Unusual effect of water vapor pressure on dehydration of dibasic calcium phosphate dihydrate

Aditya M. Kaushal, Venu R. Vangala, Raj Suryanarayanan

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Dibasic calcium phosphate occurs as an anhydrate (DCPA; CaHPO4) and as a dihydrate (DCPD; CaHPO4 2H2O). Our objective was to investigate the unusual behavior of these phases. Dibasic calcium phosphate dihydrate was dehydrated in a (i) differential scanning calorimeter (DSC) in different pan configurations; (ii) variable-temperature X-ray diffractometer (XRD) at atmospheric and under reduced pressure, and in sealed capillaries; and (iii) water vapor sorption analyzer at varying temperature and humidity conditions. Dehydration was complete by 210°C in an open DSC pan and under atmospheric pressure in the XRD. Unlike "conventional" hydrates, the dehydration of DCPD was facilitated in the presence of water vapor. Variable-temperature XRD in a sealed capillary and DSC in a hermetic pan with pinhole caused complete dehydration by 100°C and 140°C, respectively. Under reduced pressure, conversion to the anhydrate was incomplete even at 300°C. The increase in dehydration rate with increase in water vapor pressure has been explained by the Smith-Topley effect. Under "dry" conditions, a coating of poorly crystalline product is believed to form on the surface of particles and act as a barrier to further dehydration. However, in the presence of water vapor, recrystallization occurs, creating cracks and channels and facilitating continued dehydration.

Original languageEnglish (US)
Pages (from-to)1456-1466
Number of pages11
JournalJournal of Pharmaceutical Sciences
Issue number4
StatePublished - Apr 2011


  • Calorimetry (DSC)
  • Dehydration
  • Dibasic calcium phosphate
  • Excipients
  • Hydrate
  • Physical stability
  • Thermogravimetric analysis
  • Water vapor
  • X-ray diffractometry

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