Vanadium In Aqueous Solutions Research Articles

Vanadium(IV) and ‐(V) Complexes of Reduced Schiff Bases Derived from Aromatic o‐Hydroxyaldehydes and Tyrosine Derivatives

AbstractThe reduced Schiff bases of salicylaldehyde and pyridoxal (and o‐vaniline) with L‐tyrosine (Tyr) and D,L‐o‐tyrosine (o‐Tyr), designated as sal‐Tyr (1), sal‐o‐Tyr (2), pyr‐Tyr (3), pyr‐o‐Tyr (4), and o‐van‐L‐Tyr (5), as well as the oxidovanadium(IV) complex VO(sal‐o‐Tyr) (6) have been prepared. The compounds have been characterized in the solid state and in solution. The structure of 3 has been determined by X‐ray diffraction. Complexation of these ligands with vanadium in aqueous solution has been studied by pH potentiometry, UV/Vis, and circular dichroism (for the L‐Tyr derivatives), as well as by EPR for the VIVO systems and 51V NMR for the VVO2 systems. Stoichiometries and complex formation constants have been determined by pH potentiometry (25 °C, I = 0.2 M KCl) 1:1 complexes are formed in most systems with variable protonation states VOLH2 (only with 3 and 4), VOLH, VOL, and VOLH–1. Dinuclear species (VOL)2H and (VOL)2 were identified only in the case of 3. Spectroscopic data provided information about the most probable binding modes for each stoichiometry. The VIVO complexes formed with the o‐Tyr‐derived ligands are more stable than those with L‐Tyr, the more adequate coordination position of the phenolate in the o‐Tyr ligands shows a more significant stabilizing effect compared with 3 and 4.

Acid leaching of black shale for the extraction of vanadium

The feasibility of vanadium recovery from black shale was investigated using sulfuric acid and hydrofluoric acid solutions. The chemical characterization of black shale from Guizhou, China showed that vanadium mainly existed in free oxide and mica. According to E–pH diagram for V–H 2O at 25 °C, the dissolved species of vanadium in aqueous solution mainly present in form of VO 2+ and VO 2 + at high potential and low pH value. The effects of leaching time, sulfuric acid concentration, hydrofluoric acid concentration, sodium hypochlorite concentration, temperature and liquid to solid ratio on the vanadium leaching were studied. The optimum parameters obtained were a reaction time of 6 h, sulfuric acid concentration of 87.5 g/L, hydrofluoric acid concentration of 15 g/L, sodium hypochlorite concentration of 1 g/L (added after a reaction time of 3 h), leaching temperature of 95 °C and a liquid to solid ratio of 4 mL/g. Under these conditions, up to 86% vanadium was recovered. HF has an important role in the leaching vanadium process. Compared to our previous two-step countercurrent leaching, the vanadium recovery only decreased 4% despite the leaching temperature was decreased from 150 °C for the previous tests to 95 °C for the present work.

Corrosion control of vanadium in aqueous solutions by amino acids

The electrochemical behavior of vanadium in amino acid free and amino acid containing aqueous solutions of different pH was studied using open-circuit potential measurements, polarization techniques and electrochemical impedance spectroscopy (EIS). The corrosion current density, i corr, the corrosion potential, E corr and the corrosion resistance, R corr, were calculated. A group of amino acids, namely, glycine, alanine, valine, histidine, glutamic and cysteine has been investigated as environmentally safe inhibitors. The effect of Cl − on the corrosion inhibition efficiency especially in acid solutions was investigated. In neutral and basic solutions, the presence of amino acids increases the corrosion resistance of the metal. The electrochemical behavior of V before and after the corrosion inhibition process has shown that some amino acids like glutamic acid and histidine have promising corrosion inhibition efficiency at low concentration (≅25 mM). The inhibition efficiency ( η) was found to depend on the structure of the amino acid and the constituents of the corrosive medium. The corrosion inhibition process is based on the adsorption of the amino acid molecules on the metal surface and the adsorption process follows the Freundlich isotherm. The adsorption free energy for valine on V in acidic solutions was found to be −9.4 kJ/mol which reveals strong physical adsorption of the amino acid molecules on the vanadium surface.

