1. INTRODUCTION:

Vanadium is widely distributed in the earth’s crust. It occurs naturally in the form of about 70 minerals, but does not occur as metallic vanadium. In its compounds, it forms different oxidation states, the most common being +3, +4, and +5. Very little is known about the biological function of vanadium. It is most commonly found in the +4 and +5 oxidation state in the form of vanadyl (VO2+) and vanadate (VO3^-) respectively.

Spectrophotometry is the most common technique used for vanadium determinations [1–3] owing to the high sensitivity and selectivity achieved in these reactions.

Vanadium is an essential trace element for plants and animals which acts in the synthesis of chlorophyll and promotes the growth of young animals. Vanadium compounds are toxic in high concentrations or after long periods of exposure [4,5]. There are also vanadium compounds that exhibit chemotherapeutic effects in the treatment of leukemia1 and recent studies showed promising application in management of diabetes [6-9]. Vanadium in trace amounts is an essential element for cell growth at μg cm⁻¹levels, also has been shown to inhibit cholesterol synthesis and to increase the oxidation of fatty acids of higher concentrations. It is excreted through urine.

The already developed methods for the determination of vanadium with tannic acid require either extraction or use of other reagents or heating [10-12]. Hence these all methods are time consuming.

In recent years V-speciation studies have focused on the determination of V in natural waters, (Bosque-Sendra et al., 1998; DuPont et al., 1991; Miura, 1990; Yamane et al., 1998) biological systems (Elvingson et al., 1997; Hirayama et al., 1992; Kawakubo etal., 1995) and in industrial processes (De Beer and Coetzee, 1994; Murthy et al., 1989). A literature survey shows that most published analytical methods focus on the determination of total V or the determination of one species at a time. These methods include high-performance liquid chromatography (De Beer and Coetzee, 1994;Komarova et al., 1991; Miura, 1990; Miura et al., 1990),spectrophotometry (Balaji et al., 1998; Bosque-Sendra et al., 1998;Chauhan and Kakkar, 1992; Iranpoor et al., 1992; Kawakubo et al.,1995; Murthy et al., 1989; Shah et al., 1991; Zucchi et al., 1998), flow injection (Grudpan and Nacapricha, 1991; Taylor et al., 1996;Yamane et al., 1998), atomic absorption spectrometry(Chakrabortyand Das, 1994; Frankenberger et al., 1991; Yaman and Gucer, 1994),atomic emission spectrometery (DuPont et al., 1991; Hirayama et al.,1992), colorimetric (Serrat and Morell, 1994), and electrochemical methods (Ensafi and Naderi, 1997; Sander and Henze, 1996).No general accepted or standard method for the simultaneous determination of vanadium species is currently available. This is particularly true for industrial samples with complex matrices and environmental samples with low concentrations of VO₃^-.

The aim of this work was to develop a simple, fast, cost effective, and interference-free method for the determination of VO₃^-that would be useful in a routine industrial or water quality laboratory or pharmaceutical laboratory.

2. EXPERIMENTAL:

2.1 Apparatus:

All glassware’s used for experimental purpose were made up of Pyrex or corning glass. The burette, pipette and standard flasks were calibrated by the method described by Vogel [13].

2.2 Instruments:

pH meter: A digital pH meter, was used for pH measurements The pH meter was calibrated by employing the buffer solutions of pH 4.0, 7.0 and 9.2.

Spectrophotometer: The absorption measurements were carried out on a Boush and Lomb spectronic -20, using 1cm matched glass cell. The spectrophotometer is calibrated by measuring the absorption spectra of potassium chromate in KOH solution and that of potassium permanganate in sulphuric acid solution [14].

2.3 Preparation of experimental solution:

The stock solution of metavanadate (1000 ppm) was prepared by dissolving 0.116g of ammonium metavanadate in 100 cm³ of distilled water and standardized volumetrically. [15]

2.4 Tannic acid solution was prepared by dissolving weighed amount in alcohol and then diluting up to the mark.

2.5 Preparations of foreign ion solution:

The solutions of cations and anions were prepared by dissolving their A.R. grade salts in distilled water or dilute acids as required. The solutions of various anions were prepared by dissolving their A.R. grade sodium, potassium or ammonium salts in distilled water.

2.6 Procedure for spectrophotometric determination of metavanadates using tannic acid as a reagent.

To 1 cm³ of an aqueous solution containing 100 ug of VO3^-1, 1 cm³ of 0.5% tannic acid was added. The pH of the solution was between 4.5-6.00, the final volume was made unto 10 cm³ with distilled water. The amount of metavanadate was determined from a calibration curve prepared by processing solutions containing known amounts of metavanadate through recommended procedure and plotting the absorbance against the concentration of VO3^-1.

