ZINC(II) OXIDE STABILITY IN ALKALINE SODIUM-PHOSPHATE SOLUTIONS AT ELEVATED-TEMPERATURES

Zinc oxide (ZnO) is shown to transform into either of two phosphate-containing compounds in relatively dilute alkaline sodium phosphate solutions at elevated temperatures via ZnO(s) + Na+ + H2PO4- reversible NaZnPO4(s) + H2O or 2ZnO(s) + H3PO4(aq) reversible Zn2(OH)PO4(s) + H2O. X-ray diffraction analyses indicate that NaZnPO4 possesses an orthorhombic unit cell having lattice parameters a = 8.710 +/- 0.013, b = 15.175 +/- 0.010, and c = 8.027 +/- 0.004 angstrom. The thermodynamic equilibria for these reactions were defined in the system ZnO-Na2O-P2O5-H2O for Na/P molar ratios between 2.1 and 3. On the basis of observed reaction threshold values for sodium phosphate concentration and temperature, the standard entropy (S-degrees) and free energy of formation (DELTAG(f)degrees) for NaZnPO4 were calculated to be 169.0 J/(mol K) and -1510.6 kJ/mol, respectively; similar values for Zn2(OH)PO4 (tarbuttite) were 235.9 J/(mol K) and -1604.6 kJ/mol. Additions of sodium sulfite and sulfate did not alter the above reactions.