On the solubility of whitlockite, Ca9Mg(HPO4)(PO4)6, in aqueous solution at 298.15 K

• Published in: Monatshefte für Chemie Vol. 149; no. 2; pp. 253 - 260
• Main Authors: Magalhães, M. Clara F., Costa, M. Odete G.
• Format: Journal Article
• Language: English
• Published: Vienna Springer Vienna 01.02.2018; Springer Nature B.V
• Subjects: Analytical Chemistry; Aqueous solutions; Calcium; Chemistry; Chemistry and Materials Science; Chemistry/Food Science; Energy of formation; Free energy; Heat of formation; Inorganic Chemistry; Magnesium; Organic Chemistry; Original Paper; Phosphates; Physical Chemistry; Solids; Solubility; Theoretical and Computational Chemistry; Calcium–magnesium phosphate; Whitlockite; Phase diagram; Solubility constant
• ISSN: 0026-9247; 1434-4475
• DOI: 10.1007/s00706-017-2129-z

Whitlockite was assigned the ideal formula Ca 9 Mg(HPO 4 )(PO 4 ) 6 , but the synthesis of a solid phase with this exact composition is very difficult. One of the reasons arises from the fact that there are no accurate values published for its solubility constant and/or other thermodynamic parameters. These could help to delineate a stability field in relation to the most common solid calcium and magnesium phosphates which normally co-occur with whitlockite. Calcium magnesium phosphates with the structure of whitlockite containing various n Mg / n Ca ratios were synthesized from aqueous solution of pH between 5.0 and 6.0, with the ratio of the concentrations of magnesium and calcium between 0.4 and 1.5, and excess of total calcium and magnesium over total phosphate. Those solids were used to determine the solubility of whitlockite with the ideal formula [lg K s (whitlockite, solid, 298.15 K) = − 113.75 ± 2.18], which was used to determine its standard molar Gibbs energy of formation. This value, together with the previously published values for the standard molar Gibbs energy of formation of the relevant calcium and magnesium solid phosphates and other species at equilibrium, allowed the construction of a phase diagram containing all the relevant solid calcium and magnesium phosphates. Graphical abstract