Nd(III) Hydrolysis Constants and Solubilities of Nd(III) Hydroxide.

MAKINO Hitoshi, YAJIMA Tatsuya, YOSHIKAWA Hideki, Yui Mikazu, SASAKI Noriaki

doi:10.1246/nikkashi.1993.445

The solubilities of crystalline Nd(III) hydroxide was measured in dilute solutions under anaerobic condition, CO, and 02 partial pressures of O.1 and 1 ppm, respectively. The aim of this work is to get thermodynamic data (equilibrium constant of crystalline Nd(III) hydroxide and hydrolysis constants of Nd(III)) and discuss the similarity of dissolution behavior between Nd(III) and Am(III) which is important element in the view of geological isolation of high level radioactive waste. And another aim is to confirm the reliability of thermodynamic data obtained from this work. The equilibrium was attained from both over- and undersaturation as long as 68 and 64days, respectively. The experiments were carried out at the pH range of 7 to 13 at an ionic strength of O.01 and at room temperature (22 °C). The solid phase was identified as Nd(OH)3 (c) by X-ray diffraction before and after the experiments. From the solubilities of crystalline Nd(III) hydroxide, the equilubrium constant (K810 for Nd(OH)3 (c) +3 H=Nd3+ + 3 H20) and the hydrolysis constants of Nd(111) (pm?, for m N d"-Fn H20=-- Ndm, (OH)(3m-n)+-1-n H+) were estimated. The value for log Ks, is 16. O. The value s for log p, , , log p, 2 and log 1313a re 7.6, 14.3 and < 24.9, respectively. The results indicate both [Nd(OH)]2 and [Nd(OH)4]- a r e not dominant in the pH range 7 to 13. The v a lue of log ign, 7.6, was well agreed with the calculated value of 8.1 by the theory of Brown et al. The dissolution beh a v i o r o f Nd(III) hydroxide was similar to that of Am(111) reported by Rai et al. in aspect of dominant species at different pH.

Nd(III) Hydrolysis Constants and Solubilities of Nd(III) Hydroxide.

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