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Experimental Determination of Solubilities of Tri-calcium Di-Citrate Tetrahydrate [Ca3[C3H5O(COO)3]2•4H2O] Earlandite in NaCl and MgCl2 Solutions to High Ionic Strengths and Its Pitzer Model: Applications to Nuclear Waste Isolation and Other Low Temperature Environments

Xiong, Yongliang X.; Kirkes, Leslie D.; Westfall, Terry; Marrs, Cassandra M.; Knox, Jandi L.; Burton, Heather L.

In this study, solubility measurements on tri-calcium di-citrate tetrahydrate [Ca3[C3H5O(COO)3]2•4H2O, abbreviated as Ca3[Citrate]2•4H2O] as a function of ionic strength are conducted in NaCl solutions up to I = 5.0 mol•kg–1 and in MgCl2 solutions up to I = 7.5 mol•kg–1, at room temperature (22.5 ± 0.5°C). The solubility constant (log K$0\atop{sp}$) for Ca3[Citrate]2•4H2O and formation constant (logβ$0\atop{1}$) for Ca[C3H5O(COO)3]–Ca3[C3H5O(COO)3]2•4H2O (earlandite) = 3Ca2+ + 2[C3H5O(COO)3]3– + 4H2O (1) Ca2+ + [C3H5O(COO)3]3– = Ca[C3H5O(COO)3]– (2) are determined as –18.11 ± 0.05 and 4.97 ± 0.05, respectively, based on the Pitzer model with a set of Pitzer parameters describing the specific interactions in NaCl and MgCl2 media.