The constant composition method for crystallization of calcium carbonate at constant supersaturation

The exact control of supersaturation is of great importance when studying the formation of crystalline and amorphous matter. The constant composition method is suitable for the study of crystallization processes at constant supersaturation by controlled addition of titrants to a crystallizer to maintain constant pH. Not all aspects necessary for successful operation of this method are obvious from the existing literature, and the method is often used in an incorrect way. The focus of the present work is to highlight pitfalls associated with the constant composition method. The method is assessed and described in detail to show that even if the solution pH is kept constant, the supersaturation may change. First and foremost, it is illustrated how crucial it is to use a chemical composition of the titrant solutions which is in accordance with the initially prepared aqueous solution. General rules are presented for carbonates as to how the composition of the titrant solutions should be calculated based on total alkalinity in order to maintain constant supersaturation. This has - to the knowledge of the authors - not been shown before. Then, it is shown how exchange of carbon dioxide with the atmosphere corrupts the constancy of the supersaturation level during an experiment. Third, it is pointed out that the ionic strength should be kept constant throughout crystallization experiments since a change in ionic strength alters the activity of the ions in solution. Here, the determination of the thermodynamic driving force (supersaturation) is explained based on the relevant chemical equilibria, total alkalinity and calculation of the activity coefficients. The calculations are presented for the least stable polymorph of calcium carbonate, vaterite, but can easily be extended to the other polymorphs and other pH-dependent systems allowing for crystallization studies at low and maintained supersaturation levels typical of naturally occurring processes in geology and biomineralization, as well as formation of mineral scales in industry.