Electrochemical behaviour of vanadium in aqueous solutions of different pH

he electrochemical behaviour of vanadium in aqueous solutions of pH covering the range 1–13 in oxygen saturated, oxygen free and naturally aerated solutions was investigated. The behaviour of the vanadium electrode in alkaline solutions ( pH > 8) is different from that in acid or neutral solutions (pH 1–8). The equilibrium electrode potential is a linear function of the solution pH. The slope of the linear relation changes from −0.039 V/decade for solution of pH ≤ 8 to −0.058 V/decade for solutions of pH > 8. EIS investigations have shown that the metal undergoes active dissolution in aqueous solutions. In solutions of pH > 8 the corrosion behaviour of vanadium can be simulated to a Randles equivalent circuit model. In oxygen saturated solutions the electrode surface is covered with an oxide film. The interaction of this film with the H + and OH − in solutions with different pH was discussed. Polarization measurements have shown that the rate of corrosion in acid solutions is not affected by the prevailing gas. In alkaline solutions, removal of oxygen or air from the solution leads to a decrease in the corrosion rate of the metal.

Separation and preconcentration of trace vanadium in aqueous solution using trioctylmethylammonium chloride-loaded silica gel.

The separation and preconcentration of trace vanadium from an aqueous solution for a graphite furnace atomic-absorption spectrometric (GF-AAS) determination was studied using silica gel loaded with trioctylmethylammonium cloride (Capriquat). A sample solution containing V was adjusted to pH 6.0. The solution was then passed through 1.0g of Capriquat loaded in a silica gel column at a flow rate of 10 ml min-1. The V collected on the Capriquat-loaded silica gel on the column was eluted with 10 ml of 0.5 M perchloric acid, and then determined by GF-AAS. In this method, over 99% of the V was adsorbed at about pH 6.0. The calibration curve was linear from 0.05 to 2.5 μg/l of V. The proposed method has been applied to the determination of V in river water and seawater samples.

Laser-produced reduction of pentavalent vanadium in aqueous solutions and V2O5 powder

The reduction of pentavalent vanadium ions in aqueous solutions upon laser irradiation has been confirmed by using optical absorption and fluorescence techniques. X-ray photoelectron spectroscopy (XPS) study revealed that V2O5 was reduced to lower oxides upon laser treatment. Furthermore, the presence of reduced valence states of vanadium has been detected through XPS in vanadium-containing solids such as V2O5/P2O5 glass, thin films prepared separately from V2O5 powder and the glass.

Thiophene-2-hydrazide as a reagent for the spectrophotometric determination of vanadium in aqueous solution

A new chromogen, thiophene-2-hydrazide, for the determination of trace amounts of vanadium in aqueous solution has been developed. The intense yellow, water-soluble, stable and binary complex, formed in acidic medium, is suitable for the determination of 0.5–5 p.p.m. of vanadium ion, with a molar absorptivity of 12.1 × 103 mol–1 cm–1, at 410 nm, relative error –0.6 to +4.0% and relative standard deviation 0.3–0.8%, depending on the concentration level of the determinant. Moreover, the colour formation is very fast. Interferences due to foreign ions have been examined.

Computer controlled system for the automatic neutron activation analysis of vanadium in petroleum with a californium-252 source

A pneumatic system interfaced to a digital computer has been designed, programmed, and tested for the unattended and continuous determination of vanadium in aqueous solutions, petroleum, and cokes. A californium-252 source is used for neutronactivation of the samples. Under computer control, the system is capable of analyzing 48 samples in an 8-h period. Three different calibration curves are required for the three types of materials analyzed because of different thermalization effects in the different types of samples. The system requires 5 to 10 g samples to achieve a detection limit of 10 ppM +- 30%. Increased precision is obtained by reanalyzing aliquots of the sample. Accuracy studies obtained with prepared samples showed differences of up to 4%. 6 figures, 2 tables.