Complex formation in a liquid-liquid extraction-chromogenic system for vanadium(IV)

The azo dye 4-(2-thiazolylazo)orcinol (TAO) and the cationic ion-pair reagent 2,3,5-triphenyl-2H-tetrazolium chloride (TTC) were examined as constituents of a water-chloroform extraction-chromogenic system for vanadium(IV). The effects of TAO concentration, TTC concentration, pH and extraction time were examined. Under the optimum conditions the extracted complex has a composition of 1:2:1 (V:TAO:TTC). The absorption maximum, molar absorptivity and constant of extraction were determined to be λmax=544 nm, ε544=1.75×104 dm3 mol–1 cm–1 and Log Kex=4.1. The ground state equilibrium geometries of the possible monoanionic VIV-TAO 1:2 species were optimized by the HF method using 3-21G* basis functions. Their theoretical time dependent electronic spectra were simulated and compared with the experimental spectrum. The best fit was obtained for the structure in which one of the TAO ligands is tridentate, but the other is monodentate (bound to VIV through the oxygen which is in the ortho-position to the azo group) and forms a hydrogen bond N–H...O=V through its protonated heterocyclic nitrogen. Based on this unusual structure, which can explain some peculiarities of the complex formation between VIV and commonly used azo dyes, the ground state equilibrium geometry of the whole ternary 1:2:1 complex was computed at the HF and BLYP levels.

An extraction-chromogenic system for vanadium(IV,V) based on 2,3-dihydroxynaphtahlene

A liquid-liquid extraction-chromogenic system for vanadium(IV, V) containing 2,3-dihydroxynaphtahlene (DN), 2,3,5-triphenyl-2H-tetrazolium chloride (TTC), water and chloroform was studied in detail. When the vanadium is in the oxidation state of IV, the extracted species are aggregates containing three 1:2:1 (V:DN:TTC) ion-pair units composed of triphenyltetrazolium cations (TT+) and chelate anions {[VIVO(DN)(DNH)]− (I) and/or [VIV(OH)(DN)2]− (II)}. When the initial oxidation state of vanadium is V and the DN concentration is high, vanadium(V) is reduced by DN to a lower oxidation state, V(IV). However, at low DN concentration, vanadium(V) can enter the organic phase as a part of an ion-pair consisting of TT+ and [VVO2(DN)]− (III). The ground-state equilibrium geometries of the anions I, II, and III were optimized by quantum chemical calculations using BLYP/6-31++G⋆. The following characteristics were determined under the optimum conditions for VIV extraction: absorption maximum λmax = 333 nm, molar absorptivity ε333= 2.1x104 dm3 mol−1 cm−1, Sandell’s sensitivity SS = 2.4 ng cm−2, and fraction extracted E = 98%. The conditional extraction constant was calculated by two independent methods. The calibration graph was linear in the range 0.1-3.1 μg cm−3 (R2=0.9994) and the limit of detection was 0.03 μg cm−3.