Preparation, characterization and thermochemistry of trimagnesium phosphate and tricalcium phosphate

Trimagnesium phosphate Mg-3(PO4)(2) (triMg) and beta tricalcium phosphate C a-3(PO4)(2) (triCa) have been prepared and characterized by X-ray diffracti on, IR spectroscopy and chemical analysis. Using an isoperibol calorimeter the heat of solution of these products in a 9% weight nitric acid solution were determined. They are (-232.6 +/- 2.7) kJ mol(-1), (-130.3 +/- 1.2) kJ mol(-1), respectively. Combining these results with those found by dissolvi ng other products and entities involved in thermochemical cycles, allows to determine the standard enthalpies of formation of these phosphates. These quantities are -3706 kJ mol(-1) for triMg and -4056 kJ mol(-1) for triCa. T hese values together with those obtained previously for triSr and triBa wer e discussed taking into account the coordination number and the metal-oxyge n bond energy. (C) 1999 Elsevier Science B.V. All rights reserved.