Potentiometric study of Gd– and Yb–acetate complexing in the temperature range 25–80°C

The stability of aqueous complexes formed by gadolinium (Gd3+) and ytterbium (Yb3+) aqueous species with the acetate ion (AcO−) was investigated at 25°C, 40°C, 60°C and 80°C, and saturated vapor pressure, using potentiometric measurements. In the large range of [AcO]T/[REE]T molal ratio investigated (1 to 120), the results are interpreted in terms of the formation of two complexes, REEAcO2- and REE(AcO)2- (where REE denotes Gd or Yb). The corresponding association constants β11 and β12 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 increasing temperature. The regression of the obtained association constant values using either a linear equation or a simplified formulation of the revised HKF thermodynamic model allows the retrieval of thermodynamic properties for the aqueous complexes. ing these results with those obtained with lanthanum in a previous study [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°C), and La–acetate complexing (25 to 80°C). Chem. Geol. 151, 349–372.] demonstrates that the stability of the REE–AcO− complexes increases from La to Gd, then decreases to Yb.