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Threshold effect

Inhibition of precipitation in super-satured solution (eg CaCO3, Ca SO4) with sub-stechiometric inhibitor concentrations.

The inhibitor's molecules are attracted by the growing crystal surface, chiefly due to the presence of metal cations (Ca,Mg,Ba) with which they usually have great affinity. Once they have been adsorbed, the inhibitor's molecules settle on the surface and edges of the crystal, disturbing their even growth to an enormous extent.

If all this takes place during the "nucleation" phase, that is to say, during the stage when a certain number of molecules in solution begin to pack together to give rise to a crystal nucleus, the inhibitor has the capacity of interfering with the growth of the nucleus to such an extent as to dissolve it again.

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