Solubility and stability of barium arsenate and barium hydrogen arsenate at 25 °C

The inconsistency among current thermodynamic data of Ba-3(AsO4)(2)(c) and (BaHAsO4H2O)-H-.(c) led the authors to obtain independent solubility data of barium arsenate by both precipitation and dissolution experiments. Low and neutral pH (3.63-7.43) favored the formation of (BaHAsO4H2O)-H-.(c). Both (BaHAsO4H2O)-H-.(c) and Ba-3(AsO4)(2)(c) formed at the neutral pH conditions (7.47, 7.66), whereas Ba-3(AsO4)(2)(c) was the only solid phase precipitated at high pH (13.03, 13.10). The Ba-3(AsO4)(2)(c) precipitate acquired at 50 degrees C appeared as small leafy crystal, while the Ba-3(AsO4)(2)(c) solid precipitated at 25 degrees C comprised granular aggregate with some smaller crystal clusters. XRD and SEM analyses of Ba-3(AsO4)(2)(c) and (BaHAsO4H2O)-H-.(c) indicated that the solids were indistinguishable before and after the dissolution experiments. In the present work, the solubility products (K-sp) for Ba-3(AsO4)(2)(c) and (BaHAsO4H2O)-H-.(c) were determined to be 10(-23.53) (10(-23.01) to 10(-24.00)) and 10(-5.60)(10(-5.23) to 10(-5.89)), respectively. Delta G(f)(o) for Ba-3(AsO4)(2)(c) and (BaHAsO4H2O)-H-.(c) were calculated to be -3113.40 and -1544.47 kJ/mol, respectively. There was no difference between the solubility products of the leafy and the granular Ba-3(AsO4)(2)(c) solids. (c) 2004 Elsevier B.V. All rights reserved.