Kinetic-Spectrophotometric Determination of Trace

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3 concentrations [4]. The toxicity of vanadium is dependent decrease in absorbance of the reaction mixture for the first on its oxidation state [5], with vanadium (V) ...
World Applied Sciences Journal 6 (5): 624-629, 2009 ISSN 1818-4952 © IDOSI Publications, 2009

Kinetic-Spectrophotometric Determination of Trace Amounts of Vanadium (V) Based on its Catalytic Effect on the Oxidation of Victoria Blue B by Potassium Bromate in Micellar Medium Mohsen Keyvanfard Faculty of Science, Islamic Azad University, Majlesi Branch, Isfahan, Iran Abstract: A new, simple, sensitive and selective catalytic spectrophotometric method was developed for the determination of trace amounts of V (V). The method is based on the catalytic effect of vanadium (V) on the oxidation of Victoria Blue B by potassium bromate at acidic and micellar medium. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of Victoria Blue B at 632 nm with a fixed-time method. The decrease in the absorbance of Victoria Blue B is proportional to the concentration of V (V) in concentration range 1-250 ng/ml, with a fixed time of 0.5-2 min from initiation of the reaction. The limit of detection is 0.42 ng/ml V (V). The relative standard deviations of 50 and 100 ng/ml V (V) were 1.9 and 2.4%, respectively. The method was applied to the determination of vanadium (V) in natural water. Key words: Vanadium (V)

Catalytic

Victoria blue B

INTRODUCTION

Potasium bromate

Micellar

toxic than vanadium (IV). Vanadium pentoxide dust and fumes are strong respiratory irritants, owing to their capacity to lessen the viability of alveolar macrophages, which play an important role in the lung defense against environmental contaminations. Vanadium is widely distributed in nature in ores, clays, hard coal, igneous rock, limestones, sandstones and fossil fuel, but not in appreciable amount in high silicon rocks. It is emitted into the the environment through the combustion of fossil fuels. A variety of methods have been used for determination of vanadium; these include fluorometry [6], gas chromatography [7], neutron activation analysis [8,9], X-ray fluorescence spectrometry [10], emission spectroscopy [11] and atomic absorption spectroscopy [12]. In this paper a rapid, selective, sensitive and simple method is described for based on the catalytic effect of V(V) on oxidation of Victoria Blue B by bromate in micellar medium. The reaction was monitored spectrophotometrically at 632 nm by measuring the decrease in absorbance of the reaction mixture for the first 0.5-2 min from initiation of the reaction.

Vanadium is a biologically essential element [1]. Its inclusion in enzymes such as bromoperoxide and nitrogenase reveals the importace of its redox chemistry. A number of model complex systems have been investigated in order to elucidate vanadium redox mecanisms. Some tunicate fish and marin animals selectivity accumulate vanadium species from the ocean. Vanadium complexes, inclouding organovanadium, compound, exist in a variety of configurations depending on their oxidation states and coordination numbers [2]. Vanadium in the hyposphere was believed to be a conservative element due to its almost uniform distribution in both oceanic and limnetic areas. However, slight seasonal variations with the depth of water might be encountered due to biological processes and/or the geochemical cycles of particulate vanadium and phosphorus [3]. Vanadium in trace amounts is an essential element for cell growth at µg dm 3 levels, but can be toxic at higher concentrations [4]. The toxicity of vanadium is dependent on its oxidation state [5], with vanadium (V) being more

Corresponding Author: Mohsen Keyvanfard, Faculty of Science, Islamic Azad University-Majlesi Branch, Isfahan, Iran

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MATERIALS AND METHODS

RESULTS AND DISCUSSION

Reagents: Doubly distilled water and analytical reagent grade chemicals were used during all of the experimental studies. Victoria Blue Solution 1.0×10 3 M was prepared by dissolving 0.0506 g of the compound (MW: 505.1) in water and Solution was diluted to the mark in a 100ml volumetric flask. Bromate stock Solution 0.1 M, was prepared by dissolving 1.67 g of potassium bromate (MW: 167) in water and diluting to 100 ml in a 100-ml volumetric flask. Standard stock V (V) solution (100 µg/ml) was prepared by dissolving 0.0179 g of V2O5 (Merck) in sulfuric acid (concentrated) and diluted to 100 ml in a 100ml volumetric flask. Stock Solution (1000 µg/ml) of interfering ions were prepared by dissolving suitable salts in water, hydrocloric acid, or sodium hydroxide solution. All glassware were cleaned with detergent solution, rinsed with tap water, soaked in diluted HNO3 solution (2%V/V), rinsed with water and dried.

