Study of the Aqueous Chemical Interactions between a Synthetic Tetra-acid and Divalent Cations as a Model for the Formation of Metal Naphthenate Deposits

The previously presented synthetic tetra-acid model compound BP10 was used to investigate the chemistry behind the formation of metal naphthenate deposits. The interactions between BP10 and the cations Ba2+, Ca2+, H+, Mg2+, and Sr2+ were investigated using potentiometric titrations, metal ion depletion by inductively coupled plasma−atomic emission spectrometry (ICP−AES), pH measurements, and elemental analysis of precipitates, in 20−600 mM NaCl ionic medium. The interactions of BP10 with the monovalent Na+ are discussed on the basis of a previous study. The data given indicate that Ca2+ shows the strongest affinity toward BP10 and Ba2+, and Sr2+ form approximately equally stable solid phases with BP10, while Mg2+ is less tightly bound to the tetra-acid. H+ interacts more strongly than the Me2+ ions, and Na+ shows a rather small affinity for BP10. No soluble complexes could be detected, and all products in the chemical reactions are therefore believed to be solid materials. We suggest that BP10 show the following preference of cations: H+ ≫ Ca2+ > Ba2+ ≈ Sr2+ > Mg2+ ≫ Na+. This order could be due to the hydration state and size of the cations. In comparison to typical concentrations found of each in saline water, it is proposed that the dominance of Ca2+ in naphthenate deposits is dependent upon both availability and selectivity.