ZINC(II) OXIDE SOLUBILITY AND PHASE-BEHAVIOR IN AQUEOUS SODIUM-PHOSPHATE SOLUTIONS AT ELEVATED-TEMPERATURES

A platinum-lined, flowing autoclave facility is used to investigate the solubility/phase behavior of zinc(II) oxide in aqueous sodium phosphate solutions at temperatures between 17 and 287-degrees-C. ZnO solubilities are observed to increase continuously with temperature and phosphate concentration. At higher phosphate concentrations, a solid phase transformation to NaZnPO4 is observed. NaZnPO4 solubilities are retrograde with temperature. The measured solubility behavior is examined via a Zn(II) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria are obtained from a least-squares analysis of the data. The existence of two new zinc(II) ion complexes, Zn(OH)2(HPO4)2- and Zn(OH)3(H2PO4)2-, is reported for the first time. A summary of thermochemical properties for species in the systems ZnO-H2O and ZnO-Na2O-P2O5-H2O is also provided.