Victoria Blue B undergoes a oxidation reaction with bromate in acidic medium at very slow rate. We found that this reaction rate is sharply increased by addition of trace amount of V (V). This process was monitored spectrophotometrically by measuring the decrease in absorbance of the characteristic band of Victoria Blue B (632 nm) (Fig. 2). Therefore, by measuring the decrease in absorbance of Victoria Blue B for a fixed time of 0.5-2 min initiation of the reaction, the V (V) contents in the sample can be measured. It was found that in the presence of Triton X-100 as a micellar medium, this reaction rate was sharply increased by addition of trace amount of V(V). The rate equation of the catalyzed reaction is: d[Victoria Blue B] − = Rate = dt k [V(V)][Victoria Blue B]m [BrO 3− ]n

(1)

were k is the rate constant. The terms [BrO3-], [Victoria Blue B], BrO3-can be considerd to be constant and m was found to be 1. By integration of Eq [1] and by incorporating Beer’s law, the final expression was obtained:

Appartus: Absorption spectra were recorded with a CECIL model 7500 spectrophotometer with a 1.0cm quartz cell. A model 2501 CECIL Spectrophotometer with 1.0 cm glass cuvettes was used to measure the absorbance at a fixed wavelength of 632nm.A thermostat water batch was used to keep the reaction temperature at 25ºC.

A = k [V(V)]t

Recommended Procedure: All the solutions and distilled water were kept in a thermostated water batch at 25°C for 20 min for equilibration before starting the experiment. An aliquot of the solution containing 10-2500 ng/ml V (V) was transferred into a 10-ml Volumetric flask and then 1.6ml of sulfuric acid 2 M ,1 ml Triton X-100 0.1 M and 0.6 ml 1.0×10 3 M Victoria Blue B were added to the flask. The solution was diluted to ca.8 ml with water. Then, 1 ml 0.10 M bromate was added and the solution was diluted to the mark with water. The solution was mixed and a portion of the solution was transferred to the spectrophotometer cell. The reaction was followed by measuring the decrease in absorbance of the solution against water at 632 nm for 0.5-2 min from initiation of the reaction. This signal (sample signal) was labeled as As. The same procedure was repeated without addition of V (V) solution and the signal(blank signal) was labeled as Ab. Time was measured just after the addition of last drop of bromate solution.

(2)

Where t is the reaction time. There are number of methods, such as fixed-time, initial rate, rate constant and variable time methods for measuring the catalytic species. Among these, the fixedtime method is the most conventional and simplest, involving the measurement of A at 596 nm. Figure 2 shows the relationship between A and reaction time. It was found that the rate of reaction is proportional to the

Fig. 1: Structure of Victoria Blue B

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World Appl. Sci. J., 6 (5): 624-629, 2009 DataStream - Scan 2.2

107 108 109

1.7

1.2

0.7

0.2 400

460

520

580

640

700

Wavelength [nm]

Fig. 2: Variation of the Victoria Blue B-BrO3--V(V) system with time Condition:0.32 M H2SO4; 6×10 5 M Victoria Blue B; 1.0×10 2 M BrO 3; 2.0×10 2 M Triton-X-100; 100 ng/ml vanadiumV(V); temperature, 25°C. Time for each scan after initiation of the reaction :30 S , 90 S and 180 S Table 1:

The effect of the sulfuric acid concentration on the rate of reaction was studied in the range of 0.08-0.36 M (Fig. 3). The results show that the net reaction rate increases with increasing sulfuric acid concentration up to 0.32 M and decreases at higher concentrations.This mean that the rate of uncatalyzed reaction increases with) sulfuric acid concentration (>0.32M) to a greater extent than the catalyzed reaction and the difference between the rates of catalyzed and uncatalyzed reactions ( as- Ab) diminishes at higher sulfuric acid concentrations. Therefore, a sulfuric acid concentration of 0.32M was selected for further study. Figure 4 shows the effect of the Victoria Blue B concentration on the sensitivity for the range 2×10 5 to 1.4×10 4 M . This sensitivity (net reaction rate) increases with increasing Victoria Blue B concentration up to 6×10 5M and decreases at higher concentrations. This may be due to the aggreagration of the dye at higher concentrations. Therefore, a final concentration of 6×10 5M of Victoria Blue B was selected as the optimum concentration. The effect of the Bromate concentration on the rate of reaction was studied in the range of 6.0×10-3-2.0×10 2 M (Fig. 5). The results show that the net reaction rate increases with increasing Bromate concentration up to 0.01 M and decreases at higher concentrations. Therefore, a Bromate concentration of 0.01M was selected for further study.