2.7 Absorption spectra:

To determine absorption spectra, 1.017x10^-3M VO3^-1solution was used.

2.8 Effect of reagent concentration:

The effect of reagent concentration was studied by varying the reagent concentration from 0.2% -2% and keeping all the parameters constant. From the value of absorbance the minimum concentration of reagent required for the analysis of metavanadate was determined.

2.9 Stability of the complex:

The stability of the color of the complex was determined by measuring the absorbance of the complex at different time intervals at 585 nm.

2.10 Calibration curve:

A series of solutions containing known amounts of VO3^-1 treated as per procedure developed for extractive spectrphotometric determination of VO3^-. The calibration curve was prepared by plotting absorbance versus concentration of VO3^-1 in ppm. The calibration curve obtained was employed for determination of VO3^-1in unknown samples.

2.11 Precision and accuracy:

Precision and accuracy of the method developed were determined by carrying out 10 replicate analysis of the 10 cm³ of solutions containing 50 ug of VO3^-1.

The average of 10 replicate determinations was taken to calculate standard deviation, variance, and variation from mean at 95% confidence limit.

2.12 Procedure for Job’s continuous variation method:

1.017x10^-3M VO3^-1solution was prepared. 1cm³ of same molar tannic acid was mixed. Series of solutions containing 0.5-4.0 cm³ of VO3^-1 and 4.5-1.0 cm³ of reagent were taken. The total volume of each mixture was made up to 10 cm³ with distilled water. These mixtures were treated as per the procedure described for spectrophotometric determination of VO3^-1and their absorbance were recorded at 585 nm. The absorbance was plotted against the mole fraction.

2.13 Procedure for mole ratio method:

A series of solutions were prepared by mixing 1 cm³ of 1.017x10^-3M VO3^-1 and 0.5-4.0 cm³ of same molar reagent. Each mixture was treated according to the procedure developed and the absorbance of these solutions was measured against reagent blank at 585 nm. Absorbance obtained was plotted against the mole ratio of the concentration of reagent to VO3^-1.

2.14 Interference study:

The effect of diverse ions on determination of 50 ug VO3^-1 was studied by mixing a definite amount of the desired foreign ions with the VO3^-1 solutions and then treated as per the developed procedure. The tolerance limit was taken as the amount of ion causing an error of not more than +-2% in the absorbance values of VO3^-1at 5 μg mL⁻¹. These results revealed that various cations and anions can be tolerated at satisfactory levels .However Mn²⁺, Zr²⁺, Sr³⁺,Al³⁺,Ba²⁺,Ce²⁺ interfered seriously. However, the interference of some of these ions could be masked by using EDTA (1000 μg cm⁻¹) up to 25 μg cm⁻¹.Table

2.15 Determination of VO3^- in the synthetic mixtures and real samples:

A number of synthetic mixtures of vanadates and other commonly associated anions like were prepared and treated as per the developed procedure. Environmental water samples and urine were spiked with known amounts of vanadates (VO₃^-) and analyzed by the proposed general procedure.

Each filtered environmental water sample was analyzed for vanadate.

They tested negative. To these samples known amount of vanadate (VO₃^-)was added and then analyzed by the proposed procedure.

Determination of vanadates in urine:

About 50 cm³ of the urine sample was concentrated to 5 cm³ by evaporation. To this solution was spiked a known amount of vanadate and mixed with 5 cm³ of conc. HNO3 and 5 g of potassium sulfate and heated to dryness. The process was repeated 2–3 times and then HNO3 (1:3, 25 cm³) was added to the residue and digested on a water bath for 30 min [16]. The contents were again evaporated to dryness, cooled and the residue was dissolved in water, filtered, and neutralized with dilute ammonia. The mixture was diluted to a known volume with water. Appropriate aliquots of this solution were taken and the proposed general procedure was followed for the vanadate determination. They tested negative. To these samples known amount of vanadate was added and then analyzed by the proposed procedure.

Determination of vanadates in pharmaceutical samples:

A volume of 10 cm³ of neogadine elixir sample was treated with 10 cm³ of conc. HNO3, the mixture was then evaporated to dryness. The residue was leached with 5 cm³ of 0.5M H2SO4. The solution was diluted to a known volume with water, after neutralizing with dilute ammonia. An aliquot of the made up solution was analyzed by the present method for vanadate determination.

Table 1 Analytical parameters pertaining to the proposed method.

