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Indian Journal of Chemistry Vol. 48A, September 2009, pp. 1228-1234 Kinetic studies on effects of EDTA and surfactants on reduction of vanadium(V) to vanadium(IV) in sulphuric acid medium A P Mishraa, *, Raju Khanb, & Ravi Ranjan Pandeyb
a Department of Science and Technology, Ministry of Science and Technology, Technology Bhavan, New Delhi 110 016, India Email: bEngineering Material Division, National Physical Laboratory, New Delhi 110 012, India Received 17 April 2009; revised and accepted 19 August 2009 Reduction kinetics of vanadium(V) to vanadium(IV) by EDTA and effects of surfactant have been studied using thiourea as a reductant at 40±1°C in acidic medium at 350 nm. UV-vis and ESR spectral techniques have been used to confirm the reduction product of vanadium(V). Electronic absorption spectra of the reaction products suggest the formation of aqua vanadium(IV) and vanadium(IV)-EDTA complex in the absence and presence of EDTA, respectively. The oxidation of thiourea by vanadium(V) has also been studied in the presence of surfactants. The anionic sodium dodecyl sulphate and non-ionic TX-100 surfactants catalyze the reaction whereas the cationic surfactant, cetyltrimethylammonium bromide, has no effect. The single electron sequence is confirmed by the formation of vanadium(IV) aqua ion. The experimentally determined intermediate complex formation constant, Kes is 34 mol-1 dm3. Keywords: Kinetics, Reaction mechanisms, Surfactants, Reductions, EDTA
IPC Code: Int. Cl.9 C07B31/00
Inclusion of vanadium in enzymes such as literature but there is no report on the reduction of bromoperoxidase1 and nitrogenase2 reveals the vanadium by EDTA. importance of its redox chemistry. A number of model complex systems have been investigated in We report herein the results of kinetic studies on order to elucidate vanadium's redox mechanisms3-5. vanadium(V) reduction by EDTA. The oxidation of thiourea has also been studied in order to find out how pharmacological effects of biologically active thiourea differs in its kinetic features from EDTA. In vanadium complexes have been reviewed6-7 addition, the effects of anionic, cationic and non-ionic The reduction of vanadium(V) to vanadium(IV) surfactants have also been reported. by different inorganic and biologically relevant reducing agents has been the subject of investigation Materials and Methods
by several researchers8-10. Ethylenediaminetetraacetic Ethylenediaminetetraacetic acid disodium salt (SD acid (EDTA) is used as an antioxidant in foods, as a Fine India, 98%), ammonium monovanadate (99%, chelating agent in pharmaceuticals, cosmeceuticals Merck Germany), H2SO4 (Merck, India, 98%), and plant food and also as an anticoagulant11. In thiourea (Merck, India, 99%) and acrylonitrile (Merck addition, the EDTA susceptibility to biodegradation is India) were used without additional purification. an important criterion for assessing its environmental Triton X-100 (SD Fine India, 99%), sodium dodecyl impact and toxicology. The inhibitory12-14, catalytic15 sulphate (Merck India, 99%) and cetyltrimethyl- and vanadate-stimulating16,17 behavior of EDTA in the ammonium bromide (BDH England, 99%) were used redox chemistry of vanadium has been reported in the deionized and CO2-free water was used as the solvent. The vanadium(V) solutions were prepared by Present address: dissolving ammonium monovanadate in the calculated Analytical Chemistry Division, North East Institute of Science & amounts in H2SO4 solution as required with Technology, Jorhat 785 006, Assam, India.

MISHRA et al.: KINETIC STUDIES ON EFFECTS OF EDTA/SURFACTANTS ON REDUCTION OF V(V) vigorous shaking. The vanadium(V) solutions were Results and Discussion
The observed rate constant values for varying concentrations of vanadium(V) (50.0×10-3-80.0 × Solutions of vanadium(V) and the reaction 10-3 mol dm-3) and fixed concentration of EDTA mixture containing the requisite amounts of EDTA, (5.0×10-3 mol dm-3) at constant H2SO4 (1.78 mol dm-3) H2SO4 were separately thermostatted (± 0.1°C ) in a have been recorded. A plot of kobs versus three-necked reaction vessel fitted with a double [vanadium(V)] was linear passing through the origin, walled spiral condenser (to arrest evaporation). The (Fig. 1) indicating first order with respect to reaction was initiated by adding the required amount [vanadium(V)]. On the other hand, the invariance of of vanadium(V) to the reaction mixture and zero rate constants over a variation of initial [EDTA] time was recorded when half of the solution had (1.0×10-3-6.0×10-3 mol dm-3) at fixed [vanadium (V)] (60×10-3 mol dm-3), [H2SO4] (1.78 mol dm-3) and temperature (50°C) is indicative of pseudo-first-order A spectronic 21-D spectrophotometer (Bauch & dependence of the reaction in [EDTA], leading Lomb) was employed for the measurement of the progress of the reaction using a cell of 1 cm path length. Vanadium(V) reduction was carried out in the presence of excess of vanadium(V) at 760 nm. The pseudo first-order rate constants (k determined from the plots of log (A versus time. Throughout the experiment, no other where T is the total concentration. species except vanadium(IV) absorbed at 760 nm. The reaction rate is found to increase with increase Reproducible results giving good first order plots were obtained for each reaction run (r ≥ 0.998). 