Calcium carbonate solubility: A reappraisal of scale formation and inhibition

Considerable disparity exists in the published thermodynamic data for selected species in the Ca2+/CO2/H2O system near 25 degrees C and 1 atm pressure. Some authors doubt the significance of CaCO30(aq) complexes although there is experimental evidence of their occurrence. Evaluation of all the published experimental and estimated data for aqueous calcium carbonate species confirms that the consistent set of constants given by Plummer and Busenberg in 1982 is the best available, and suggests a formation constant log beta = 3.22 for CaCO30(aq). This value was confirmed by additional experimental data and calculations using a specially developed computer program. The solubility s and solubility product K-s are critically evaluated for each solid polymorph (amorphous CaCO3, ikaite, vaterite, aragonite and calcite) using a hydrated ion pair model and we give coherent explanations for the calcium carbonate precipitation/dissolution process and the existence of supersaturated waters. The practical cases of scale formation and its inhibition by phosphonate-type compounds are discussed and explained with the same model, taking into account the CaCO30(aq) species.