Thermodynamic Stability of Zirconium(IV) Complexes with Hydroxy Ions

Abstract

A protometric study on the speciation of zirconium(IV) was performed in an aqueous solution at 25 °C in KNO3 and four ionic strengths (0.2, 0.5, 1, and 2 mol·dm−3). This study indicates the formation of the soluble species Zr(OH)3+, Zr2(OH)7+, and Zr(OH)4 in the pH range 1.5–3.5. The use of an appropriate titration technique allows the precipitation of the zirconium hydroxide to be limited. The overall stability constants βmlh of these three hydroxo complexes were computed and refined, by the program PROTAF, for each ionic strength. The solubility product Ks of Zr(OH)4 was calculated by taking these βmlh constants into account. The thermodynamic values were determined by extrapolation at zero ionic strength (Specific ion Interaction Theory method): log β°10–3 = –0.52 (0.01); logβ°20–7 = –2.22(0.07); logβ°10–4 = –4.52 (0.07); logKs° = –55.26(0.08).

On the Hydrolysis of Tetravalent Metal Ions

“…This behavior is supported by the results of Brown et al [26] for thorium hydrolysis, where evidence was found for the species Th(OH) 3+ , Th 6 (OH) 15 9+ and Th 4 (OH) 12 4+ . In the present study, the data support the formation of the U 4 (OH) 12 4+ species but not the U 6 (OH) 15 9+ species (Table 2). However, this result may not be entirely surprising.…”

“…In a recalculation of this work, Baes and Mesmer [17] found that the data were consistent with the formation of U(OH) 3+ and U 6 (OH) 15 9+ but that systematic deviations remained at high pH values. These deviations point to the need for an additional species that has a q/p ratio higher than 2.5 at these higher pH values [17].…”

“…These deviations point to the need for an additional species that has a q/p ratio higher than 2.5 at these higher pH values [17]. This behavior is supported by the results of Brown et al [26] for thorium hydrolysis, where evidence was found for the species Th(OH) 3+ , Th 6 (OH) 15 9+ and Th 4 (OH) 12 4+ . In the present study, the data support the formation of the U 4 (OH) 12 4+ species but not the U 6 (OH) 15 9+ species (Table 2).…”

“…Of the studies that have been performed [8][9][10][11][12][13][14][15][16], there is little agreement between either the species that form or the magnitude of the stability constants for individual species. Two reviews [17,18] of the hydrolytic behavior of zirconium(IV) have indicated that the only reliable estimate of Zr(OH) 3+ is that from Noren [8] who used a modified potentiometric technique.…”

Zirconium complexes of amino(polyphenolic) ligands and their hydrolytic stability

“…In order to operate efficiently as 'pH-swing' extractants for Zr IV , loading and stripping, e.g., L org þ Zr 4þ ½ZrðL-4HÞ org þ 4H þ ðL ¼ 1Þ 2L org þ Zr 4þ ½ZrðL-2HÞ 2 org þ 4H þ ðL ¼ 3 or 4Þ must occur over a pH-range to allow the ligand to be fully loaded at a pH lower than that at which insoluble zirconium(IV) oxy/hydroxides precipitate from aqueous solution [10], and be fully stripped at a low pH which is conveniently accessible with hydrochloride or similar acids. Ligand 1 was synthesised by a modified literature procedure [8] outlined in Scheme 1.…”