K. Gavazov, V. Lekova, A. Dimitrov 217 Journal of the University of Chemical Technology and Metallurgy, 41, 2, 2006, 217-220 EXTRACTION STUDIES ON THE SYSTEM VANADIUM(V) – 4-(2-THIAZOLYLAZO)- RESORCINOL – NEOTETRAZOLIUM CHLORIDE – WATER – CHLOROFORM K. Gavazov, V. Lekova, A. Dimitrov Department of General and Inorganic Chemistry, Plovdiv University “P. Hilendarski” 24 Tsar Assen Str., 4000 Plovdiv, Bulgaria, E-mail: kgavazov@abv.bg ABSTRACT The system vanadium(V) 4-(2-thiazolylazo)-resorcinol (TAR)  neotetrazolium chloride (NTC)  water  chloro- form has been studied by using an extraction-spectrophotometric method. The optimum conditions for extraction of vanadium have been found: pH, concentration of reagents, extraction time. Molar absorptivity calculated using the Beers law is ε 555 = 2.39x10 4 l mol -1 cm -1 . Under the optimum extraction conditions the dominating vanadium species is estab- lished to be (NT) 3 [VO 2 (TAR) 2 ] 2 . The following constants have been calculated: distribution constant (LogK D = 1.21), association constant (Logβ = 7.0), extraction constant (LogK ex = 8.2), recovery factor (R = 93.78 %). The effect of foreign ions on the extraction has been studied as well. Keywords: vanadium(V), 4-(2-thiazolylazo)-resorcinol, neotetrazolium chloride, extraction-spectrophotometry. INTRODUCTION Vanadium(V) forms intensively coloured, easily extracted into organic solvents ternary complexes with azoderivatives of resorcinol and organic bases [1-16]. Of interest are the ion-association complexes between vanadium(V)-azoderivative of resorcinol anionic che- lates and tetrazolium cations [9-14] which are appro- priate for extraction-spectrophotometric determination of vanadium in complex matrices. The present paper aims at studying the system V(V) - 4-(2-thiazolylazo)- resorcinol (TAR) - 2,2;5,5-tetraphentyl-3,3-(p-biphe- nyl) - ditetrazolium chloride (neotetrazolium chloride, NTC) - water - organic solvent. Preliminary investiga- tions indicate that a stable ternary complex is formed and vanadium is extracted for a short time into chloro- form with recovery factor R > 90%. EXPERIMENTAL Reagents and Apparatus A standard V(V) solution with a concentration of 2x10 4 mol l -1 was prepared by dissolving of NH 4 VO 3 (Apolda). TAR (Aldrich Chem. Co) and NTC (Fluka) aqueous solutions were prepared with concentrations of 2x10 3 mol l -1 . The other reagents were CH 3 COOH (0.1 mol l -1 ), CH 3 COONa (0.1 mol l -1 ), chloroform and solutions of diverse ions and reagents. All reagents used were of analytical grade. A Specol  11 spectrophotometer (Carl Zeiss, Germany) supplied with 0.5 cm cells was employed for reading the absorbance. Procedure Aliquots of vanadium(V) solution, buffer solu- tion, TAR solution and NTC solution were introduced into 125 ml separatory funnels. The resulting solutions were diluted with distilled water to a total volume of 10 ml. Then 10 ml of organic solvent was added and the funnels were shaken for 2 min. A portion of the organic layer was filtered through a filter paper into a cell and the absorbance was read against a blank. RESULTS AND DISCUSSION Optimum extraction-spectrophotometric conditions Fig. 1 shows the absorbance spectra of the ternary com- plex and of the blank at different pH values in chloro- Received 16 November 2005 Accepted 10 January 2006