Hardly soluble calcium phosphate compound is able to dissolve easily by stirring with an aqueous suspension containing a strongly acidic ion exchange resin, H-R. However, CaHPO
4·2H
2O (DCPD), an intermediate, precipitates during the dissolution process of tetracalcium phosphate [Ca
4 (PO
4)
2O, TetCP] or α-tricalcium phosphate [Ca3 (PO
4)
2, α-TCP], accompanied with a decrease of electric conductivity. In this paper, the dissolution process of six kinds of calcium phosphates was followed by simultaneous measurement of calorimetry, conductivity and pH. Precipitation of DCPD by the reaction between TetCP and Ca (H
2PO
4)
2 solution proceeds quantitatively. The fact is applied to determine the standard formation of enthalpy (-4779.0 kJ · mol
-1) of TetCP.
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