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Investigation of formation and stability of V(IV)- EDTA and V(V)-EDTA complexes using Hanging Mercury Drop Electrode

Vanadium is considered a relatively abundant element, where its share in natural samples is estimated to be around 0.019 %.[1] Interest in Vanadium is derived from the occurrence of the common redox states in Earth surface environments(+3, +4, +5) as a consequence of Vanadium`s geochemical evolution. Vanadium chemical speciation and solubility is a complex function of pH, Eh, concentration, solution chemistry and many other factors.[2] Reduced species have a strong tendency to oxidize quickly, making the speciation of Vanadium very demanding. Based on former studies on Vanadium speciation, it has been found that Vanadium reduced species form stable complexes with EDTA. In order to optimize chromatographic separation of V(III)-EDTA, V(IV)- EDTA and V(V)-EDTA we investigated the stability of V-EDTA complexes through electrochemical behavior of solutions containing V(IV) and V(V). Analysis were made using differential pulse voltammetry (DPV) at a hanging mercury drop electrode (HMDE). The experiments were carried out on 2.78, 3.0, 3.3, 3.6, 4.1, and 6.3 pH values of solutions, respectively. Results show strong pH dependence of V electrochemical behavior at the HMDE.In more acid conditions for both species, i.e. V (IV) and V (V) one pronounced voltammetric peak is observed, whereas on more alkaline pH values, same species are characterized by two peaks. It has been found that on lower pH values, single peak is a result of V(IV) to V(II) reduction. However, on higher pH values observed reduction peaks can be assigned to the one electron reduction of V(V) to V(IV) ions and again to the two electron reduction of V(IV) to V(II) ions, respectively. This is in agreement with the reported behavior using differential pulse polarography (DPP) and cyclic voltammetry (CV) for solutions containing V(V).[3] Also the solutions containing V(IV) and V(V) were titrated with EDTA in order to examine stability of formed complexes. During titrations it can be seen that the wave of free V-species reduces due to the complexation with EDTA. In the same time another wave is forming which can be attributed to V(IV)-EDTA complex orV(V)-EDTA complex, respectively. Derived stability constants of formed complexes are found to be in agreement with available literature data. [4] It has been found that complexation of V species with EDTA is also pH dependent. At lower pH values voltammetric wave characteristic for free V(IV) and V(V) is diminishing after EDTA concentration added are approximately equal to the starting free V concentration indicating 1:1 V vs. EDTA stoichiometry. At higher pH values concentration of added EDTA needed to titrate free V voltammetric wave are two or more times higher. Moreover, also the shift free V wave towards negative values of potential with increasing pH of the solution is observed. This behavior is in accordance with the formation of stable hydrolyzed polynuclear complexes of V species on higher pH values.[1] All the listed results will be compared with the spectrophotometry measurements of V(IV) and V(V) solutions behavior at different pH values.

Vanadium ; EDTA complexes ; hanging mercury drop electrode

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