Extractive Spectrophotometric Determination and Theoretical Investigations of Two New Vanadium(V) Complexes.

Two new vanadium (V) complexes involving 6-hexyl-4-(2-thiazolylazo)resorcinol (HTAR) and tetrazolium cation were studied. The following commercially available tetrazolium salts were used as the cation source: tetrazolium red (2,3,5-triphenyltetrazol-2-ium;chloride, TTC) and neotetrazolium chloride (2-[4-[4-(3,5-diphenyltetrazol-2-ium-2-yl)phenyl]phenyl]-3,5-diphenyltetrazol-2-ium;dichloride, NTC). The cations (abbreviated as TT+ and NTC+) impart high hydrophobicity to the ternary complexes, allowing vanadium to be easily extracted and preconcentrated in one step. The complexes have different stoichiometry. The V(V)-HTAR-TTC complex dimerizes in the organic phase (chloroform) and can be represented by the formula [(TT+)[VO2(HTAR)]]2. The other complex is monomeric (NTC+)[VO2(HTAR)]. The cation has a +1 charge because one of the two chloride ions remains undissociated: NTC+ = (NT2+Cl-)+. The ground-state equilibrium geometries of the constituent cations and final complexes were optimized at the B3LYP and HF levels of theory. The dimer [(TT+)[VO2(HTAR)]]2 is more suitable for practical applications due to its better extraction characteristics and wider pH interval of formation and extraction. It was used for cheap and reliable extraction-spectrophotometric determination of V(V) traces in real samples. The absorption maximum, molar absorptivity coefficient, limit of detection, and linear working range were 549 nm, 5.2 × 104 L mol-1 cm-1, 4.6 ng mL-1, and 0.015-2.0 µg mL-1, respectively.

Complex Formation in a Liquid-Liquid Extraction System Containing Vanadium(IV/V), 2,3-Dihydroxynaphtahlene and Thiazolyl Blue.

Liquid-liquid extraction systems for VIV/V containing 2,3-dihydroxynaphtahlene (DN) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (thiazolyl blue, MTT) were studied. The optimum conditions for VIV and VV extraction were found. VIV is extracted in chloroform as a 1:2:2 complex (V:DN:MTT) with lmax = 570 nm and e570 = 2.9´104 dm3 mol-1 cm-1. However, this wavelength was found unsuitable for precise spectrophotometric measurements due to time dependent absorbance changes. VV forms predominantly a 1:1:1 complex with lmax = 335 nm. The calibration graph for this oxidation state is linear in the range of 0.06-1.5 mg cm-3. The molar absorptivity, Sandell's sensitivity and limit of detection were calculated to be 1.6 ´ 104 dm3 mol-1 cm-1, 3.2 ng cm-2 and 0.02 mg cm-3, respectively. The ground-state equilibrium geometries of the anionic parts of the extracted ion-associates, [VIVO(DN2-)2]2-and [VVO2(DN2-)]-, were optimized at the BLYP/6-31++G* level of theory.

Liquid-Liquid Extraction-Chromogenic System for Vanadium(V) Based on 4-(2-thiazolylazo)orcinol (TAO) and Aliquat 336.

The quaternary ammonium salt Aliquat 336 (A336; R4N+Cl-) and the azo dye 4-(2-thiazolylazo)orcinol (ТАО) were examined as constituents of a water-isobutanol extraction-chromogenic system for vanadium(V). Under the optimum conditions (cTAO = 1.6´10-3 mol dm-3, cA336 = 1.3´10-2 mol dm-3 and pH 5.0), vanadium(V) is extracted as a ternary complex which can be represented by the formula (R4N+)[VO2(TAO2-)]. The key extraction-spectrophotometric characteristics were determined. The interfering effect of foreign ions was studied as well. The wavelength of maximum absorption (λmax), molar absorptivity (εmax), constant of extraction (Kex) and fraction extracted (E) were found to be λ = 547 nm, ε = 3.1×104 dm3 mol-1 cm-1, Log Kex = 2.8 and E = 90.2%, respectively. Beer's law was obeyed in the range of 0.084-2.0 mg cm-3 and the limit of detection was 25 ng cm-3 of vanadium(V).