Modelling the dependence on medium and ionic strength of molybdate acidbase properties, and its interactions with phytate

The importance of molybdenum from a biological, environmental and technological point of view is very well known since many decades [15]. In particular, it is mainly present in aqueous solutions as molybdate (MoO42), which is the biologically active form, entering in the cells by active transport systems. Though molybdate is the major species in neutral to basic pH conditions, at lower pH it undergoes protonation and, chiefly, polymerization, even at millimolar concentration levels [2]. Consequently, the modelling of its speciation and acidbase properties is not very simple, as demonstrated by the nonhomogeneity of available literature data. In this light, our group has started a systematic study aimed at the evaluating the dependence on medium and ionic strength of the acidbase properties of molybdate. This contribution reports some results relative to the protonation constants of molybdate at T = 298.15 K, in NaClaq and NaNO3aq at different ionic strengths (0 < I / mol dm3 ≤ 5.0 in NaClaq, 0 < I / mol dm3 ≤ 3.0 in NaNO3aq), by potentiometric and spectrophotometric titrations. The two techniques allowed the investigation of different ligand concentration ranges, in order to define the acid base behaviour of molybdate below and above the concentration limits for the formation of the polynuclear species. The dependence of the protonation constants on ionic strength has then been modelled by means of classical approaches (Extended DebyeHückel, Specific ion Interaction Theory, Pitzer). Moreover, it is well known that molybdate, other than as a classical anion, behaves as Lewis acid too, leading to relevant interactions with both cations and ligands in aqueous solution [4]. As a consequence, a better insight on the coordination behaviour and the speciation of molybdate in aqueous systems containing various cations and ligands is of great concern from many different point of view. For this reason, its interactions with phytate [Phy, 1,2,3,4,5,6hexakis(dihydrogen phosphate) myoinositol] have also been investigated at T = 298.15 K in NaClaq at different ionic strengths, with the aim of defining the speciation of these systems and evaluating the sequestering ability of this ligand toward molybdate. In fact, analogously to MoO42, also phytate is a very important molecule from the biological, environmental and technological point of view, due to its ability to form quite stable complex species with several metal and organometal cations, as well as other ligands [6,7].