Abstract
H-R represents a strongly acidic ion exchange resin with hydrogen form. Dissolution mechanism of two modifications of Ca3(PO4)2, α- and β-TCP, in the presence of H-R was studied to compare their reactivities. Chemical analysis of liquid phase, simultaneous measurement of calorimetry and conductimetry, XRD and SEM observation were carried out to discuss the dissolution process. Hydroxyapatite (HAp) and brushite (DCPD) were also used for reference. The results obtained are summarized as follows; (1) The heat of dissolution (exothermal in kJ/mol) at 25°C measured by introducing a sample into aqueous suspension containing an excess amount of H-R was estimated as follows; β-TCP: 122.8, α-TCP: 137.1, HAp: 310.8, DCPD: 2.4. Consequently, the heat of transition from α- to β-TCP was calculated to be 14.3kJ/mol. (2) The dissolution of β-TCP proceeds by the following two-stage reactions; Ca3(PO4)2+4(H-R)→2(Ca-R2)+Ca(H2PO4)2 (aq)……(1), Ca(H2PO4)2(aq)+2(H-R)→Ca-R2+2H3PO4(aq)……(2). (3) In contrast with the β-phase, the dissolution of metastable α-phase proceeds by the three-stage reactions of Eqs. (1), (3) and (4). An initial addition of H-R gives Ca(H2PO4)2 solution (Eq. (1)), which was soon followed by crystallization of intermediate DCPD according to the Eq. (3). Two distinct thermal flows due to the dissolution (Eq. (1)) and crystallization (Eq. (3)) processes are accompanied by the corresponding variations of specific conductivity. Ca3(PO4)2+Ca(H2PO4)2(aq)+8H2O→4[CaHPO4⋅2H2O]……(3). Finally, the dissolution of DCPD is completed to give phosphoric acid and Ca-absorbed resin (Eq. (4)). CaHPO4⋅2H2O+2(H-R)→Ca-R2+H3PO4+2H2O……(4).