Molar absorptivity	7070 L mol ^–1 cm ^–1
Sandell’s sensitivity	0.01498 5 ug cm^–1
Beer’s law range	0.2 –8.0 ug/cm-¹
Regression coefficient of VO3^- on absorbance =	17.106
Regression coefficient of absorbance on VO₃^- =	0.058
Correlation coefficient =	0.996

3. RESULTS AND DISCUSSION:

3.1 Absorption spectra:

The proposed method involved the formation of a blue color between Vanadate and tannic acid in a medium of pH 4.5-6.0. The absorption spectra of solution containing blue colored complex against the reagent blank, and that of tannic acid solution against corresponding buffer blank is shown in the Fig. 1. The figure revealed that the complex has maximum absorbance at 585 nm. The reagent has no appreciable absorbance at specified wavelengths. Hence further studies were carried out at 585 nm. The color reaction was instantaneous and the absorbance of the complex solution was found to remain constant for at least for six hours.

Fig 1. Absorption spectra (series1=Complex, series2=reagent)

3.2. Effect of pH:

The effect of pH on the intensity of the color reaction is shown in the Fig. 2. The absorbance was found to be maximum in the pH range 4.0-6.0. Hence further analytical investigations were carried out in media of pH 5.5.

Fig 2.

3.3 Effect of reagent tannic acid concentration and stability of the complex:

The studies of effect of various concentrations of the reagent on the color reaction reveal that, quantitative analysis is obtained with concentration of 0.5% and above. (Fig 3.)However it was found that the presence of excess of the reagent solution does not alter the absorbance of the color reaction. Hence 1 ml of 0.5% tannic acid solution was employed throughout the experimental work. The complex was found to be stable fot six hours.

Fig 3.

Fig 4. Calibration curve

3.4. Calibration curve:

The adherence to Beer’s law was studied by measuring the absorbance value of the series of solutions containing different concentrations of the metal ion. A linear calibration graph drawn between absorbance and the metal ion concentration indicates that VO3- may be determined in the range 0.02- 8.0ug/ml. The calibration graph is shown in the Fig. 4.

3.5. Stoichiometry of the VO3^- -Tannic acid complex

The stoichiometry of the complex was determined by Job’s method of Continuous variation and by mole ratio method. It was found to be 1:1(Metal : Ligand). The plots are shown in Figure 5 and 6 respectively.

3.6 Precision and accuracy:

The precision and accuracy of the developed method for the determination of vanadate was tested by analyzing the 10 cm3of solutions containing 50 ug of VO₃^-. The average of 10 replicate analyses was 49.8 which vary between 49.8 ±0.738 at 95% confidence limit. The standard deviation was 1.032 and the variance was 1.065.

\In conclusion the analytical parameters pertaining to the proposed method are given in the table 1

3.7 Effect of diverse ions:

The effect of interfering ions on the proposed method was investigated. The tolerance limit was considered to be the amount that caused a + -2% deviation in the absorbance value. The results are summarized in Table 2. These results reveal that various cations and anions can be tolerated at satisfactory levels.

Table 3

Synthetic mixtures(10 cm³)	Amount ^afound (ug/ml)	% Recovery
VO3^-(50Ug)+SO3^-(100ug)	51.00 ± 0.57	102
VO3^-(50Ug)+CH3COO^-(100ug)	48.2 ± 0.68	96.4
VO3^-(50Ug)+WO4^-(100ug)	49.0 ± 0.48	98.0
^bVO3^-(50Ug)+IO4^-(100ug)	50.70 ± 0.41	101.4

a^aMean ± standard deviation (n=5)

Fig 5. Job’s continuous variation method

Table 4

Sample	Vanadate added	Proposed method			Reference method ¹⁷
Sample	Vanadate added	^aFound ±SD	% Recovery		^aFound ±SD	% Recovery
a Natural water i)Juhu, ii)alibag, b Urine sample c Pharmaceutical sample	2.0 4.0 2.0 ---	1.97±0.98 3.97±0.83 1.96±0.16 1.81±0.31		98.50 99.25 98.00 98.36	1.98±0.04 3.98±0.02 1.96±0.02 ------	99.00 99.50 98.00 ------

a Mean ± S.D. (n = 5). ; b Gave no test for vanadium.

c Neogadine Elixir®, Raptakos Brett&Co. Ltd., India (each 10 ml contains Iodised peptone 19.33 mg, magnesium chloride 13.33 mg, magnesium sulfate 2.66 mg, sodium metavanadate 0.44 mg, zinc sulfate 4 mg, pyridomine HCl 0. 5 mg,cyanocobalamin 0.33 μg, nicotinamide 6.66 mg, alcohol (95%) 0.63 ml, total alcohol 6% v/v), vanadium taken 1.84μg.

Fig: 6. Mole ratio method

3.8 Analytical Applications:

The proposed method under the already established optimum conditions was applied for the determination of VO3- in Synthetic samples, environmental water samples and pharmaceutical products. The results presented in Table 3 indicate the successful applicability of the proposed method to real sample analysis.

Separation of Vanadates in presence of periodates:

^bA synthetic mixture of vanadate(50ug) and periodate (100 ug) was treated as per the procedure developed where 10ml of 0.5% thiourea was used as masking agent for periodate.

