Modeling the acid-base properties of molybdate(VI) in different ionic media, ionic strengths and temperatures, by EDH, SIT and Pitzer equations

Abstract

This paper reports the results of a study on the determination of the protonation constants of MoO42 −, in NaClaq, NaNO3aq, KClaq, at different ionic strengths (0 < I/mol dm− 3 ≤ 5.0 in NaClaq, 0 < I/mol dm− 3 ≤ 3.0 in NaNO3aq and KClaq) and temperatures (278.15 ≤ T/K ≤ 318.15 in NaClaq, only 298.15 K in NaNO3aq and KClaq), by potentiometric (ISE-H+ glass electrode) and spectrophotometric (UV/Vis) titrations. After a critical analysis of results and literature findings, the proposed speciation model takes into account the formation of two monomeric and four heptameric species, namely: MoO4H−, MoO4H2, (MoO4)7H86 −, (MoO4)7H95 −, (MoO4)7H104 − and (MoO4)7H113 −. Due to the complexity of the system, and to discrepancies of literature findings, particular attention has been paid to the determination of the stability of monomeric species, by performing measurements at very low concentrations, in which the polymerization processes do not occur. Data obtained have then been used to model the dependence of the acid-base properties of molybdate on medium, ionic strength and temperature by models commonly used for these purposes, namely the Extended Debye-Hückel (EDH), the classical Specific ion Interaction Theory (SIT), Pitzer equations, and van't Hoff (for the dependence on temperature). These last results, some of them obtained for the first time in this work, can be exploited for the modeling of the speciation of MoO42 − in a wide range of conditions typical of natural waters and biological fluids, as well as in many aqueous systems of technological/industrial importance, where molybdate can play key roles.