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)

The solubilities of pure synthetic La and Gd hydroxides were measured in aqueous solutions over a wide range of pH (5-9.5) and temperature ( 40-150 degrees C). The regression of these data is used to determine t he first dissociation constant( K-S0) of the minerals as a function o f temperature. The logarithm of these constants is found to be linearl y related to the reciprocal temperature, which allows their reliable e xtrapolation to 25 degrees C. In the range of temperature and pH inves tigated, only the REE3+ ion(where REE denotes La or Gd) is present in solution, indicating that the first hydrolysis constants proposed by B aes and Mesmer (1976), which are generally admitted, are overestimated . The complexation of La with acetate ion (AcO-) was studied by both s olubility and potentiometric measurements. Potentiometric titrations a re interpreted in terms of the formation of two complexes: LaAcO2+ and La(AcO)(2)(+), and are used to generate the corresponding stability c onstants in the temperature range 25-80 degrees C. These data allow ac curate description of the La(OH), solubility increase, evidenced in ac etate-bearing solutions. (C) 1998 Elsevier Science B.V. All rights res erved.