Raman spectroscopy of aqueous ZnSO4 solutions under hydrothermal conditions:: Solubility, hydrolysis, and sulfate ion pairing

Raman spectra have been measured for aqueous ZnSO4 solutions under hydrothermal conditions at steam saturation to 244 degrees C; solubility has been recorded as a function of temperature from 25 to 256 degrees C. The high-temperature Raman spectra contained two polarized bands, which suggest that a second sulfate complex, possibly bidentate, is formed in solution, in addition to the 1:1 zinc(II) sulfate complex, which is the only ion pair identified at lower temperatures. Under hydrothermal conditions, it was possible to observe the hydrolysis of the zinc(II) aquo ion by measuring the relative intensity of bands due to SO42- and HSO4- according to the equilibrium reaction [Zn(OH2)(6)](2+) + SO42- reversible arrow [Zn(OH2)(5)OH](+) + HSO4- The precipitate in equilibrium with the solution at 210 degrees C could be characterized as ZnSO4. H2O (gunningite) by x-ray diffraction (XRD) and Raman and infrared spectroscopy. At 244 degrees C the equilibrium precipitate could be identified as ZnSO4 (zincosite).