ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2011, том 66, № 7, с. 732-737
^=ОРИГИНАЛЬНЫЕ СТАТЬИ =
УДК 543
FLAME ATOMIC ABSORPTION SPECTROMETRIC DETERMINATION OF TRACE AMOUNTS OF PALLADIUM, GOLD AND NICKEL AFTER CLOUD POINT EXTRACTION
© 2011 г. Sayed Zia Mohammadi*, Daryoush Afzali**, Davood Pourtalebi*
* Department of Chemistry, Payame Noor University (PNU) Kerman, Iran
** Environment Department, Research Institute of Environmental Sciences, International Center for Science,
High Technology & Environmental Sciences
Kerman, Iran Received 26.01.2010; in final form 05.05.2010
In this article, a sensitive cloud point extraction procedure for the preconcentration of trace amounts of palladium, gold and nickel prior to their determination by flame atomic absorption spectrometry has been developed. The cloud point extraction method is based on the complexation of Pd(II), Au(II) and Ni(II) ions with 1-(2-pyridyla-zo)-2-naphthol and entrapping in non-ionic surfactant Triton X-114. The main factors affecting cloud point extraction efficiency, such as pH of sample solution, concentration of 1-(2-pyridylazo)-2-naphthol and Triton X-114, equilibration temperature and time, were investigated in detail. Under the optimized conditions, calibration curves were constructed for the determination of palladium, gold and nickel according to the general procedure. Linearity was maintained from 0.01 to 1.0 ^g/mL for palladium, 10.0 ng/mL to 1.5 ^g/mL for gold and 10.0 ng/mL to 0.5 ^g/mL for nickel. Detection limits based on three times the standard deviation of the blank divided by the slope of analytical curve (3Sb/m) for Pd(II), Au(III) and Ni(II) ions were 3.4, 3.9 and 2.4 ng/mL, respectively. Seven replicate determination of a mixture of 0.5 ^g/mL palladium and gold and 0.2 ^g/mL nickel gave a mean absorbance of 0.174, 0.150 and 0.201 with relative standard deviation ±1.5, ±1.3 and ±1.8%, respectively. The high efficiency of cloud point extraction to carry out the determination of analytes in complex matrices was demonstrated. The proposed method has been applied to the determination of trace amount of palladium, gold and nickel in certified reference material and water samples with satisfactory results.
Keywords: flame atomic absorption spectrometry, palladium, gold, nickel, cloud point extraction.
The major increase in the use of heavy metals has resulted in an increased concentration of metals in aquatic systems. There are numerous sources of industrial effluents leading to heavy metal discharges apart from the mining and metal related industries [1—3]. Because of their toxicity and non biodegradable nature, metals are of special significance. The presence of heavy metals in wastewater and surface water is becoming a severe environmental and public health problem.
Water pollution by heavy metals is causing serious ecological problems in the world, therefore, the determination of heavy metals such as palladium, gold and nickel in the environmental samples is needful today, and requires analytical techniques exhibiting low detection limits for these toxic elements. Analytical chemists are searching for sample preparation procedures that are faster, easier, safer, less expensive and provide accurate and precise data with reasonable detection limits [2, 3].
A high number of analytical methods such as flame (FAAS) or graphite furnace atomic absorption spectrometry (GFAAS), inductively coupled plasma atomic emission or mass spectroscopy (ICP-AES, ICP-MS), and stripping voltammetry have been proposed for the deter-
mination of heavy metal ions at low concentration level. These methods need relatively high cost apparatus [4].
Flame atomic absorption spectrometry is widely employed in the determination of heavy metals due to the ease ofoperation, good selectivity and low instrument and operation cost. However, direct determination of trace level heavy metals presented in environmental samples is difficult because their amount is always lower than the detection limit of instruments and in addition, the problem of nonsuitable matrix occurs. These limitations can be overcome by applying a clean up and/or preconcentration step prior to the determination step [5].
Various separation and preconcentration methods have been proposed to achieve these goals. Among them are solid phase extraction [6], liquid-liquid extraction [7], cloud point extraction [8], membrane filtration [9], ion exchange [10] and coprecipitation [11] techniques.
Liquid-liquid extraction and other conventional separation methods are time consuming and labor-intensive approaches, besides requiring relatively large amounts of high-purity and frequently toxic solvents, which have to be disposed off properly. Cloud point extraction (CPE) is
an alternative suitable separation technique, which is based on the phase behavior ofnon-ionic and zwitter ionic surfactants in aqueous solutions, which exhibit phase separation after an increase in temperature or the addition of a salting-out agent [12, 13].
