ANALYTICAL SCIENCES OCTOBER 2003, VOL. 19 2003 © The Japan Society for Analytical Chemistry
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Notes
Preconcentration and Speciation of Chromium in Water Samples by Atomic Absorption Spectrometry after Cloud-Point Extraction Farzaneh SHEMIRANI,*† Shiva Dehghan ABKENAR,* Aazam Alsadat MIRROSHANDEL,* Masood Salavati NIASARI,** and Reyhaneh Rahnama KOZANIA* *Department of Chemistry, Faculty of Science, University of Tehran, Tehran, Iran **Department of Chemistry, Kashan University, Kashan, Iran
A rapid, sensitive and accurate atomic absorption method for Cr(III) and Cr(VI) ions was developed based on the cloudpoint extraction (CPE) technique. Cr(III) reacts with a new Schiff’s base ligand to form the hydrophobic complex, which is subsequently entrapped in the surfactant micelles. The Cr(VI) assay is based on its reduction to Cr(III) by the addition of concentrated H2SO4 and ethanol to the sample solution. The condensed surfactant phase containing the metal chelates is introduced into an atomic absorption spectrometer. The relative standard deviations were 2.1% for both species and the limits of detection were around 0.17 µg l–1. (Received November 22, 2002; Accepted July 14, 2003)
Introduction In recent years, the determination and speciation of chromium due to the different properties and toxicities of the chemical forms of chromium have become very important in environmental samples.1 The direct determination of chromium in water may not be possible with sufficient sensitivity by also using expensive analytical methods, such as inductively coupled plasma atomic emission spectrometry2 or electrothermal atomic absorption spectrometry3 because of low concentrations and/or matrix interferences. For this purpose, various separation and preconcentration methods, such as liquid-liquid extraction,2,3 coprecipitation,4,5 ion exchange,6–8 adsorption9–12 and solid-phase extraction,10,13–15 have been developed. In recent years, several methods for the preconcentration and separation of Cr(III) and Cr(VI) have been developed by using a micelle-mediated methodology.16–18 Cloud-point extraction is probably the most versatile and simple method for the preconcentration and extraction of hydrophobic species from water. The technique is based on the property of most non-ionic surfactant in aqueous solutions to form micelles and to become turbid when heated to a temperature known as the cloud-point temperature (CPT). Above this temperature, the micellar solution separates in a surfactant-rich phase, in which the surfactant concentration is close to the critical micellar concentration.19 This phenomenon, which is especially observed with polyoxyethylene surfactants, can be attributed to ethyl oxide segments in a micelle that repel each other at low temperature and attract each other at high temperature.20 The cloud-point phenomenon is reversible, and when the temperature falls below the CPT a single phase appears again. † To whom correspondence should be addressed. E-mail:
[email protected]
Taking full advantage of the analytical merits of the cloudpoint extraction (CPE) approach, the concept of using this technique for the speciative analysis of chromium was put forward. The complexation was done with appropriate chelating agents with the aim to form water-insoluble or sparingly soluble complexes.15,16 Compared with the recent developments in the preconcentration, speciation and determination of Cr(III) and Cr(VI), such as solid-phase extraction and cloud-point extraction, the proposed method is simple and sensitive.10,12–17,21,22 In the present work we developed a CPE procedure for the speciation of chromium.
Experimental Apparatus A varian (Madrid, Spain) Model AA-1475 atomic absorption spectrometer equipped with deuterium background correction and a chromium hollow cathode lamp, as the radiation source, was recommended by the manufacture. A thermostated bath maintained at the desired temperatures was used for cloud-point temperature experiments, and phase separation was assisted using a centrifuge (Hettich, Universal). Reagents and solutions All of the chemicals used were of analytical reagent grade, free from chromium traces. Stock solutions of Cr(III) and Cr(VI) were prepared by dissolving appropriate amounts of Cr(NO3)3 and K2Cr2O7 (Sigma-Aldrich Ltd.) in double-distilled water. The non-ionic surfactant Triton X-114 (Fluka Chemie AG, Switzerland) was used without further purification. NaOH, HCl, H2SO4 and the solutions used for the interference study were obtained from Sigma-Aldrich Ltd. A chelating-agent solution was prepared by dissolving 0.296 g of a Schiff’s base in 100 cm3 of 99.5% methanol. The Schiff’s
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ANALYTICAL SCIENCES OCTOBER 2003, VOL. 19
Fig. 1 Effect of the pH on the CPE–preconcentration performance: Cr(III) [or Cr(VI)] 20 µg l–1; Triton X-114 0.12% (w/v); Schiff’s base 2 × 10–4 mol dm–3.
Fig. 2 Effect of the Triton X-114 concentration on the CPE–preconcentration performance: Cr(III) [or Cr(VI)] 20 µg l–1; pH 8.0; Schiff’s base 2 × 10–4 mol dm–3.
base (bis(2-methoxybenzaldehyde) ethylene diimine) was synthesized and purified as described in the “Synthesis of ligand” section.
trapped in surfactant micelles. The total chromium was determined as Cr(III) by the described method after reducing Cr(VI) to Cr(III). Then, the concentration of Cr(VI) was calculated by subtracting the concentration of Cr(III) from the total chromium concentration. Any parameter affecting the proposed reactions and micelle formation was considered in the optimization experimental design.
