Influence of foreign salts on the CaCO₃ pre-nucleation stage: application of the conductometric method

The calcium carbonate crystallization process has been studied for more than a century. Nevertheless, little is known about the early stages of nucleation since in situ observations are difficult at the nanometer scale. In this paper, an original method based on electrical conductivity modeling applied to the fast controlled precipitation (FCP) method data is used to underline the existence of different ion pairs during the prenucleation stage as a function of the foreign salts (NaCl, MgCl₂, Na₂SO₄ and MgSO₄) added to the calco-carbonic pure water (CCPW). It is shown that after the addition of each foreign salt a resistivity variation is detected during the aggregation time due to formation of several ion pairs (CaCO⁰₃, CaHCO₃⁺, NaHCO⁰₃, NaCO₃⁻ NaCO₃⁻, NaSO₄⁻, CaSO⁰₄, MgSO⁰₄, MgHCO₃⁺ and MgCO⁰₃). The foreign salt, without completely inhibiting the formation of ion pairs, acts on their formation rate and subsequently influences the nucleation kinetics and thermodynamics. A meaningful comparison is obtained between the resistivity-pH curves obtained experimentally in a FCP test and modeled using the McCleskey equation in the prenucleation stage after the addition of NaCl, MgCl₂, Na₂SO₄ and MgSO₄ to CCPW.