Micelle-mediated extraction procedures have found wide applications in different areas of analytical chemistry, and their advantages over the conventional liquid-liquid extraction technique have been well documented in the literature [14—16]. The formation of micelles consists on the aggregation of a certain number of surfactant monomers [17]. Aqueous solutions containing a non-ionic or zwitterionic surfactant above its critical micellar concentration (CMC) become turbid, because the surfactant molecules associate spontaneously, forming aggregates of colloidal dimensions [18]. Any species that originally present associates and binds to these micellar aggregates can be extracted from the initial solution and preconcentrated in a small volume of the surfactant-rich phase. The clouding phenomenon can be induced by changing the temperature, additive content, or pressure by which results the separation ofa single isotropic micellar phase into two isotropic phases: (i) a surfactant-rich phase of small volume composed mainly of surfactant, and (ii) an aqueous phase containing surfactant with the concentration level near to CMC [16]. CPE has been developed in the recent two decades and wide applications for the extraction of metal species and organic molecules have been found [18—25].
In this work, cloud point extraction method was developed and optimized for the separation and preconcentra-tion of palladium, gold and nickel in the aqueous samples. Pd(II), Au(III) and Ni(II) ions were changed into their chelates in the presence of 1-(2-pyridylazo)-2-naphthol (PAN) followed by extraction into Triton X-114 as a non-ionic surfactant. Finally, simultaneous determination of these elements was performed by FAAS and the proposed method was successfully applied to the real samples.
EXPERIMENTAL
Reagents and solutions. All chemicals were of analytical-reagent grade and were used without previous purification. The laboratory glassware was kept overnight in a 1.4 M HNO3 solution. Before the use, the glassware was washed with de-ionized water and dried. The stock solution ofpalladium at a concentration of 1000.0 ^g/mL was prepared by dissolving an appropriate amount of PdCl2 (Merck, Darmstadt, Germany) in 2 M HCl. Stock solution of gold at a concentration of 1000.0 ^g/mL was prepared by dissolving an appropriate amount of HAuCl4 • • 3H2O(Merck) in de-ionized water. Stock solution of nickel at a concentration of 1000.0 ^g/mL was prepared by dissolving appropriate amount of Ni (NO3)2 • 6H2O (Merck) in de-ionized water containing 1 mL of concentrated nitric acid (Merck). Working reference solutions were obtained daily by stepwise dilution from stock solution. A solution of 0.2% PAN (Merck) was prepared by
Table 1. The operating parameters for palladium, gold and nickel
Element Slit bandwidth, nm Current mA Wavelength, nm
Pd 0.2 5.0 244.8
Au 0.5 4.0 242.8
Ni 0.2 4.0 232.0
dissolution of 0.20 g of the chelating agent in 100 mL of ethanol. A solution of1.5% (v/v) Triton X-114 (Sigma, St. Louis, USA) in de-ionized water was used as surfactant agent. Buffer solutions of pH value from 3 to 10 were prepared. Solutions of alkali metal salts (1%) and various metal salts (0.1%) were used for studying the interference of anions and cations, respectively.
Instrumentation. A SensAA GBC (Dandenong, Australia) atomic absorption spectrometer equipped with deuterium background correction and air-acetylene burner was used for absorbance measurements according to instrument instruction. Palladium, gold and nickel hollow cathode lamps were used as light sources. The acetylene flow rate and the burner height were adjusted in order to obtain the maximum absorbance signal, while aspirating the analyte solution. A Metrohm 692 pH meter (Her-isau, Switzerland) was used for pH measurements. The operating parameters of elements were set according to the manufacturer recommendation. These conditions are presented in Table 1. A Centurion scientific centrifuge model 1020 D.E. (West Sussex, United Kingdom) was used to accelerate the phase separation. An ultrasonic bath with temperature control (FALC instruments S.Vl Treviglio, Italy) model LBS2 was used to adjusting of incubation temperature.
Cloud point extraction method. Aliquots of8.0 mL water samples or standard solutions were pipetted to centrifuge tubes. Then, 1 mL of 0.2 M acetate buffer (pH 3.0), 1.0 mL of0.5% Triton X-114, 0.3 mL of0.125% PAN and 0.5 mL of 5% NaCl were sequentially added and completely mixed with the sample or standard solutions. The centrifuge tubes containing the mixed solutions were heated in a thermostatic water bath at 50°C for 15 min. Separation of aqueous and surfactant rich phases were accomplished by centrifuging at 3000 rpm for 5 min. Then, supernatant aqueous waste in the tubes was removed with a pipette and 1.0 mL of1.0 M HNO 3 was added to it. The final solution was aspirated directly into the flame ofAAS.
RESULTS AND DISCUSSION
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