1998 Volume 14 Issue 1 Pages 89-98
A variety of lipophilic amines incorporated in PVC matrix liquid membranes exhibited anionic potentiometric responses to phenolic compounds at the pH conditions under which the phenols exist mainly or exclusively in their undissociated, neutral forms. The examined lipophilic amines include a macrocyclic pentaamine, tri(decyl)amine, 4,7-diphenyl-1,10-phenanthroline (bathophenanthroline), 4-octadecylpyridine, and sapphyrin. The potentiometric selectivities of the membranes based on lipophilic aliphatic amines (B) reflected the acidity (hydrogen bond donor activity) and lipophilicity (extractability) of the phenols (ArOH), similarly as membranes based on lipophilic quaternary ammonium salts (Q+X-). The anionic responses were explained on the basis of a decrease in the charge separation of protonated amines (BH+) and their counteranions (X-) across the membrane interface. Possible processes leading to a decrease in the charge separation between BH+ and X- are (i) complexation between ArOH and BH+X-, followed by proton dissociation and ejection of HX into the aqueous phase, as well as (ii) complexation between ArOH and B. The membrane based on sapphyrin showed a high potentiometric selectivity to catechol, possibly due to geometrical discrimination of the ortho dihydroxy structure of catechol by the nitrogen(s) on the rigid macrocyclic structure of sapphyrin.