Synthesis of ligand The Schiff’s base [(CH3)2Salen] was prepared according to the literature through a well-known method, as follows: Ethylenediamine (0.30 g (5.0 mmol)), was dissolved in 40 cm3 of ethanol, and then transferred into a 250 cm3 three-necked flask. Under reflex, 1.36 g (10.0 mmol) of 2methoxybenzaldehyde in 30 cm3 of ethanol was added dropwise to the flask. The stirred mixture was kept reacting for 60 min under reflux, and then cooled to room temperature. The solid product was filtered, and the product was recrystallized from ethanol and vacuum-dried for 12 h. Anal. Calcd. for C18H20N2O2: C, 72.86; H, 6.73; N, 9.52%. Found: C, 72.95; H, 6.80; N, 9.45%. This Schiff’s base is sparingly soluble in water. Its solubility in methanol is relatively high (more than 10–1 mol dm–3). This compound is very stable in the solid state and in methanol. This ligand has two nitrogens and two oxygens as σ-donors and πacceptors in complexation with chromium. In acidic media (pH < 4.0), its nitrogen atoms were protonated. Procedures For the CPE, aliquots of 60 cm3 of a cold solution containing Cr(III) [and/or Cr(VI)], 0.12% (w/v) Triton X-114 and 2.5 × 10–4 mol dm–3 Schiff’s base was adjusted to the appropriate pH value (pH = 8.0) with either NaOH or HCl. When Cr(VI) was to be determined, 0.3 cm3 of concentrated H2SO4 and 0.3 cm3 of ethanol (95% (v/v))10 had to be added prior to the complexation reaction. The mixture was kept for 15 min in a thermostatic bath maintained at 65˚C. Separation of the phases was achieved by centrifugation at 3500 rmp, for 10 min. The phases were cooled down in an ice bath in order to increase the viscosity of the surfactant-rich phase. The bulk aqueous phase was easily decanted. The remaining micellar phase (200 µl) was dissolved in 300 µl of a methanolic solution of 1 mol dm–3 HNO3 in order to reduce its viscosity. The final solution was aspirated directly into the flame of the AAS.
Results and Discussion It was demonstrated that the employed reaction schemes function in the following way: Cr(III) reacts with a Schiff’s base, forming a hydrophobic complex, which is subsequently
Effect of the pH The pH was the first critical parameter evaluated concerning its effect on the determination of the two species. As can be seen in Fig. 1, the highest recoveries for Cr(III) were obtained in the pH range 7.5 – 8.5. While the recovery of Cr(III) was quantitative, at pH 8.0 the recovery of Cr(VI) was rather low. This could make it possible to separate of Cr(III) from Cr(VI) and to determine Cr(III) by adjusting pH to 8.0. Effect of the Triton X-114 concentration The variation in the extraction efficiency upon the surfactant concentration was examined within the following range: CTritonX-114 = 0.03 – 0.42% (w/v). The results are shown in Fig. 2. Triton X-114 was chosen for the formation of the surfactantrich phase due to its low cloud-point temperature and high density of the surfactant-rich phase, which facilitates phase separation by centrifugation. It was proved that Triton X-114 effectively extracts Cr(III) from liquid samples at a concentration of 0.12% (w/v). With an increase of the Triton X-114 concentration above 0.12%, the signals decreased because of the increment in the overall analyte volumes and the viscosity of the surfactant phase. The optimum surfactant concentration used for the Cr(III) was 0.12% (w/v) Triton X-114, in order to achieve the optimal analytical signal in conjunction with the highest possible extraction efficiency. Effect of the Schiff’s base concentration Under the optimum pH, the complexation efficiency of Cr(III) with the Schiff’s base as a function of the concentration of the chelating agent was studied, the results are shown in Fig. 3. The signal increased up to a concentration of 2 × 10–4 mol dm–3, and reached a near quantitative extraction efficiency. The concentration level of the ligand must remain lower than 2.5 × 10–4 mol dm–3, and the placement of any excess may adversely affect the system performance.
ANALYTICAL SCIENCES OCTOBER 2003, VOL. 19
1455 Table 1
Analytical characteristics of the method Parameter
Analytical feature a
Preconcentration factor LODb (µg l–1) R.S.D. (%) Regression equation, C (µg l–1) Correlation coefficient (r) Linear rangec (µg l–1)
Fig. 3 Effect of the Schiff’s base concentration on the CPE–preconcentration performance: Cr(III) [or Cr(VI) 20 µg l–1; pH 8.0; Triton X-114 0.12% (w/v).
57 0.17 2.1 y = 11.08 × 10–3C + 0.014 0.9985 0.17 – 150
a. The ratio of the concentration of analyte without the application of CPE technique to that after the CPE, giving the same analytical response. b. Limit of detection defined as three times the signal-to-noise ratio. c. For surfactant concentration: 0.12% (w/v).
