| Abstract: |
Acid/base reactions and pyrocatechol violet complexation at the boehmite–solution interface have been investigated in 0.10 M K(Cl) solution at 298.2 K in the range 5.0≤−log h≤9.0. Equilibrium measurements were performed as potentiometric titrations complemented by spectrophotometric analysis of a ligand (batch adsorption experiments). The experimental data were evaluated on the basis of the extended constant capacitance model, allowing for inner-sphere and/or outer-sphere complexation. The acid/base properties are described by the equilibria: H++≡AlOH⇋ ≡AlOH2+; log β1,1,0(int)=7.46±0.04 and ≡AlOH ⇋≡AlO−+H+; log β−1,1,0(int)=−9.87±0.12. The specific capacitance was determined as 1.00 F m−2. The binding of pyrocatechol violet {PCV, 2-[(3,4-dihydroxyphenyl)(3-hydroxy-4-oxocyclohexa-2,5-diene-1-ylidene)methyl]benzenesulfonic acid}, H4L, was described by the formation of a single inner-sphere complex according to the reaction ≡AlOH+H4L⇋≡AlL3−+3H++H2O. Allowing for a charge distribution of this complex between the inner plane and β-plane (−2, −1), a significant improvement of the fit was obtained. The adsorbed complex exhibited a dark blue color over the pH range 5–8.5, indicative of proton loss from the 1,2-dihydroxyl moiety. Furthermore, DRIFT spectra showed the bonding environment of PCV bound to the surface to exhibit similarities with corresponding catechol adsorption to different aluminium (hydr)oxides. These spectral features turned out to be very similar to soluble catechol complexes of Al(III). Thus, it is suggested that PCV forms a mononuclear chelating complex at the boehmite–water interface. The maximum surface coverage reached was ca. 33%, indicating that the high negative charge of the species may result in surface charge “saturation” rather than saturation of surface binding sites. Steric hindrances caused by the large size of PCV may also contribute to the low surface coverage. |
