SOLUBILITY BEHAVIOR OF TITANIUM(IV) OXIDE IN ALKALINE MEDIA AT ELEVATED-TEMPERATURES

A platinum-lined, flowing autoclave facility was used to investigate the solubility behaviour of titanium dioxide (TiO2) in aqueous sodium phosphate, sodium hydroxide and ammonium hydroxide solutions between 17 and 288-degrees. Baseline Ti(IV) solubilities were found to be on the order of one nanomolal, which were enhanced by the formation of anionic hydroxo- and phosphato-complexes. The measured solubility behaviour was examined via a titanium(IV) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from a least-squares analysis of the data. The existence of three new Ti(IV) ion complexes is reported for the first time: Ti(OH)4(HPO4)2-, Ti(OH)s(H2PO4)2- and Ti(OH)5(HPO4)3-. The triply-charged anionic complex was the dominant Ti(IV) species in concentrated, alkaline phosphate solutions at elevated temperatures. This complex is expected to exhibit C.N. = 4 (i.e. Ti(OH)2OPO43-). A summary of thermochemical properties for species in the systems TiO2-H2O and TiO2-P2O5-H2O is also provided.