Role of CaCO3° Neutral Pair in Calcium Carbonate Crystallization

The molecular structure of the units that get incorporated into the nuclei of the crystalline phase and sustain their growth is a fundamental issue in the pathway from a supersaturated solution to the formation of crystals. Using a fluorescent dye we have recorded the variation of the pH value in time along a gel where CaCl 2 and NaHCO 3 counter-diffuse to crystallize CaCO 3 . The same pH-space-time distribution maps were also computationally obtained using a chemical speciation code (phreeqc). Using data arising from this model we investigated the space-time evolution of the activity of the single species (ions and ion pairs) involved in the crystallization process. Our combined results suggest that, whatever the pathway from solution to crystals, the neutral pair CaCO 3 ° is a key species in the CaCO 3 precipitation system.