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 hydrothe rmal conditions at steam saturation to 244 degrees C; solubility has been r ecorded as a function of temperature from 25 to 256 degrees C. The high-tem perature Raman spectra contained two polarized bands, which suggest that a second sulfate complex, possibly bidentate, is formed in solution, in addit ion to the 1:1 zinc(II) sulfate complex, which is the only ion pair identif ied at lower temperatures. Under hydrothermal conditions, it was possible t o 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 rea ction [Zn(OH2)(6)](2+) + SO42- reversible arrow [Zn(OH2)(5)OH](+) + HSO4- T he precipitate in equilibrium with the solution at 210 degrees C could be c haracterized as ZnSO4. H2O (gunningite) by x-ray diffraction (XRD) and Rama n and infrared spectroscopy. At 244 degrees C the equilibrium precipitate c ould be identified as ZnSO4 (zincosite).