Physical characterization of dibasic calcium phosphate dihydrate and anhydrate.

The dehydration of different commercial brands of dibasic calcium phosphate dihydrate (DCPD; CaHPO4.2H2O) was examined over a range of temperatures and water vapor pressures.   To determine the main factors affecting the physical stability of DCPD, the baseline characterization of DCPD and dibasic calcium phosphate anhydrate (DCPA; CaHPO4) was conducted by thermogravimetric analysis, differential scanning calorimetry and X-ray diffractometry.   The surface area and the DCPA content (present as an impurity) depended on the commercial source of DCPD.   The larger particles contained a higher concentration of DCPA and the anhydrate exhibited a concentration-dependent acceleratory effect on the dehydration of DCPD.   Unlike DCPD, DCPA is physically stable and resisted hydration even when dispersed in water for over 7 months in the temperature range of 4-50 degrees C.   In dosage forms containing DCPD, there is a potential for phase transformation to DCPA, while the reverse transition, that is, DCPA --> DCPD appears to be extremely unlikely.   Thus, the risk of physical transformation can be minimized by using DCPA in formulations.