The understanding of the precipitation mechanisms of calcium phosphates (CaPs) remains extremely challenging despite its fundamental interest for material sciences and its practical implications in medicine, paleosciences, and chemistry, especially for heterogeneous catalysis. The competitive precipitation of CaPs, such as hydroxyapatite, octacalcium phosphate and dicalcium phosphate, has been shown experimentally to be strongly impacted by the pH of the precipitating medium. However, the thermodynamic validation of this observation, based on the solubility isotherms reported in the literature, could not be comprehensively substantiated. In the present study, we developed a thermodynamic prediction model adapted to the synthesis of CaPs in open systems (dropwise synthesis). By overcoming the theoretical limits of the solubility isotherms to explain their precipitation features, this new thermodynamic model makes it possible to shed light on the influence of the common ion effect on the stability of these materials in aqueous solution. Finally, this work paves the way to a methodological approach that could be of broad interest in the field of the precipitation of inorganic materials when a continuous flow addition is used.