Oct 16, 2012 - antagonistic relationship with copper; hence it's excess or deficiency ... ion, per chlorate or bromated on the electro reduction of molybdenum ...
International Journal of Chemical and Analytical Science ISSN: 0976-1206 Short Communication www.ijcas.info
Adsorptive Stripping Voltammetric Determination of Molybdenum in Medicinal Leaves and Soil Samples N.V.V.Jyothi*, B.Natesh kumar, S. Venkateswarlu, T. Balaji Department of Chemistry, S.V.University, Tirupati, A.P., India The determination of ultratrace concentrations of molybdenum (VI) by adsorptive cathodic stripping voltammetry was described. The method involves a controlled preconcentration of the element by interfacial accumulation as molybdenum-2, 5dichloro-3, 6-dihydroxy-1, 4-benzoquinone (Chloranilic acid) complex on the electrode followed by a cathodic stripping voltammetric measurement. The optimum analytical conditions for the measurement of molybdenum by this method include the use of HCl at pH 2.7, 1×10-3mol/l chloranilic acid, adsorption time 3min., accumulation potential of -0.20 to 1.0 v (Vs Ag/AgCl) and a rotating electrode at 3000rpm. Under these conditions, the linear concentration range and lowest detectable concentration obtained with a 3 min. accumulation were 0-200µg/L (R2=0.998) and 0.4µg/L (Relative standard deviation (RSD) =12.0%) respectively. The presence of most other metal ions does not interfere with the molybdenum determination, except for Pb (II) Cd (II) which were subsequently masked by addition of 3 µM EDTA. The interference of surface active substance such as triton-x100 was overcome by UV irradiation of the sample. The use of the adsorptive stripping voltammetric technique, after decomposition by wet digestion and UV treatment, successfully demonstrates that the determination of ultra trace molybdenum in medicinal leaves and soil samples is possible. Keywords: Adsorptive stripping voltammetry, Molybdenum, Chloranilic acid.
INTRODUTION
MATERIALS AND METHODS
Molybdenum is an essential component of various enzymes involved in plant and animal metabolism. It has an antagonistic relationship with copper; hence it’s excess or deficiency can disrupt biological functions [1]. The main sources of introduction of molybdenum into the environment and biological systems are from industrial smoke and mining activities [2]. The accurate determination of molybdenum is important, because helps us to understand its status and fate in the environment, as well as in establishing pollution control measures.
Sampling and sample pre-treatment:
Several electrochemical methods for the determination of molybdenum based on catalytic effect of nitrate, hydrogen ion, per chlorate or bromated on the electro reduction of molybdenum are well known [3-12]. The approach of catalytic adsorptive stripping voltammetry has recently been explored for the direct determination of molybdenum using complexing agents such as mandelic acid [10-11] and its derivatives 3-methoxy-4-hydroxy mandelic acid [12].Current methods for determination of molybdenum are exclusively on the hanging mercury drop electrode (HMDE) [10-13], nano molar amounts of molybdenum in various samples can be successfully determined. Pico molar detection limits have been achieved after a few minutes of adsorptive accumulation. This article describes a method for the determination of molybdenum in biological and environmental materials by adsorptive cathodic stripping voltammetry (CSV) on a HMDE. The method involves a controlled pre concentration of chloranilic acid complex of molybdenum by interfacial scan, which enables stripping of the complex. Factors that affect the reliability of this method, such as concentration and pH of supporting electrolyte, concentration of the complexing agent, accumulation potential and period, inorganic and organic interferences, linear concentration range of metal and detection limit, are described. The successful applications of this method to detection concentrations of molybdenum in wet biological and environmental material are also discussed.
Sampling has been carefully carried out to avoid elemental loss and contamination [14]. The medicinal leaves, leaf vegetables and soil samples were collected 2-5 Kilometers away from the regular automobile road connecting Tirupati and Tirumala. The soil was alluvial type. About 1 kg of the medicinal leaves/soil/leaf vegetables was taken into a pre cleaned polyethylene bags which were then sealed. The bags were treated inside and outside with 1:10 hydrochloric acid for 24 hrs and rinsed with pure triple distilled water. The leaves/soil/leaf vegetables were air-dried for 24 hrs at 600 C, the homogenized subsamples were mixed in a plastic dish and transferred into precleaned bags and stored in the darkness. About 1g of finely powdered sample (leaf/soil/vegetable) was digested under pressure in PTFE vessels with 2 ml of HNO3 (65%) in a high pressure ashers (HPA, kurner, Rosenheim, Germany) at 1800 C for 3hrs. the analyte solution, resulting from the digestion, was made up to 10 ml with triple distilled water. Apparatus and Reagents: Voltammetric measurements were carried out with a Metrohm 757-VA computrace, employing a conventional three-electrode-cell; it consists of a working electrode (hanging mercury drop), a reference electrode (saturated calomel electrode) and an auxiliary electrode (platinum wire). The pH measurements were carried out with a metrohm-623pH meter. The temperature of the voltammetric cell was kept at 20.0 ± 0.5c and desecrated with pure nitrogen for min. prior to measurements. All reagents were Merck analytical grade. Aqueous stock solution (1000mg/l) Mo was diluted with triple distilled water for obtaining intermediate concentration solutions. The Teflon voltammetric cell was rinsed with supra pure concentrated nitric acid to prevent any contamination prior to use.