2SO4] at constant [oxidant] and [reductant]. The plot of log k obs versus log [H2SO4] resulted in two straight line portions with slopes 2.29 and 0.72 The presence of free radical in the reaction mixture indicating the order with respect to [H was evaluated by using acrylonitrile monomer. When more than two at lower and fractional at higher acid concentrations respectively. The k (60.0×10-3 mol dm-3), EDTA (5.0×10-3 mol dm-3), obs values, initially increased and then tend toward a limiting value with acrylonitrile (20% v/v) and H2SO4 (1.78 mol dm-3) was allowed to stand for 24 h, the reaction mixture 2SO4] (Fig. 1). Further, the plot yields a curve concave in nature. became a thick white precipitate, indicating in situ generation of free radical during the oxidation of EDTA by vanadium(V). Controlled experiments without EDTA or vanadium(V) did not give such polymerization with acrylonitrile. In order to confirm the reduction product of (60.0×10-3 mol dm-3), H2SO4 (1.78 mol dm-3) and EDTA (5.0×10-3 mol dm-3) were mixed at 50°C and UV-vis spectra of reaction mixture was recorded. At the end of the reaction a sharp peak was observed at 760 nm, characteristics of the vanadium(IV) ion (d-d transition). The yellow (λmax = 365 nm) reaction mixture became blue (λmax = 760 nm) after completion 103 [V(V)] mol (dm-3)
of the reaction. Thus, vanadium(IV) ion was confirmed as the product under the experimental Fig. 1—Plots of pseudo first order rate constants (kobs) versus [vanadium(V)] and [H conditions. Carbon dioxide and formaldehyde were 2SO4] for the oxidation of EDTA by vanadium(V). [React. cond.: [EDTA] = 5.0×10-3 mol dm-3; identified by the standard methods as the other temp. = 50°C. 1, Variation of [vanadium(V)]; 2, Variation of reaction products. concentration of [H2SO4]. INDIAN J CHEM, SEC A, SEPTEMBER 2009 The plot of 1/kobs versus 1/[H2SO4] is also linear Effect of temperature was also studied within the ([H2SO4]=1.04-2.51 mol dm-3) with a positive range 35-60°C at constant [V(V)]T, [thiourea]T and intercept and slope, indicative of Michaelis-Menten [H2SO4]. Activation parameters were calculated from behavior and complex formation between the Arrhenius and Erying equations and are given reactants and H2SO4. Due to the existence of proton-dependent 2SO4] on kobs1 at constant [V(V)]T (5.0×10-3 mol dm-3), [thiourea] dependence on [H+] is complicated. The observation T (60.0×10-3 mol dm-3) and temperature (40°C) (Table 1) was studied. The is in agreement with the fact that the vanadium- EDTA reaction in aqueous H obs1 increases from 0.5×10-4 to 11.3×10-4 s-1 when catalyzed by [H+]. On protonation, the positive charge 2SO4] is increased from 0.02 to 0.58 mol dm-3 and the order is one with respect to [H on the vanadium(V)-sulphate species increases, and 2SO4]. This is in agreement with the fact that the vanadium-thiourea reaction in aqueous H vanadium(V) center. 2SO4 medium is catalyzed by [H+]. Protonation leads to the generation of more Effect of thiourea in absence of EDTA
Kinetics of the reduction of vanadium(V) by
vanadium(V), which enhances the rate of the reaction. Also, the positive charge on the vanadium 2SO4 aqueous medium was investigated. The invariance of k (V)-sulphate species increases with decrease in pH obs1, over varying initial [V(V)],
(1.0×10-3-6.0×10-3 mol dm-3) at fixed [thiourea] and facilitates the electron transfer towards the 60.0× 0-3 mol dm-3, [H vanadium (V) center. Again, the vanadium (III) state 2SO4] = 0.39 mol dm-3 and temperature = 40°C is indicative of first order becomes more favorable as a result of pH dependence dependence of the reaction in [V(V)] of the reaction: VO2 + 4H+ + 2e- → V3+ + 2H2O. T (Table 1). kobs1 increases with increase in [thiourea]T and the plot In order to further confirm the formation of aqua between kobs1 versus [thiourea]T shows that the vanadium(IV) ion, the EPR spectrum of the reaction reaction is second-order with respect to [thiourea]T mixture was recorded after completion of the reaction. (a double logarithmic plot with a slope of 1.85 with The room temperature (303 K) spectrum shows a average linear regression coefficient, γ = 0.998). distinct eight-line pattern indicating that a single vanadium (I = 7/2) is present in the molecule, i.e., it is Table 1—Pseudo-firstorder rate constants for a monomer of vanadium(IV) aqua ion. Our results are thiourea-vanadium(V) reaction at 40°C in good agreement with the observations of other investigators12. The above reaction mixture was allowed to cool down. On cooling, the yellow crystals of CC′-dithiobis (formamidinium) were obtained. The Table 2—Pseudo-firstorder rate constants (kobs1) and activation parameters for thiourea-vanadium(V) reaction. [[V(V)] = 5.0×10-3 mol dm-3; [thiourea] = 60.0×10-3 mol dm-3; [H2SO4] = 0.39 mol dm-3] obs1 × 104 (s-1) Activation parameters Ea (kJ mol-1) ∆H#(kJ mol-1) ∆S# (JK-1 mol-1) ∆G#( kJ mol-1)



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