ROLE OF METASTABLE PHASES IN THE SPONTANEOUS PRECIPITATION OF CALCIUM-CARBONATE

This paper reports a study of the spontaneous precipitation of calcium carbonate from aqueous solutions which are very supersaturated with respect to calcite, both in the absence of inhibitors, and in the presence of triphosphate as inhibitor. The sequence of events during precipitation is governed strongly by the initial supersaturation, the temperature, and the presence or absence of triphosphate. At high supersaturation, the first-formed phase is an amorphous calcium carbonate. It is observed only above a well defined ionic activity product, is homogeneously nucleated, and its formation is not inhibited by triphosphate. Over the temperature range 289-333 K, this amorphous phase has a solubility product, K1, defined by the equation: log K1 = (1247.0/T) - 10.224. At concentrations insufficient to produce the amorphous phase, the first-formed solid nucleates heterogeneously. In the absence of triphosphate it is calcite. In the presence of triphosphate, the first solid is calcium carbonate hexahydrate at lower temperatures, and a vaterite/calcite mixture at higher temperatures. CaCO3 . 6H2O is formed only at temperatures below ca. 25-degrees-C, and decomposes to vaterite. These crystalline phases also result from the decomposition of the amorphous CaCO3 at higher initial supersaturation. In the course of the work, the solubility product for CaCO3 . 6H2O (K2) was measured in the range 277-312 K as: log K2 = (1696/T) + 0.9336. The quantitative basis of the study was consistent with a re-evaluation of the CaCO3O pair constant K3, as: log K3 = 7.722 - (1347.2/T). These results show that at temperatures below 25-degrees-C the binding of CO32- to calcium is significantly greater than earlier publications have suggested: the implication for the distribution of carbonate species in the oceans has yet to be fully assessed.