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 bee n 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 s pontaneous precipitation occurring at S-0 > 2. At pn 7 the growth rate incr eases with S up to S-0 approximate to 3, but thereafter growth is increasin gly inhibited as S-0 increases. Spontaneous precipitation occurs after long er induction times at pH 7 than at pH 5. Contrary to other ionic species, c oncentrations of CaP2P72- and P2O74- ions in solution are considerably high er in experiments at pH 7 than at pH 5, and the concentration of CaP2P72- i ncreases strongly with S-0 for S-0 > 3 at pH 7. This indicates that chelati on of Ca2+ and P2O74- on the crystal surface inhibits growth and precipitat ion 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.