Growth and precipitation of a monoclinic calcium pyrophosphate tetrahydrate indicating auto-inhibition at pH 7
Abstract
The rate of growth of a stable form of calcium pyrophosphate tetrahydrate, found to be monoclinic and not orthorhombic as previously proposed, has been studied at pH 5 and 7. At low supersaturation ( S<2), the rate of growth at pH 5 is about 20 times higher than at pH 7 for the same supersaturation. At pH 5 the growth rate increases with S as expected, with simultaneous spontaneous precipitation occurring at S0>2. At pH 7 the growth rate increases with S up to S0≈3, but thereafter growth is increasingly inhibited as S0 increases. Spontaneous precipitation occurs after longer induction times at pH 7 than at pH 5. Contrary to other ionic species, concentrations of CaP 2P 72- and P 2O 74- ions in solution are considerably higher in experiments at pH 7 than at pH 5, and the concentration of CaP 2P 72- increases strongly with S0 for S0>3 at pH 7. This indicates that chelation of Ca 2+ and P 2O 74- on the crystal surface inhibits growth and precipitation at pH 7. To our knowledge such an auto-inhibition of crystal growth has not been observed previously.
- Publication:
-
Journal of Crystal Growth
- Pub Date:
- May 2000
- DOI:
- 10.1016/S0022-0248(00)00231-1
- Bibcode:
- 2000JCrGr.212..500C