The chapter presents a thermodynamic approach for the complex chemical and redox equilibria investigation of mono- and two-phase systems containing metal ions of slightly soluble solid phases in different oxidation states. For heterogeneous systems, this approach utilizes thermodynamic relationships combined with original mass balance constraints. The forms of occurrence of metal ions are determined by the major chemical reactions in the aquatic environment such as hydrolysis, oxidation, reduction, and precipitation. On the basis of the introduced generalized equation of the overall redox reaction the physical meaning of the overall electrode potential under conditions other than standard ones, was revealed. The diagrams of complex chemical equilibria have been used for the graphical representation of complex equilibria in aqueous solutions. The developed approach can be applied to calculating potential−pH (Pourbaix) diagrams, based on the thermodynamic analysis of chemical equilibria in homogeneous and heterogeneous multicomponent systems.
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Although most chemical reactions occurring in natural environments are considered acid–base processes (weathering of aluminosilicates, the solubility of carbonates) involving the transfer of protons, many other reactions involve the transfer of electrons and are known as oxidation–reduction processes. The forms of occurrence of metal ions are determined by the significant chemical reactions in the aquatic environment such as hydrolysis, oxidation, reduction, and precipitation. Depending on pH, redox potential, and total concentration of inorganic and organic ligands, metal ions may undergo various transformations to produce a whole range of chemical species in aqueous systems. Furthermore, depending on the redox conditions, organisms utilize different oxidized constituents as electron acceptors.
Complexing agents change the redox potential of a redox system involving metal ions. Frequently, in the presence of a complexing agent, oxidized species can form more stable complexes than reduced species. Therefore, the redox potential is modified in the presence of a suitable ligand (Bashir et al., 2018). This phenomenon has the potential to develop new various redox reactions in the presence of an appropriate complexing agent (Bashir et al., 2018; Rizvi et al., 2011a; Serjeant, 1984). This process has been applied to potentiometric titrations of some metal ions (Rizvi et al., 2011b; Safavi et al., 2002; Zhang & Zhou, 2019). For example, to determine the suitable pH range for the titration of vanadium (V) with iron (II) in the presence of a ligand, the formal potentials of equimolar mixtures of iron (II) and iron (III), as well as vanadium (IV) and vanadium (V), were measured at various pH, respectively (Umetsu, 1991). By using this aspect, Japanese researchers developed new methods for the potentiometric determination of chromium (VI) with iron (II), copper (II) with iron (II) (Itabashi et al, 1990), vanadium (V) with iron (II) (Umetsu, 1991) and sequential titration of chromium (VI) and iron (III) with cobalt (II) (Katsumata et al., 1997) in the presence of suitable complexing agents. The flow injection analysis methods for the determination of iron (II) (Itabashi et al., 1992) and vanadium (IV) (Itabashi et al., 1991) were developed with photometric detection and elaborated as well for the determination of complexing agents such as EDTA, DTPA, NTA, diphosphate, and others, using ligand effects (Itabashi et al., 1992; Teshima et al., 1993). Based on this ligand effect, Teshima et al. (1992) elaborated a new instantaneous flow injection analysis method for the determination of vanadium (IV) and vanadium (V) using redox reactions of vanadium (IV) with iron (III) and vanadium (V) with iron(II).
Redox processes in the presence of complexing agents control the chemical speciation, along with bioavailability, toxicity, mobility, and adsorption of water pollutants in the environment. Coordination of the redox action of transition metals can be an effective approach for the development of new redox systems for specific applications. A priori assessment of the complexation effect of selected ligands on the redox potential is significant for future trace-level applications of potentiometric polymeric membrane electrodes, which are useful analytical tools for heavy metal ion determinations in drinking water at nanomolar total concentrations (Ceresa et al., 2001; Lisak, 2021; Maksymiuk et al., 2020).