Thermodynamic Model for the Solubility of TcO2⋅xH2O in Aqueous Oxalate Systems

The room temperature solubility of amorphous, hydrous technetium(IV) oxide (TcO2⋅xH2O) was studied across a broad range of pH values extending from 1.5 to 12 and in oxalate concentrations from dilute (10−6 mol⋅kg−1) to complete saturation with respect to sodium bioxalate at lower pH values, and to saturation with respect to sodium oxalate at higher pH values. The solubility was measured to very long equilibration times (i.e., as long a 1000 days or longer). The thermodynamic modeling results show that the dominant species in solution must have at least one more hydroxyl moiety present in the complex than proposed by previous investigators (e.g., TcO(OH)Ox− rather than TcO(Ox)(aq)). Inclusion of the single previously unidentified species TcO(OH)Ox− in our aqueous thermodynamic model explains a wider range of observed solubility data for TcO2⋅xH2O(am) in the presence of oxalate and over a broad range of pH values. Inclusion of this species is also supported by the recently proposed thermodynamic data for the TcO(OH)+ hydrolysis species that indicates that this species is stable at pH values as low as one.