Chinese Journal of Chemistry, 2006, 24, 557—562 Full Paper * E-mail: hamidrezapouretedal@yahoo.com; Tel.: 0312-5220520; Fax: 0312-5225068 Received May 6, 2005; revised October 14, 2005; accepted December 5, 2005. © 2006 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Determination of Trace Amounts of Vanadium by Kinetic-Catalytic Spectrophotometric Methods POURETEDAL, H. R.* KESHAVARZ, M. H. Department of Chemistry, Malek-ashtar University of Technology, Shahinshahr, Iran Kinetic-catalytic spectrophotometric methods were proposed for the determination of trace amounts of vana- dium element as vanadium(IV) and/or V(V) ions. The vanadium(IV) as VO 2＋ ion and/or vanadium(V) as 3 VO － ion showed a catalytic effect on the kinetic reactions between a color reagent such as methylthymol blue (MTB) or SPADNS and bromate in acidic media. The rate of decrease in the absorbance of the reagent MTB at 440 nm or SPADNS at 510 nm was proportional to concentration of V(IV) and/or V(V) ions in the solution. The linear ranges for determination of vanadium were obtained in the range of 1.0—150 and 5.0—100.0 μg/L by the fixed-time and slope methods, respectively, with using MTB as reagent. In the presence of SPADNS as reagent, the calibration curves were made in the amplitude 1.0—200.0 and 5.0—150 μg/L of vanadium ion by the fixed-time and slope methods, respectively. Using fixed-time method, the limits of detection were obtained to be 0.5 and 0.7 μg/L of va- nadium in the presence of MTB and SPADNS as reagents, respectively. Detection limits of vanadium by slope method and reagents of SPADNS and MTB were obtained to be 3.5 and 3.8 μg/L of vanadium, respectively. The proposed methods were applied successfully to determination of vanadium in synthetic and real samples. Keywords kinetic-catalytic method, SPADNS, methylthymol blue, vanadium Introduction Environmental pollution due to vanadium has be- come a serious problem. Vanadium in air originates mainly from the combustion of fuel oil, especially re- sidual oils, which are known to be rich in this element. 1 Vanadium exists in the ＋2 to ＋5 oxidation states and can be toxic when present at high concentration. 2 The toxicity of vanadium depends on the oxidation state; V(V) as vanadate is more toxic than V(IV), present as a vanadyl ion. 3 The determination of vanadium is there- fore important both to assess its toxicity in environ- mental and biological samples as well as for industrial quality control. A variety of methods have been used for the deter- mination of vanadium, including colorimetry, 4-6 fluorimetry, 7,8 voltammetry, 9 ion selective electrode, 10 gas chromatography, 11 neutron activation analysis, 12,13 X-ray fluorescence spectrometry, 14 emission spectros- copy 15 and atomic absorption spectroscopy. 16 Kinetic method has progressed because of its extremely high sensitivity, selectivity and low detection limit of detec- tion. Numerous catalytic methods have been reported for determination of vanadium, 17-21 however many of these kinetic methods lack a suitable sensitivity or have a limited dynamic range. This paper presents simple, highly selective and sen- sitive methods for the determination of vanadium with- out preconcentration, based on the catalytic effect of vanadium as V(IV) or V(V) on the oxidation of methyl- thymol blue (MTB) and/or SPADNS as color reagents by bromate in the sulfuric acid media, with a very low detection limit and a wide linear range. Experimental Reagents A 1000 μg/mL stock solution of V(IV) or V(V) was prepared by using VOSO 4 •2H 2 O and NH 4 VO 3 (Merck) in water acidified with conc. H 2 SO 4 and diluted to de- sired volume. Potassium bromate solution (0.4 mol/L) was prepared by dissolving 6.6808 g of KBrO 3 (Merck) in water in a 100-mL volumetric flask, stock solution of MTB (1.0×10 －3 mol/L) was by dissolving methylthy- mol blue sodium salt (Merck), C 37 H 40 N 2 Na 4 O 13 S, in distilled water, stock solution SPADNS (1.0 × 10 － 3 mol/L) was by dissolving 1,8-dihydroxy-2-(4-sulfo- phenylazo)-3,6-naphthalenedisulfonic acid, trisodium salt (Merck), C 16 H 9 N 2 Na 3 O 11 S 3 , in distilled water, sul- furic acid solution (1 mol/L) was by diluting the appro- priate volume of concentrated acid (Merck) with water and stock solutions (1000 μg/mL) of interfering ions were prepared by dissolving appropriate amounts of their suitable salts in water. Doubly distilled water was used throughout the study. Apparatus A UV-Vis spectrophotometer (Carry 100) with 1-cm cells was used for absorption measurements at a fixed