Cryst. Res. Technol. 51, No. 1, 99–109 (2016) / DOI 10.1002/crat.201500182 Original Paper Conductometric study of calcium carbonate prenucleation stage: underlining the role of CaCO 3 o ion pairs Raghda Hamdi 1 and Mohamed Mouldi Tlili 1,2,∗ Received 28 June 2015, revised 26 August 2015, accepted 2 September 2015 Published online 23 September 2015 Calcium carbonate crystallization process, especially the prenucleation stage, has increasingly been the subject of several works. In the present work, a simple method based on electrical conductivity modeling applied to the FCP (Fast Controlled Precipitation) method data is used to highlight the role of CaCO 3 o ion pairs on calcium carbonate prenu- cleation stage. A good agreement was obtained between the resistivity vs pH curves estimated by the McCleskey model equation and obtained experimentally in a FCP test. Results showed that the nucleation process begins with the for- mation of CaCO 3 o ion pairs as pre-nuclei as soon as the calcite-equilibrium pH is reached. Additionally CaCO 3 o content increases with pH to form aggregates, which de- pend on the saturation state of the solution. Basing on our thermodynamic data, these aggregates do not form amor- phous calcium carbonate ACC as an intermediate phase. They lead to the formation of stable calcium carbonate nu- clei which will further evolve to crystallize. Furthermore we demonstrate that in addition to their inhibitory efect on the Ca 2+ and CO 3 2− association to form ion pairs, the two scale inhibitors sodium triphosphate (STP) and sodium poly- acrylate (RPI) reduce ion pairs aggregation rate. 1 Introduction Crystallization is an important formation process by which molecules in a solution are transformed into a solid phase of regular lattice structure. During precipi- tate ageing, a gradual transformation of an initially pre- cipitated metastable phase into a final crystalline form often occurs. The metastable phase may be an amor- phous precipitate, a polymorph of the final material, a hydrated species or some system-contaminated sub- stance [1]. Controlling this process can have an undeni- able importance to improve or to prevent the formation of solid phases. One of the most undesirable crystallization phenom- ena is the one that leads to the scaling process which consists on the formation of sparingly soluble salts, es- pecially calcium carbonate, in natural water treatment and distribution systems and in industrial equipment. The appearance of this phenomenon is responsible for numerous technical and economical problems such as pipes obstruction, seizure of valves, fouling of mem- branes and heat transfer surfaces in desalination plants [2, 3]. For over a century and still today, CaCO 3 crystalliza- tion process, especially the nucleation step, attracts the attention of several researchers worldwide [4–8]. Its for- mation is governed by the thermodynamic imbalance of the calco-carbonic system CaCO 3 -CO 2 -H 2 O. The com- plexity of this system is not only limited to the numer- ous equilibriums between ionic species, but also to the formed precipitate. Indeed, calcium carbonate precip- itates as hydrated forms: [amorphous (ACC), monohy- drate (MCC) and hexahydrate calcium carbonate] and anhydrous forms: [calcite, aragonite and vaterite]. Inves- tigations on the thermodynamics of the calco-carbonic system showed the occurrence of a large super-saturated domain with respect to calcite, which is the most stable phase of CaCO 3, called the metastable domain, where the precipitation does not occur [9–12]. In this metastable domain, in accordance with Ost- wald’s step rule, it was shown that the onset of massive CaCO 3 precipitation occurs only if the solubility prod- uct of one of the hydrated forms was reached [12–15]. Hence, Elfil and Hannachi [11] have defined a new scal- ing index (MLSI = pH − pH s ) by reformulating the one proposed by Langelier [16]: LSI = pH − pH s . The pH s is ∗ Corresponding author: e-mail mohamed.tlili@certe.rnrt.tn, Phone: +216 79 325 044 1 Lab of Natural Water Treatment, Center of Water Research and Technologies, Ecopark of Borj Cedria, BP 273 Soliman, 8020, Tunisia 2 Department of Chemistry, Faculty of Sciences - King Khalid Uni- versity 9033 Abha, Saudi Arabia 99 C  2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim