SOLUBLE COMPLEXES IN THE SYSTEMS DICALCIUM PHOSPHATE DIHYDRATE OR DIMAGNESIUM PHOSPHATE TRIHYDRATE EQUILIBRATED WITH AQUEOUS SALT SOLUTIONS

Abstract

Dicalcium phosphate dihydrate (CaHPO4∙2H2O) (DCPD) or dimagnesium phosphate trihydrate (MgHPO4∙3H2O) (DMPT) was equilibrated with water or aqueous solutions of CaCl2, CaSO4, MgCl2, MgSO4, KCl or K2SO4 at 25 ± 1 C for 24 hr. Values of pCa + pHPO4 and pMg + pHPO4 corrected for complex formation, using dissociation constants published for CaHPO4, MgHPO4, CaSO4 and MgSO4 soluble complexes, were calculated. The average corrected value of pCa + pHPO4 = 6.68 ± 0.03, calculated for the systems in which DCPD was equilibrated with water or aqueous CaCl2, CaSO4, KCl or K2SO4, was larger than pCa + pHPO4 = pKDCPD = 0.36 at 25 C calculated assuming an absence of soluble complexes. However, it showed excellent agreement with pKDCPD = 6.66 ± 0.02 at 37.5 C calculated assuming CaH2PO4+ and CaHPO4 complexes to be present. The average corrected value of pMg + pHPO4 = 5.82 ± 0.01, calculated for the systems in which DMPT was equilibrated with water or aqueous MgCl2, MgSO4, KCl or K2SO4 showed excellent agreement with pMg + pHPO4 = pKDMPT = 5.82 ± 0.03 at 25 C calculated assuming the MgHPO4 complex to be present. For the systems in which DCPD was equilibrated with aqueous MgCl2 or MgSO4 and DMPT equilibrated with CaCl2 or CaSO4, some corrected values of pCa + pHPO4 and pMg + pHPO4 showed close agreement with the respective values of pKDCPD and pKDMPT. However, most did not, presumably because with some of these systems DMPT could not exist at equilibrium, or equilibrium with respect to DCPD was not attained within the 24-hr period of equilibration.