Study of the hydrolytic properties of the trivalent Y-ion in chloride medium

Hydrolysis reactions of YCl3 aqueous solutions were studied by potentiometric titrations at 310 K in the pH range ca.3–9 and total Y3+ ion concentration range 0.5–10.0 mmol/dm3. The experimental data were evaluated with the aid of the Hyperquad2006 suite of programs, which indicated the formation of the following hydrolytic complexes (log conc. overall stability constants βp,q defined as βp,q = Mp(OH)q[H]q/[M]p), Y(OH)2+ (−7.71(2)); Y(OH)2+ (−16.42(2)); Y(OH)3 (−26.01(3)); Y2(OH)24+ (−14.23(4)); Y3(OH)52+ (−34.20(4)); Y4(OH)62+ (−37.10(5)). The hydrolysis of the Y3+ ion was also monitored by electrospray ionization ion-trap mass spectrometry (ESI-MS) in the pH range 2.63–7.10. The influence of the sample cone voltage on the product distribution was significant, indicating the contribution of ion source fragmentation reactions. The optimal cone voltage was set to 70 V for cationic and 100 V for anionic spectra. The cationic and anionic ESI mass spectra of the Y3+ solutions produced strong evidence for the presence of a variety of monomeric and polymeric hydrolytic complexes. The Cl− ion appeared to be a strong competitor to the OH− ion for the oxo-yttrium core. The hydrolytic precipitation reactions with NaOH or urea as precipitating agents were studied. The obtained precipitates were characterized by the potentiometric technique, elemental analysis, thermal analysis (TG-DSC), infrared spectroscopy, X-ray diffractions and the SEM technique. Hydrothermal synthesis of the precipitate in an autoclave at T = 420 K and P = 10 bar with NaOH as the precipitating agent produced a crystalline precipitate in the form of hexagonal needles (P63/m space group). At atmospheric pressure only an amorphous precipitate was produced with both precipitating agents. The concentration solubility product K∗so of Y(OH)3 was evaluated from the potentiometric titrations curves and log K∗so = −22.85(4). The synthesized precipitate with NaOH as a precipitating agent had a composition of Y(OH)3·2H2O, while the amorphous precipitate obtained with urea had a composition of Y2(OH)2(CO3)2·3H2O.

The hydrolysis speciation scheme for the Y3+ ion in aqueous chloride medium, consisting of Y(OH), Y(OH)2, Y(OH)3, Y2(OH)2, Y3(OH)5, Y4(OH)6 and Y(OH)3(s) hydrolytic complexes, was established by potentiometric and ESI-MS measurements. The hydrolytic precipitate of the composition Y(OH)3·2(H2O) was prepared by a hydrothermal synthesis and is shown to belong to the P63/m crystal class.Figure optionsDownload as PowerPoint slide