Fast Ionic Reactions in Solution VIII. The Fission and Formation Kinetics of the Dimeric Isopolybase (UO2OH)22+

Abstract

AbstractThe fission of the dimeric species (UO2OH)22+ has been investigated at μ = 0.5 M and 3°C with the stopped‐flow‐method. Upon mixing prehydrolyzed UO2(NO3)2 solutions with HClO4 solutions in a fast protolytic step the conjugate acid (UO2OHUO2)3+ is formed. The dissociation constant of this acid is (1.3 ± 0.2) · 10−3 M. In the rate determining step the protonated dimer (UO2OHUO2)3+ dissociates with a rate of (65 ± 10) sec−1.The fission reaction of the protonated species is enhanced further at higher concentrations of hydrogen ions. Therefrom it can be concluded that the species (UO2OH)22+ can be protonated twice. The doubly protonated species is relatively stable and its first acid dissociation constant can be estimated from the experiments to be 2.5 · 10−2 M. Thus the dimeric species (UO2OH)22+ behaves quite dissimilar to the other dimeric isopolybases hitherto investigated (dimers of Fe3+ and VO22+) as its conjugate acid and diacid are remarkably stable. The rate of fission of the diacid proceeds with k ∼ 1000 sec−1.From the kinetics of the fission reaction the following conclusions for the formation reaction of the dimer (UO2OH)22+ can be drawn:1. As with the dimerization reactions of the ferric and vanadyl ion the rate determining step for the fusion of one uranyl mono hydroxo cation and one uranyl ion to form the dimerUO22+ + UO2OH+ → (UO2OHUO2)3+ is the rate of spontaneous water exchange on the cation UO22+.2. The dimerization reaction of two uranyl ions to form (UO2H2OUO2)4+ is remarkably slower than the rate of water exchange on UO22+. Since the reaction investigated was fast (relative to the dead time of the flow apparatus used) the investigation was restricted to 3 °C and no activation parameters were obtained.From T‐jump measurements of acidified UO2(NO3)22 + KSCN‐solutions an estimation of the rate of water exchange on UO22+ ions is given (K(Uo2+)/H2O‐exch · 105 sec−1, 3 °C).