THE AQUEOUS COMPLEXATION OF ND(III) WITH MOLYBDATE - THE EFFECTS OF BOTH MONOMERIC MOLYBDATE AND POLYMOLYBDATE SPECIES

The complexation of Nd(III), with molybdate was studied by examining t he solubility of three distinct solid phases, NdPO4(c), NaNd(MoO4)(2)( c), and Nd-2(MoO4)(3) . xH(2)O over wide ranges of concentrations of N a2MoO4 and NdCl3, and/or of pH values, and over equilibration times ex tending to 120 days. The solubility data for NdPO,(c) in different con centrations of dissolved molybdate showed that at dissolved molybdate concentrations of greater than or equal to 10(-3) M and pH values less than similar to 5, the observed NdPO4(c) solubility can increase seve ral orders of magnirude. These enormous increases in solubility and th eir very strong pH dependence indicate that polymolybdate complexation of Nd3+ is occurring in these solutions. It appears that Nd(III) comp lexation with polymolybdate species is much stronger than Nd(III) comp lexation with monomeric molybdate species. A preliminary aqueous therm odynamic model was developed based principally on the NdPO4 solubility data in different molybdate concentrations. This thermodynamic model, which included both monomeric and polymolybdate complexes of Nd3+, wa s then tested on, and found to be consistent with, the NaNd(MoO4)(2)(c ) solubility data and the solubility data for the apparent Nd-2(MoO4)( 3) . xH(2)O phase.