Complexation of the lanthanides Eu3+, Gd3+, and Tb3+ with 1,4,7,10-tetrakis(carboxymethyl)-1,4,7,10-tetraazacyclododecane (dota) has been studied in solution by using potentiometry, luminescence spectrometry, and EXAFS. Three series of successive complexes were characterized by at least two of these methods: the immediate [LnHn(dota)](n-1)+** and intermediate [LnHn(dota)](n-1)+* complexes with 0

The stability of the various complexes formed by racemic solutions of the title ligand (L) with Gd 3+ , Eu 3+ and Tb 3+ was investigated by potentiometry. The reaction of complexation proceeds through the quick formation of metastable species leading, after a slow reorganisation of the macrocycle, to thermodynamically stable complexes. The mean numbers of water molecules coordinated to the lanthanides were determined by luminescence and EXAFS spectroscopy. This last method, applied to solutions of complexes, allowed us to precisely determine the nature of the atoms that surround the metal atom and the distance between the lanthanide ion and the various ligands. These structural data that are in good agreement with the results found using quantum mechanics

A protometric study on the speciation of zirconium(IV) was computed and refined, by the program PROTAF, for each ionic strength. The solubility product Ks of Zr(OH) 4 was performed in an aqueous solution at 25°C in KNO 3 and four ionic strengths (0.2, 0.5, 1, and 2 mol·dm -3 ). This study calculated by taking these β mlh constants into account. . Increasing zirconium concentration induces the formation of polynuclear species outstanding resistance to corrosion, as well as to a low neutron capture cross-section. It is an essential constituent of Zr m (OH) h (4mϪh)ϩ . In accordance with studies from Zielen and Connick [2] , Johnson and Kraus [11] , and Angstadt and the core of the nuclear power station and for confining fissile materials.Tyree [12] , Baes and Mesmer conclude that the existence of a trimer, Zr 3 (OH) 4 8ϩ or Zr 3 (OH) 5 7ϩ , and of the tetramer The running of nuclear installations relies on the proZr 4 (OH) 8 8ϩ is probable. cessing of nuclear fuels and fission by-products, but also on the long-term storage, in protected sites, of confined materiAja et al. [13] also published a review concerning the aqueals which have acquired an appreciable radioactivity.ous chemistry of zirconium(IV). They principally take into In the long-term one may expect the possibility of cor-account the results obtained by Russian scientists [5] [6] [8] [14] . rosion and fissuring of the confining containers leading to They consider that the values of the formation constants of a contamination of soils and waters.the mononuclear hydroxide complexes depend little on the ionic strength and the nature of the supporting electrolyte. Modelling programs which take into account the whole For these constants, they have retained average values which local physical parameters as well as the physicochemical are very different from those given by Baes and Mesmer. data allow the quantification of these contamination risks. The values used for these simulations, especially those relatMore recently, Adair et al. [15] published an article entitled ing to zirconium, have to be as reliable as possible in order "A review of the aqueous chemistry of the zirconium-water to yield realistic estimations.system" in which they simply give the same values of conThe chemistry of zirconium(IV) in aqueous media is stants that Baes and Mesmer indicated for the mono-and dominated by the complexation of Zr 4ϩ with OH Ϫ . The polynuclear zirconium hydroxide complexes at 25°C. data currently available on zirconium hydroxide complexes In short, numerous studies deal with the formation of comes from studies carried out (essentially) before zirconium-hydrolysed complexes in aqueous solution but. The advanced species formulae many discrepancies exist from one author to another conand, above all, the values of the corresponding comcerning the major species formed and the values of the corplexation constants, seem questionable.responding constants of formation. It must be pointed that Baes and Mesmer [10] have reviewed bibliographical re-in these studies the stoi...

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).

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