Table 2 Effect of foreign ions on the preconcentration and determination of Cr(III) (50 µg l–1)
Effect of equilibration and time It was desirable to employ the shortest equilibration time and the lowest possible equilibration temperature, as a compromise between completion of extraction and efficient separation of the phases. It was found that 65˚C is adequate for these analyses. The dependence of the extraction efficiency upon the equilibration time was studied for a time span of 5 – 25 min. An equilibration time of 15 min was chosen as the optimal to achieve quantitative extraction. Effect of the ionic strength The influence of the ionic strength was examined by studying the response for the KCl concentration in the range of 0 – 1.0 mol dm–3. The ionic strength had no significant effect on the extraction efficiency and the sensitivity up to 0.5 mol l–1. This is in agreement with the results reported in the literature, which demonstrate that an increase in the ionic strength in micellemediated systems does not seriously alter the extraction efficiency of the analyte.23,24 Effect of the viscosity on the analytical signal Since the surfactant-rich phase obtained after cloud-point extraction is rather viscous, methanol containing 1 mol dm–3 nitric acid was added to the surfactant-rich phase after separation of the phases in order to facilitate its introduction into the nebulizer of the spectrometer. An optimal volume of 300 µl of a methanolic solution of 1 mol dm–3 HNO3 was added to the remaining micellar phase (200 µl). This added volume of methanol was chosen in order to ensure a sufficient volume of the sample for aspiration. Calibration, precision and detection limits Calibration graphs were obtained by the preconcentration 60 cm3 of a sample in the presence of 0.12% Triton X-114 under the optimum experimental conditions. Table 1 gives the calibration parameter, the relative standard deviation obtained for 10 analyte samples subjected to the complete procedure and the detection limits. The preconcentration factor, calculated as the ratio of the concentration of analyte after preconcentration to that prior to preconcentration, which gives the same absorbance peak area, is 57. The precision of the method was established by repeated assays (n = 10) using 5.0 µg l–1 solutions of Cr(III) and Cr(VI). The relative standard deviations were 2.1% for both species. The limits of detection are sufficiently low as compared to those attained by FAAS without preconcentration, and lie around 0.17
Ion
Concentration/mg l–1
Recovery, %
Na+ K+ Ca2+ Mg2+ Ba2+ Al3+ Mn2+ Co2+ Cu2+ Ag+ Hg2+ Ni2+ Zn2+ Cd2+ Pb2+ Fe3+
30 × 103 30 × 103 100 100 5 5a 5 5 5 5 5 2 2 2 2 0.5
101.3 103.1 98.3 100.3 100.3 102.1 96.7 99.6 99.2 100.6 98.7 101.5 99.2 99.6 103.4 103.1
a. In the presence of fluoride (5 × 10–3 mol dm–3).
µg l–1. Interferences In view of the high selectivity provided by atomic absorption spectrometry, the only interferences studied were those related to the preconcentration step. The results are shown in Table 2, and prove that the Cr(III) recoveries are almost quantitative in the presence of interfering cations. However, Al3+ interferes with the preconcentration of the Cr(III) ion. This interference was eliminated or considerably reduced in the presence of a proper masking reagent, such as fluoride. This reagent forms a stable complex with Al3+, but does not interfere with the reaction between Cr(III) and the chelating agent. Preconcentration and determination of Cr(III) and Cr(VI) in water samples The proposed method was applied to the speciation of Cr(III) and Cr(VI) in tap water and river water. As shown in Table 3, the proposed method could be successfully applied for the preconcentration and speciation of trace amounts of chromium in tap water, river water and spiked water samples. The relative error was lower than 2.1% for both Cr(III), Cr(VI) and the total chromium. Validation of the method was performed using a certified reference material, BCR 544, which is synthetic water certified for both species. The agreement of the certified values with those obtained using the proposed method is acceptable, as
1456 Table 3
ANALYTICAL SCIENCES OCTOBER 2003, VOL. 19 Determination of Cr(III) and Cr(VI) in water samples –1
Added/µg l
a
–1
Found /µg l
Recovery, %
Table 4 Application of the proposed method to the speciation of Cr in a reference material
Sample Cr(III) Cr(VI) Cr(III) Tap water
River water
— 4 10 — 4 10
— 4 10 — 4 10
0.4 4.3 10.4 15.4 19.5 26.1
b
Cr(VI) 0.6 4.7 10.5 8.4 12.6 18.3
Cr(III) Cr(VI) — 97.7 100 — 100.5 102.7
— 102.1 99.0 — 101.6 99.4
Sample volume 60 cm3. a. Mean of three determinations. b. Calculated by subtracting Cr(III) from total Cr.
can be seen from Table 4.
Acknowledgements The authors thank the research council at the University of Tehran for financial support.
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Certified value (µg l–1) Found (µg l–1)
Cr(III)
Cr(IV)
26.8 ± 1.0 24.9 ± 0.6
22.8 ± 1.0 21.9 ± 0.6
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