In Situ Study of the Precipitation and Crystallization of Amorphous Calcium Carbonate (ACC)

The precipitation and crystallization of amorphous calcium carbonate (ACC) from metastable calcium carbonate solutions in the concentration range 2-10 mM have been investigated in situ in solution in the absence of additives, and in the presence of Mg2+ and poly(acrylic acid). We demonstrate that measurement of the intensity of transmitted light provides an effective method for monitoring the early stages of precipitation of calcium carbonate in solution, where the nature of the scattering particles was confirmed with transmission electron microscopy. The recorded changes in light transmission with time and concentration can be related to the precipitation and aggregation of ACC, its subsequent crystallization to calcite (or vaterite), and the gravitational sedimentation of the growing crystals. Addition of Mg2+ or poly(acrylic acid) is shown to retard the process, delaying the precipitation of ACC and increasing the lifetime of this phase. These results show that measurement of turbidity provides an effective route for monitoring the complex precipitation and crystallization behavior of aqueous CaCO3 and suggests that this method has widespread applicability for studying solution precipitation and crystallization.