Corresponding Author: N.V.V. Jyothi, Department of Chemistry, Electro analytical lab, S.V.University, Tirupati, A.P., India; e-mail: Received 26-08-2012; Accepted 16-10-2012 October, 2012
International Journal of Chemical and Analytical Science, 2012, 3(10), 1587-1589
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Jyothi et al.: Adsorptive Stripping Voltammetric Etermination of Molybdenum in Medicinal Leaves and Soil Samples
Volumetric measurements:
mol l-1 was found to be ideal. The parameters of the differential pulse mode of the volumetric determination were investigated; best results were obtained with pulse amplitude of -25mV and a scan rate of 60-80 mV/s. the dependence of the peak height on the pulse amplitude is plotted in figure 3. An accumulation potential of +50 mV was chosen. This potential ensures the selective adsorption of the chloranilic acid complexes, while most surface active compounds in the sample do not absorb at this positive potential. The peaks potential under the chosen conditions are -610mV for molybdenum. Calibration plot was recorded under optimized conditions as described above. The calibration plot, the standard deviation of the replicate was shown in figure 4. The linearity maintains up to 50 µg/l, the accumulation time (at +50mV) was 15s. It is understood that, the linear concentration range is dependent on the duration of enrichment. The 3σ-detection limits were calculated using an accumulation time of 60s and 0.07µg/l for molybdenum (VI).
An aliquot of the sample solution, containing 10-100 µg/ml of Mo(VI), was transferred into an electrolytic cell, the supporting electrolyte was HCl at pH 2.7, the concentration of complexing agent (chloranilic acid) used was 1x10-3 mol/l. the optimum experimental conditions for Mo(VI) determination by DPAdSV are at adsorptive time, scan rate of 10m V/s and pulse amplitude of 50mV. The peak potential under these conditions was at -0.62V. The stripping peaks for Mo(VI) were registered; the magnitude of the peak current was used as a measure of Mo(VI) concentration.
RESULTS AND DISCUSSION Compound 1 (Fig. 1) was isolated as a white amorphous solid In this study, an ultra trace concentration of Mo (VI) by adsorptive cathodic stripping voltammetry was described. Prior to the analysis of standard reference materials, a preliminary study was carried out employing reference solutions. The formation of the complexes, their stability and the potentials of reduction are strongly pH dependent. The optimized pH of the supporting electrolyte was 2.7 for the determination of Mo (VI). In figure I, dependence of the peak potential and current peak height on pH is plotted. With respect to sufficient resolution and sensitivity of the peaks, a pH of 2.7 was choosen for the experiment.
Applications: The procedure was applied to environmental samples such as leaf, soil and standard reference materials. The described method was applied to soil and leaf samples collected in Tirupati, including certified reference materials. The samples were subjected to UV irradiation and wet digestion for 2 h before the determination. The results correspond to mean values of five replicate determinations. Table 1 shows the results obtained for soil and leaf samples. The values obtained for certified reference materials agree satisfactorily with certified values. Further the recovery of 98.33-99.58% for Mo was obtained with the proposed method for the soil and a leaf sample indicates its accuracy and reproducibility.
The concentration of the ligand chloranilic acid also plays an important role in the determination. As seen in figure 2, maximum peak heights are reached at a chloranilic acid concentration of 1x10-4 mol l-1. The peak decreases when additional ligand is added. The ligand concentration of 1x10-3 Table 1: Analytical results for Mo in various samples (mg/kg) S.N
Samples
Found
Added
Recovery (%)
Leaf Vegetables 1. Karivepaku
0.74
5
98.04
Kothimeera
0.60
5
99.00
0.52
5
98.60
0.64
5
99.20
Soil Samples 1. Renigunta
7.20
10
98.00
M.R.Palle
6.80
10
97.60
8.20
10
98.90
2.
