations of the three modified silica gels, possibly because of diffusion or other kinetic phenomena. The bound amines possess 2:1 ligand-to-copper(II) stoichiometry. The bound 8-quinolinol exhibits a 1:1 ratio. A lower surface density and steric constraints of bound groups on QSG are possible reasons for the difference between 8-quinolinol and the amines.