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


Whitlockite was assigned the ideal formula Ca9Mg(HPO4)(PO4)(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.



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Magalhes, MCF; Costa, MOG

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We are grateful to Prof. D. Brynn Hibbert for the suggestions on data treatment and statistical analysis, and to the anonymous reviewer that made a careful editing of the text, whose suggestions allowed to improve it. We want to acknowledge also the help from Dr. Rosario Soares with the XRD spectra and analysis, and MSc. Celeste Azevedo with the thermogravimetric analysis and FTIR spectra. The financial support from the Department of Chemistry and CICECO-Aveiro Institute of Materials POCI-01-0145-FEDER-007679 (FCT-UID/CTM/50011/2013), financed by national funds through the FCT/MEC and when appropriate co-financed by FEDER under the PT2020 Partnership Agreement.

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