Description
Calcium pyrophosphate has the molecular formula of
Ca2P2O7 and the molecular weight of 254.1378 g/mol. It
has the CAS number of 7790-76-3. It may be prepared by
the solid-state reaction of the dibasic orthophosphate, or
alternately by the precipitation of a soluble Ca salt with
aqueous pyrophosphoric acid:
2CaHPO4 + heat ? Ca2P2O7 +H2O
CaCl2 (aq) +H4P2O7 (aq) ? Ca2P2O7 (solid) + 2HCl (aq)
The solid-state reaction produces an amorphous crystal
at 240–500°C. b-Ca2P2O7 forms >750°C and a-Ca2P2O7
forms in the temperature range of 1140 to 1350°C. The
free energy of formation, △Gf=-3132 kcal/mol.
The product of the aqueous method is the dihydrate,
Ca2P2O7·2H2O. It is a white powder and has the CAS
number of 7790-76-3. It is insoluble in water but soluble
in dilute acids. The anhydrate decomposes at 1353°C to
form a metaphosphate:
Ca2P2O7 + heat ? Ca(PO3)2 + CaO
Calcium pyrophosphate is dimorphic. The phase
transformation and sintering behaviors of Ca2P2O7 with
different phase composition has been investigated by
using X-ray powder diffraction (XRD), dilatometery and
SEM techniques. It was found that although α-Ca2P2O7
(high-temperature form) could be maintained during its
cooling process, it was metastable and retransformed
into β-Ca2P2O7 (low-temperature form) at about 950 °C
during its reheating process.
