Growth and precipitation of a monoclinic calcium pyrophosphate tetrahydrate indicating auto-inhibition at pH 7

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 S-0 > 2. At pn 7 the growth rate increases with S up to S-0 approximate to 3, but thereafter growth is increasingly inhibited as S-0 increases. Spontaneous precipitation occurs after longer induction times at pH 7 than at pH 5. Contrary to other ionic species, concentrations of CaP2P72- and P2O74- ions in solution are considerably higher in experiments at pH 7 than at pH 5, and the concentration of CaP2P72- increases strongly with S-0 for S-0 > 3 at pH 7. This indicates that chelation of Ca2+ and P2O74- 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. (C) 2000 Published by Elsevier Science B.V. All rights reserved.