The Solubility of Calcium Hydrogen Phosphate Dihydrate and Magnesium Hydrogen Phosphate Trihydrate and Ion Pair Formation in the System M(OH)2‐H3Po4‐H2O (M = Ca or Mg) at 37°C

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Abstract
The solubility of CaHPO4·2H2O and MgHPO4·3H2O is determined in the system M(OH)2‐H3PO4‐H2O at 37°C with M = Ca or Mg and in the range 4.1 < pH < 6.3 and 4.9 < pH < 7.2 respectively. The solubility behaviour of both solids can be described consistently only by taking into account the formation of the ion pairs MH2PO4+ and MHPO4. The thermodynamic solubility product of CaHPO4·2H2O and the thermodynamic association constants for CaH2PO4 + and CaHPO4 with their standard error are determined as (2.06 ± 0.05)×10−7 mol2/L2, 8.4 ± 1.7 L/mol and 542 ± 46 L/mol. The corresponding constants for MgHPO4·3H2O, MgH2PO4 + and MgHPO4 are (1.15 ± 0.07)×106 mol2/L2, 43.0 ± 6.7 L/mol and 1004 ± 89 L/mol.