4. CONCLUSIONS:

The proposed method for the determination of vanadium is simple, rapid, sensitive and has the advantage of enabling a wide range of determination without the need for extraction or heating. The satisfactory applicability of the proposed procedure to the determination of vanadates in various samples shows the utility of the method. The method can be used in the form of kit in pathological laboratories for the determination of metavanadates, when used as antidiabetic agents.

Although many sophisticated techniques; ICP-MS, AAS, etc. are available for the determination of vanadates at trace levels, the factors such as the low cost of the instrument, ease of handling, lack of need for consumables, and almost no maintenance have caused spectrophotometry to remain a popular and inevitable technique, particularly in the laboratories of developing countries with a limited budget

5. ACKNOWLEDGEMENT:

The author is thanking to the Mumbai University for financial assistance and sincere thanks to the principal, Dr.(Mrs.) M.K.Pejaver, B.N.Bandodkar College of science for providing the necessary facilities to carry out the present work.

6. REFERENCES:

1. Pyrzynska K ,Recent developments in spectrophotometric methods for determination of vanadium. ,Mikrochim Acta 149: 159(2005).

2. Taylor M J C, Staden J F V Spectrophotometric determination of Vanadium (IV) and vanadium (V) in each other's presence. Review. Analyst 119: 1263 ,(1994).

3. Valero J .Determination Fotometrica de Vanadio Mediante Sistemas Ternarios. Bol Soc Quim Peru 57: 23, (1991).

4. Welz B, Sperling M .Atomic Absorption Spectrometry, 3rd edition, Wiley VCH; Weinheim, (1999)

5. Sabbioni E, Kueera J, Pietra R, Vesterberg .A critical review on normal concentrations of vanadium in human blood, serum, and urine Sci. Total Environ.188: 49, (1996)

6. Barceloux D G ,Vanadium. Clin. Toxicol. 37: 265, (1999)

7. Tsiani E, Fantus I G ,Vanadium compounds: biological actions and potential as pharmacological agents. Trends Endocrinol. Metab. 8: 51,(1997).

8. Badmaev M D V, Prakash S, Majeed M ,Vanadium: A review of its potential role in the fight against diabetes. J Altern Compl Med 5: 273, (1999)

9. Poucheret P, Verma S, Grynpas M D, McNeill J H ,Vanadium and diabetes. Mol Cell Biochem. 188: 7, (1998).

10. F. Bosch Serrat and G. Bosch Morell, Colorimetric method for the determination of vanadium with tannic acid in water and oils. Fresenius' journal of analytical chemistry 349, 717-721. (1994)

11. Abdel-Gawad, F. M.; Issa, R. M. Spectrophotometric determination of vanadium (V) with tannic acid and its application to biological fluids. Egyptian Journal of Pharmaceutical Sciences, 28(1-4), 137-47, (1987),

12. Banerjee, Samaresh; Sinha, B. P.; Dutta, R. K. Spectrophotometric determination of vanadium and application to gas-turbine fuel-oils. Talanta 22(8), 689-91, (1975),

13. A.I. Vogel , Text book of quantitative Inorganic analysisi,4^th Edn; ELBS and Longmann, London, (1978).

14. Sandell, E.G;. Colorimetric Determination of Traces of metals, Edn.3 Interscience New York (1965).

15. G.H. Jeffery, J.Bassett, J. Mendhame and R.C. denny,. A.I. Vogel, Text book of quantitative Inorganic analysisi,”5^th Edn; ELBS and Longmann, London, (1989).

16. Ahmed M J, Banoo .Spectrophotometric method for determination of Vanadium and its application to industrial, environmental, biological and soil samples. Talanta 48: 1085, (1999)

17. Badiadka Narayanaand Kenchaiah Sunil, Facile and Sensitive Spectrophotometric Method for the Determination of Vanadium. Eurasian J. Anal. Chem. 4(2): 141-151. 349, 717-721, (2009).

Received on 28.07.2012 Modified on 14.08.2012

Asian J. Research Chem. 5(9): September, 2012; Page 1123-1128

Anions	Tolerance limit(ppm)	cations	Tolerance limit (ppm)
SO4^2-,SCN,NO3^-, CH₃OO^-	5000	Cu³⁺,Fe²⁺, Ni²⁺	1000
SO₃^2-,S₂O₃^2-,IO3^-, BrO3^-,citrate	1000	Fe³⁺	500
S₂O₈^2-	500	Mg²⁺, Ca²⁺, Hg²⁺	100
WO₄^-,PO4^2-, EDTA	100	Cd²⁺, Zn²⁺	50
IO4^2-	50	^aCe²⁺, Ba²⁺	50