Potentiometric study of Gd- and Yb-acetate complexing in the temperature range 25-80 degrees C

The stability of aqueous complexes formed by gadolinium (Gd3+) and ytterbiu m (Yb3+) aqueous species with the acetate ion (AcO-) was investigated at 25 degrees C, 40 degrees C, 60 degrees C and 80 degrees C, and saturated vapo r pressure, using potentiometric measurements. In the large range of [AcO]( T)/[REE](T) molal ratio investigated (1 to 120), the results are interprete d in terms of the formation of two complexes, REEAcO2- and REE(AcO); (where REE denotes Gd or Yb), The corresponding association constants beta(11) an d beta(12) that we proposed are successfully used to predict high Gd(OH)(3) (cr) solubilities measured in acetate containing solutions in this study. The data show an increasing stability of the REE-acetate complexes with inc reasing temperature. The regression of the obtained association constant va lues using either a linear equation or a simplified formulation of the revi sed HKF thermodynamic model allows the retrieval of thermodynamic propertie s for the aqueous complexes. Combining these results with those obtained with lanthanum in a previous st udy [Deberdt, S., Castet, S,, Dandurand, J.-L., Harrichoury, J.-C., Louiset , I., 1998. Experimental study of La(OH)(3) and Gd(OH)(3) solubilities (25 to 150 degrees C), and La-acetate complexing (25 to 80 degrees C). Chem. Ge ol. 151, 349-372.] demonstrates that the stability of the REE-AcO- complexe s increases from La to Gd, then decreases to Yb. (C) 2000 Elsevier Science B.V. All rights reserved.