Medicinal Leaves 1. Utichettu Suryakanthi
2.
2.
3. Alipiri NBS Samples 1. Tomato Leaves (NIST-SRM 1573a) Apple Leaves (NIST-SRM 1515) Soil (JRC, Ispra)
2. 3.
0.48±0.02
(0.46±0.01)*
0.096±0.01 60.0±1.0
(0.094±0.05)* (62.0±1.0)*
* Certified Values are given in the brackets.
Fig. 1. Dependence of peak heights (a) and peak potentials (b) on pH: Mo (VI) = 5 µg/l, con = 1× 10 4 mol/, tacc = 15s, Uacc = + 50V, scan rate = 14 mV/s, Uampl = -50mV.
October, 2012
International Journal of Chemical and Analytical Science, 2012,3(10), 1587-1589
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Jyothi et al.: Adsorptive Stripping Voltammetric Etermination of Molybdenum in Medicinal Leaves and Soil Samples
Fig. 3. Influence of accumulation potential (a) and differential pulse amplitude (b) on the peak heights. Mo (VI) con = 5 µg/l, pH =2.3, CAA con. = 1×10-4 mol / l, tacc = 15s, Scan rate = 14 mV / s, Uacc = + 50 V (only for b), Uampl = -50mV only for a.
CONCLUSIONS The described method enables the determination of Mo (VI) in the real matrices such as soil and leaf samples. The advantages of the method are the low detection limits and time saving aspect in comparison to the single determination with several complexing reagents. Beside, the high selectivity of chloranilic acid as complexing reagent, which already has been discussed in several publications [15-17] is advantageous for the oligo element analysis. The application of the method to soil and leaf samples has sown that it is well suited for routine analysis.
REFERENCES 1.
Fig. 2. Influence of chloranilic acid concentration on molybdenum signal. Mo(VI) = 5 µg/l, pH =2.3 tacc = 15s, Uampl = -50mV, scan rate = 14 mV/s, CAA con =0(A…), 1× 106 mol-1 B( ----),1× 10-4 mol-1 C(__),1× 10-3 mol-1D(-..-…-…-).
2.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
Fig. 4. Calibration plot of Mo (VI) = 5 µg/l, CAA Con. = 1× 10-4 mol/, tacc =15S, U acc = + 50 mV, scan rate = 14 mV/s.
15. 16. 17.
Rollinson C.L, in Comprehensive Inorganic chemistry. Vol.3, (Eds: J.C. Bailar, H.J.Emeleus, Nyholm, A.F. Trotman - Dickenson, Pergamon, Oxford 1973. Purves D., Fundamental aspects of pollution control and environmental science, Trace element contamination of the environment, Elsevier, Amsterdam, 1985, Vol.7. Alary .J, Esclassan .J, Vandaele.Y, Analyst, 1986, 1111, 593. Pardo. R, Barrado.E, Castrillejo. Y, Electro analysis, 1989, 1, 367. Prabhu V.G., Zarapkar. I.R, Das M.S., Micro Chem. Acta, 1980, II, 67. Hsieh A.K., Ong T.H.. Anal. chem.Acta, 1992, 256, 237. Hidalgo J.L., Gomez M.A., Caballero M., Cela R., Perez J.A. Bustamante, Talanta, 1988, 35, 301. Lanza P., Ferri D., Buldini P.L, Analyst, 1980, 105, 379. Magyar B., Wunderli S., Micro Chem...Aacta, 1992, 257, 293. Yokoi K., Van den Berg C.M.G.. Anal. Chem. Acta, 1992, 257, 293. Pelzer J., Scholz F., Henrion G., Heininger P., Fresenius Z. Anal. Chem.1989, 334, 331. Wang J., Lu T., Taha, Analyst, 1992, 117, 35. van den Berg C.M.G., Anal. Chem. 1985, 57, 1532. Brainina KH., Schafer H., Ivanova A., Khanina R., Anal. Chem. Acta, 1996, 330, 175. Sander S., Henze G., Fresenius, J. Anal. Chem. 1994, 349, 654. Heppler F., Sander S., Henze G., Anal. Chem. Acta, 1996, 319, 19. Sander S., Wagner W., Henze G., Anal. Chem.Acta, 1997,349, 93.
Source of support: Nil, Conflict of interest: None Declared
October, 2012
International Journal of Chemical and Analytical Science, 2012,3(10), 1587-1589
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