The complex-formation reaction of hydroxycalix[
n]arene-
p-sulfonates, H
j1nn- (
n=4,8;
j=
n/2,
n=6;
j=2), with various metal ions were studied while focusing on the uranyl ion, UO
22+, by the pH-titration method at 25°C and
μ=0.1. From the pHtitration data for H
j1nn-, the acid-dissociation constants were determined to be as follows: p
Ka1=3.25, p
Ka2=12.8 for H
2144-, p
Ka1=3.28, p
Ka2=4.86 for H
2166-, and p
Ka1=3.73, p
Ka2=4.39, p
Ka3=8.07, p
Ka4=10.1 for H
4188-. These results indicate that the present ligands are relatively strong multi-protonic acids which release j mol of proton from one mol of ligand, H
j1nn-, having
n mol of hydroxyl group in a weakly acidic-to-alkaline aqueous solution. The tetramer ligand, H
21
44- formed two unstable 1:2 (UO
22+:ligand) complexes, [(UO
22+)(
146-)
2(H
+)]
9- and [(UO
22+)(
146-)
2]
10-, and a hexamer ligand, H
2166- formed four 1:1 complex species, such as [(UO
22+)(
168-)]
6-, [(UO
22+)(
168-)(H
+)
-1]
7-, [(UO
22+)(
168-)(H
+)
-2]
8-, and [(UO
22+)(1
68-)(H
+)
-3]
9- with UO
22+. Their selective reactivity to UO
22+ over other various metal ions was evaluated based on their p
Ka values, stability constants and the compositions of the complexes including that for the octamer ligand, H
4188-, for which the complex-formation reactions with various metal ions were already studied and found to form an extremely stable 2:1 complex, [(UO
22+)
2(
1812-)(H
+)
-3]
11-.
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