AQUEOUS SOLUBILITY OF Y, BA AND CU OXALATES IN THE SYSTEM [Y(OH)3,BA(OH)2,CU(OH)2]-H2C2O4-[HNO3/NAOH]-H2O

Solubility isotherms have been calculated for the system [Y(OH)3-Ba(OH)2-Cu(OH)2]-H2C2O4-[HNO3/NaOH]-H2O at T=25-degrees-C, using a numerical algorithm involving the equilibria between soluble complexes, and including the effect of ionic strength. Using this algorithm, the solubility isotherms of several precipitates in simultaneous equilibrium with the same solution have been calculated. The results highlight the importance of complex formation in determining the solubility of Y, Ba and Cu oxalates, particularly the hydroxo and oxalato complexes, and their sensitivity to pH. Neglecting the effect of ionic strength can lead to erroneous solubility estimations; the magnitude of the error depends on the nature of the precipitates and the equilibrium conditions. In specific cases, the solubility of certain metal oxalates and hydroxides can be strongly affected by the presence of other precipitates. This effect depends on the exact pH of the solution. The strongest effects are found for CuC2O4.0.5H2O and Cu(OH)2 in the presence of BaC2O4.2H2O, and for Y2(C2O4)3.9H2O and Y(OH)3 in the presence of BaC2O4.2H2O. The examples presented show that each case must be treated specifically.