Another Uranophile Sodium Hydroxycalix[8]arene-p-sulfonate: Its Acid Dissociation and Divalent Metal Ion Binding Properties

Abstract

Acid dissociation constants of the hydroxyl groups of sodium hydroxycalix[8]arene-p-sulfonate (18=Na8H4L) were measured by the pH titration method in an aqueous solution. The acid dissociation constants determined were as follows: pKa1=3.73±0.05, pKa2=4.39±0.05, pKa3=8.07±0.02, pKa4=10.1±0.1 under the conditions of μ=0.1 with KNO3 and 25°C. Furthermore, the complex formation reactions of 18 with some divalent metal ions such as Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, and UO22+ (M2+) were investigated by the pH titration method in aqueous solutions and the stability constants of the complexes were evaluated. No complex formation was observed for Cu2+. The slight formation of complexes having a 1:1 molar ratio (M2+:18) was observed for all other divalent metal ions except for UO22+. In contrast, UO22+ formed a relatively stable 2:1 complex in addition to an unstable 1:1 complex. These results indicate that 18 is a highly selective metal ligand for UO22+ among the various divalent metal ions.