Surfactant tested us a potential micellar catalyst for the enhanced rate of Victoria Blue B-BrO3--V(V) reaction

Surfactant

Type

c.m.c (M)

Micellar catalysis

Trition-x-100 SDS CTAB CPC

Nonionic Anionic Cationic Cationic

3.0×10 8.1×10 1.3×10 1.2×10

Positive Positive Inert Inert

4 3 3 4

V (V) concentration. The reaction rate was monitored spectrophotometrically by measuring the decrease in absorbance of the characteristic band of Victoria Blue B at 632 nm. In many reactions, suitable micelles can affect the rate of reactions [13-22]. Nonionic micelles (such as trition-x-100), anionic micelle (sodium dodecyl sulfate, SDS) and cationic micelle (CTAB) and cetyl pyridinium chloride (CPC) were tested at concentration above their critical micelle concentration (CMC). The results are shown in Table 1. Therefore, between this micelles, Triton X-100 and sodium dodecyl sulfate have a positive effect but Triton X-100 has a more positive effect than sodium dodecyl sulfate, therefore Triton-X-100 was selected for practical purposes. Influence of Variables: In order to take full advantage of the procedure, the reagent concentrations must be optimized. The effect of acid concentration, Victoria Blue B and bromate concentration, type of surfactants and their concentration and temperature on the rate of catalyzed reaction (with V (V)) and uncatalyzed reaction (without V (V)) was studied. 626

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0.18

0.14

0.1

0.06

0.02

0.05

0.13

0.21

0.29

0.37

sulfuric acid concentration, M

Fig 3:

Influence of Sulfuric acid coccentration on the sensitivity, conditions: 100.0 ng/ml V(V), 6 ×10–5 M Victoria Blue B ; 6.0×10–3 M bromate and Triton-X-100 1.0×10–2 M at 25°C, in fixed time of 0.5–2.0 min from initiation of reaction. 0.2

0.16

0.12

0.08

0.04

0 0

5

10

15

Victoria Blue B x 10

Fig. 4:

5

Effect of Victoria Blue concentration on the sensitivity. Conditions:100.0 ng/ml V(V), 0.32 M H2 SO4 ; 6.0×10–3 M bromate and Triton-X-100 1.0×10–2 M at 25°C, in fixed time of 0.5–2.0 min from initiation of reaction 0.4 0.3 0.2 0.1 0 2

6

10

14

18

22

26

30

3

Fig. 5:

Bromate concentration x 10 , M

Effect of Bromate concentration on the reaction rate. Conditions:100.0 ng/ml V(V), 0.32 M H2 SO4 ; 6 ×105 M Victoria Blue B and Triton-X-100 1.0×10–2 M at 25°C, in fixed time of 0.5–2.0 min from initiation of reaction 0.4 0.3 0.2 0.1 0 0

0.005

0.01

0.015

Triton-X-100 concentration, M

0.02

Fig 6: Effect of Triton-X-100 concentration on the reaction rate. Conditions:100.0 ng/ml V(V), 0.32 M H2 SO4; 6 ×10–5 M Victoria Blue B;1.0×10–2 M BrO3 at 25°C, in fixed time of 0.5–2.0 min from initiation of reaction 627

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absorbance for the sample reaction for 0.5-2.0 min from initiation of the reaction (catalytic reaction) and C is V (V) concentration in ng/ml. The limit of detection from YLOD =Yb + 3 Sb is 0.42 ng/ml, where, YLOD is signal for limit of detection, Yb is average blank signal (n=10) and Sb is standard deviation of blank signal (n=10, uncatalyzed reaction). The relative standard deviation for six replicate determination of 50 and 100ng/ml V (V) was 1.9 and 2.4% respectively.

Table 2: Effect of foreign ions on the determination of 100 ng/ml V(V) Tolerance Limit(wion/wV(V))

Species Na+ ,K+,Ca2+, Mg2+, Rb+, Zn(II),Ba+2, Cu(II), Te(IV) Se(IV), C2O4 2, S2O82-, HSO4-, ClO3-,CO32-,NO3-Tatarate, Borate,Cl-,Ru(III) ,Pb(II), Mn(II),Rh(III) Pd(II), Ag+ Co(II) Ni(II) I-

1000

800 400 5

Table 3: Determination of V(V) added to water samples Sample

V(V) added (ng/ml)

V(V) found (ng/ml)

River water

-

Less than detection limit 47 53 48

River water 50.0 Drinking water 50 Drinking water 50 + Ni+2 (20µg/ml)

Recovery (%) -

94 106 96

Interference Study: In order to assess the application of the proposed method to synthetic samples, the efffect of various ions on the determination of 100.0 ng/ml V(V) was studied. The tolerance limit was defined as the concentration of an added ions causing a relative error less than 3% and the results are summarized in Table 1. Many ions did not interfere, even when they were present in 800 fold excess over V(V). The results show that method is relatively selective for vanadium(V) determination.

