Indian Journal of Chemical Technology Vol. 18, May 2011, pp. 183-187
Solid phase extraction for selective separation/preconcentration of copper using N-benzoyl N-phenylhydroxyl amine as sorbent modifier Nasser Dalali*, Leila Farhangi & Mehdi Hosseini Phase Separation & FIA Lab, Department of Chemistry, Faculty of Sciences, Zanjan University, Zanjan, 45195-313, Iran Received 3 May 2010; accepted 9 December 2010 A simple and relatively fast method for the selective separation and preconcentration of trace amounts of copper in real samples, for determination by flame atomic absorption spectrometry is reported. By passing the aqueous solutions of pH 5 through an octadecyl-bonded silica membrane disk modified by N-benzoyl N-phenyl hydroxyl amine (BPHA), copper is retained quantitatively while almost all matrix elements are passed through. The retained copper ions are then eluted from the disk by minimal amount of HNO3. Maximum capacity of the disk modified by 5 mg BPHA is found to be 217± 6 µg of Cu2+ ions. The method permitted separation of copper ions from coexisting metal ions with an enrichment factor of 200. A good precision for 10 µg L-1 copper (2.5% R.S.D, n = 8) and detection limit (3Sb m-1) 0.3 µg L-1 are obtained. The accuracy is assessed by analysis of two standard reference materials (NBS-364) and (JSS-651-7). The proposed method is applied to the determination of copper in water, waste water and steel samples. Keywords: Solid phase extraction, Copper, N-benzoyl N-phenyl hydroxyl amine
Although trace amounts of copper is essential for many biological systems, its higher concentration is considered as a wide spread anthropogenic pollutant1,2. Thus, monitoring of trace or ultra trace copper in real samples is important because of the increased interest in environmental pollution. Analytical techniques such as electro-thermal atomic absorption spectrometry (ETAAS) and inductively coupled plasma mass spectrometry (ICP-MS) are available for direct determination of trace metals with sufficient sensitivity. Flame atomic absorption spectrometry (FAAS) is more accessible technique and has wide applications for determination of metal ions in solutions, because of its speed and availability in most routine laboratories3. However, it has the major drawback of low sensitivity for direct determination of metals in many matrices. Consequently, a preconcentration and matrix elimination step is usually required. The most widely used techniques for the separation and preconcentration of copper are co-precipitation4, and liquid-liquid extraction (LLE)5-7. Liquid-liquid extractions are generally time consuming, labor intensive and require large amounts of high-purity solvents for extraction. Solid phase extraction (SPE) is an attractive alternative technique to LLE which ————————— *Corresponding author (E-mail:
[email protected])
reduces organic solvent consumption and exposure, disposal costs and extraction time for sample preparation8. Solid phase extraction can be performed with different formats9. However, there are some disadvantages with SPE cartridges and tubes10. Columns with a narrow internal diameter limit the flow rates to a range of (1-10 mL min-1) that necessitates long trace-enrichment time for large sample volumes. Samples with particulate matter will plug the cartridge and channeling may occur. The major advantages of SPE disks are: faster sample processing due to the high cross-sectional area of the disk and decrease in pressure drop, allowing higher sample flow rates; less chance of plugging by particulate matter due to wide bed; reduced channeling and improved mass transfer because of small packing particles11. Therefore, such sorbents can be used for preconcentration of target analytes from the environmental samples. Octadecylbonded silica membrane disks have been used for extraction/pre-concentration of metal ions in presence of complexing agents12,13. In most of such procedures the modifier (ligand) is desorbed during elution of the analyte, which reduces the life time of the modified sorbent. The aim of this work was to develop a selective preconcentration method using a durable sorbent with constant adsorption-desorption process for precise determination of trace amounts of
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copper by FAAS. The proposed method was successfully applied to the determination of copper in water and wastewater samples. Experimental Procedure Reagents
All chemicals were of analytical reagent (AR) grade obtained from Merck (Darmstadt, Germany) and all aqueous solutions were prepared in doubly distilled de-ionized water. The stock standard solution of Cu (II) (1000 mg L-1) was obtained from Merck. The solutions of other metals were prepared from their respective salts. Working standard solutions were prepared by appropriate dilution of the stock standard solutions. BPHA was synthesized by the reaction of N-phenyl hydroxylamine and benzoyl chloride14. A buffer solution of pH 5 was prepared by using sodium acetate and hydrochloric acid of appropriate concentrations. The low alloy steel samples, i.e., NBS- 364 and JSS-651-7 were obtained from National Bureau of Standards, USA and Japanese Standards of Iron and Steel, Japan, respectively. Apparatus
The copper determination was carried out on a Varian Model SpectrAA220 (Mulgrave, Victoria, Australia) flame atomic absorption spectrometer equipped with a deuterium background corrector and a copper hollow cathode lamp operating at 324.7 nm using an air-acetylene flame. The other cations were determined under recommended conditions by the manufacturer. A Shimadzu 160 UV-Vis spectrophotometer was used for Ce, V, U and Mo determination6. The pH measurements were accomplished with a Metrohm model 692 pH/ion meter furnished with a combined glass electrode. Solid phase extractions were performed with 47 mm× 0.5 mm (diameter × thickness) Empore membrane disks containing octadecyl-bonded silica (8 µm particles, 60 Å pore size) from 3M (Oakdale, MN, USA). The disks were used in conjunction with standard Millipore 47 mm filtration apparatus. Preparation of extraction disks
After placing the membrane disk in filtration apparatus it was washed with 10 mL methanol (to remove all contaminants arising from the manufacturing process and from the environment). The solvent was passed through the disk under slight vacuum, and then the disk was dried by passing air through it for several
minutes. Then a solution of 5 mg BPHA in 1.5 mL methanol was introduced onto the disk surface and allowed to penetrate inside the membrane completely in absence of vacuum. The solvent was evaporated at 50°C inside an oven. The disk modified by BPHA was then ready for sample extraction. Method
The modified disk was washed with 15 mL of water to pre-wet the surface of the disk prior to extraction. Then 50 mL of sample solution containing 25 µg Cu2+ whose pH was adjusted to 5, was passed through the membrane at a flow rate of 25 mL min-1. The analyte was then stripped from the membrane disk using 5 mL of 0.5 M nitric acid at a flow rate of 2.5 mL min-1. The copper content was then determined by FAAS using a calibration graph obtained in similar matrix. Results and Discussion BPHA is a well known weak acidic chelating agent. Its chelates with metal ions are stable and hydrophobic; hence it has been used for the solvent extraction of metal ions15. The chelate formation and extraction by BPHA undergoes cation exchange mechanism, therefore as shown by Eq. (2) the extraction yield and selectivity depend upon the pH of sample solution. In this study we used BPHA as a modifier (extractant) for the selective solid phase extraction and determination of copper by the membrane disk and FAAS. Some preliminary experiments were carried out in order to investigate the quantitative retention of Cu2+ ions by the preconditioned disks, in the absence and presence of BPHA. It was found that while the non-modified disk itself does not show any tendency towards copper ions (