RSDì (%) n=5 -

2.2 2.8 2.6

DeviationìRSD=Relative Standard

Sample Analysis: In order to evaluate the applicability of the proposed method, water samples and synthetic water, samples were analyzed to determine V (V) contents. The results are presented in Table 2. A good recovery of sample with precise results showed good reproducibility and accuracy of the method.

The effect of the Triton-X-100 concentration on the rate of reaction was studied in the range of 0-2.0×10 2 M (Fig. 6). This sensitivity increases with increasing TritonX-100 concentration up to 2.0×10 2 M and decreases at higher concentrations. Therefore a final concentration of 2.0×10 2 M was selected as the optimum concentration of Triton-X-100 for further study. The effect of the temperature on the sensitivity was studied in the range 20-45°C with the optimum of the reagents concentrations. The results showed that, as the temerature increases up to 25°C, the net reaction rate increases, whereas higher temperature values decrease the sensitivity ( A= As- Ab).This means that the rate of uncatalyzed reaction increases with temperature to a greater extent and the uncatalyzed reaction occurred at a suitable rate. Therefore, 25°C was selected for further study.

CONCLUSION The catalytic-spectrophotometric method developed for the determination of V (V) is inexpensive, uses readily available reagents, allows rapid determination at low operating costs and shows simplicity, adequate selectivity, low limit of detection and good precision and accuracy compared to other catalytic procedures. With this method, it is possible to determine Vanadium (V) at levels as low as 0.42 ng/ml without the need for any preconcentration step.

Calibreation Graph, Precision and Limit of Detection: Calibration graphs were obtained using the fixed-time method. This method was applied to the change in absorbance over an interval of 0.5-2 min from intiation of the reaction because it provided the best regresion and sensitivity. Under the optimum conditions described above, a linear calibration range 1-250.0 ng/ml of V (V) was obtained. The equation of the calibration graph is A=0.216+ 1.5×10 3 C (n=6, r =0.9996), where A is change in

ACKNOWLEDGMENTS The author are Thankful to the Islamic Azad University-Majlesi Branch for the support of this work. REFERENCES 1.

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14. Ensafi, A.A. and M. Keyvanfard, 2002. KineticSpectrophotometric Determination of Tellurium(IV) By Its Catalytic Effect On The Reduction of Thionin By Sodium Sulfide In Cationic Micellar Medium. Intl. J. Environ. Analytical Chem., 83(5): 397-404. 15. Ensafi, A.A. and M. Keyvanfard, 2003. Kinetic Spectrophotometric Method for the Determination of Rhodium by Its Catalytic Effect on the Oxidation of O-Toluidine Blue by Periodate in Micellar Media. J. Analytical. Chem., 58(11): 1060-1064. 16. Keyvanfard, M. and A. Sharifian, 2006. Kinetic Spectrophotometric Method for the determination of Selenium(IV) by Its Catalytic Effect on the Reduction of Spadns by sulphide in Micellar media. J. Analytical Chem., 61(6): 646-650. 17. Keyvanfard, M., 2007. Kinetic-CatalyticSpectrophotometric Determination of Trace Amounts of Tellurium(IV) by its Catalytic Effect on the Reduction of Brilliant Cresyl Blue with Sodium Sulfide in Cationic Micellar Medium. Asian J. Chem., 19 (1): 435-442. 18. Keyvanfard, M., 2007. Spectrophotometric Reaction Rate Method for the Determination of Ultra Trace Amounts of Vanadium(V) by its Catalytic Effect on the Oxidation of Naphthol Green B by Bromate in Micellar Medium. Asian J. Chem., 19(4): 2729-2736. 19. Keyvanfard, M., 2007. Spectrophotometric Reaction Rate Method for the Determination of Ultra Trace Amounts of Rhodium by its Catalytic Effect on the Oxidation of Thionine by Metaperiodate in Micellar Media. Asian J. Chem., 19(3): 4977-4984 20. Keyvanfard, M., 2007. Kinetic-CatalyticSpectrophotometric Determination of Ultra Trace Amounts of Rhodium(III) by Its Catalytic Effect on The Oxidation of SPADNS by Metaperiodate in Micellar Media. Asian J. Chem., 19(6): 4777-4784. 21. Keyvanfard, M., 2007. Kinetic CatalyticSpectrophotometric Determination of Trace Amount of Iodide. Asian J. Chem., 19(7): 5220-5228. 22. Keyvanfard, M., 2009. Kinetic-Catalytic Spectrophotometric Determination of Ruthenium(III) Using the Oxidation of Neutral Red by Metaperiodate in Cationic Micellar Medium. Asian J. Chem., 21(2): 989-996.

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