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Sensitized spectrophotometric determination of Cr(III) ion for speciation of chromium ion in surfactant media using α-benzoin oxime
Spectrochimica Acta Part A 63 (2006) 182–188
Sensitized spectrophotometric determination of Cr(III) ion for speciation of chromium ion in surfactant media using ␣-benzoin oxime Mehrorang Ghaedi ∗ , Enayat Asadpour, Azam Vafaie Chemistry Department, Yasouj University, Yasouj 75914-353, Iran Received 27 January 2005; accepted 22 April 2005
Abstract A simple and accurate micellanized spectrophotometric method for determination of trace amounts of Cr(III) ion in tab and top water and a synthetic mixture has been described. The micellar method is based on effect of organized molecular assemblies such as micelles in spectrophotometric measurement due to their effect on the systems of interest. The ability of micellar system in solubilizing of sparingly soluble ligand or complexes has been used for increasing figures of merit of an analytical method. Due to solubility increasing in aqueous media requirement for a primary extraction can be eliminated. Using the ␣-benzoin oxime (␣-BO) spectrophotometric determination of Cr(III) ion has been performed and results are compared. The spectrophotometric determination of Cr(III) ion using ␣-BO in the presence of non-ionic surfactant Triton X-100 has been performed. The influence of type and amount of surfactant, pH, complexation time and amount of ligand were examined. Finally, the repeatability, accuracy and the effect of interfering ions on the determination of Cr(III) ion was evaluated. The proposed methods successfully with recovery yield of almost 100% have been applied to the rapid and simple determination of Cr(III) ion in the real samples. There is a good agreement between methods and atomic absorption spectrometry. The Beers law is obeyed over the concentration range of 0.1–13.7 g mL−1 for micellar media. The detection limit is 0.8 ng mL−1 . The molar absorptivity of complex is 5350 L mol−1 cm−1 . © 2005 Published by Elsevier B.V. Keywords: ␣-Benzoin oxime; Spectrophotometric method; Surfactant media; Triton X-100; Cr(III) ion
1. Introduction The importance of the determination of trace metal concentration in natural water samples is increasing in contamination monitoring studies. Chromium compounds are widely used by modern industries, resulting in large quantities of this element being discharged into the environment. Some of the main uses for chromium compounds are as follows: (1) plastic coating of surfaces for water and oil resistance, (2) electroplating of metal for corrosion resistance, (3) leather tanning and finishing and (4) in pigments and for wood preservative. Until a few years ago, it also was the favored corrosion control agent in cooling towers whose water blow down (waste) was dumped into rivers, pits, lakes ∗
Corresponding author. Tel.: +98 741 2223048; fax: +98 741 2223048. E-mail address: m [email protected] (M. Ghaedi).
1386-1425/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.saa.2005.04.049
and oceans. Chromium occurs in wastewaters resulting from these operations in both trivalent [Cr(III)] and hexavalent [Cr(VI)] forms. There is a rapidly increasing demand for fast and reliable analytical methods for the determination of chemical forms of elements, especially chromium ion, in environmental samples. The interest in chromium is governed by the fact that its toxicity depends critically on its oxidation state. While chromium(III) is considered essential for mammals for the maintenance of glucose, lipid and protein metabolism, chromium(VI) is known to be toxic to humans [1,2]. The rationale for this approach is based on the assumption that inorganic Cr(III) solids are very insoluble and Cr(III) is not considered to be toxic, whereas Cr(VI) is known to be toxic, mutagenic and carcinogenic [3]. However, the possibility that the presence of organic ligands and/or acidic conditions in the environment
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increases Cr(III) mobility, and also MnO2 , present in soil, could oxidize Cr(III) to the more toxic and mobile Cr(VI) forms. Therefore, it is of major concern to understand the behavior of chromium in natural aquatic systems. In view of the difference between the oxidation states, and in order to follow the pathways for inter conversion in the environment, it is increasingly important to monitor the concentration of the individual chemical species as well as the total concentration of chromium in the environment. In view of the marked differences in the chemical behavior and toxicity of chromium in its different oxidation states, numerous methods have been tried in the continuing effort to quantify the chromium species in natural waters. In recent years, the determination and speciation of chromium due to the different properties and toxicities of the chemical form of chromium have become very important [4]. The direct determination of chromium in water not is possible with sufficient sensitivity by also using expensive analytical method such as inductive coupled plasma emission spectroscopy or electro thermal atomic absorption spectrometry [5], because of low concentration and matrix interference. Due to the presence of chromium in environmental samples at low levels and matrix effects with notation to detection limit of lead with traditional method spectrophotometry, in addition to separation from matrix elements. Extractive and micellar sensitized spectrophotometric with ability to separate and preconcentrate of chromium solve these problems and lead to a higher confidence level and easy determination of the trace elements by less sensitive, but more accessible instrumentation such as spectrophotometer. For this purpose various preconcentration method of chromium in micellar media [6,7] and spectrophotometric and extraction method based on different ligands [8–20] has been described for chromium determination. Many of these methods are time consuming or require complicated and expensive instrument and some of them has low repeatability. Due to low concentration of Cr(III) ion content above mention problems, development of new sensitive and selective methods for selective, sensitive, rapid and convenient determination of this ion in sub-micron levels is still a challenging requirement. In recent years extractive spectrophotometric determination [21,22] and organic micellar media as powerful tools are very useful in analytical application, especially in UV–vis spectrophotometry [23–26] and fluorescence process, because they are stable in aqueous solution and transparent optically, enhance sensitivity and readily available [27]. The utility of surfactants in sensitized spectrophotometry emerged from the various pathway that among them formation of ternary complex with concomitant shift in the analytical wavelength [28,29] and increasing the solubility of insoluble complexes and/or ligands [30,31] has been benefited. In this view, the selectivity and sensitivity of numerous analytical spectrophotometric-based reactions can be improved using certain surfactants. In surfactant media complexes of metal ions with complexing agent are most stable and formed
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aggregates that cause an improvement in sensitivity and detection limits [27]. For determination of Cr(III) ion at trace level with UV–vis spectrometry various chelating agent have been used. Recently, sensitized spectrophotometric determination of Cr(III) ion based on surfactant media have been reported. In the present work, a simple and highly selective and sensitive spectrophotometric method for determination of Cr(III) ion using ␣-BO in surfactant media (Triton X-100) was established.
2. Experimental 2.1. Instrumentation A Shimadzu UV–vis 160 spectrophotometer was used to measure the absorbance of complex in Triton X-100 media. To adjust the pH and prepare the buffer solution a 691 pH/ion meter with a combined glass and calomel electrode has been used. 2.2. Reagent and solution All chemical such as nitrate of Cr(III) ion and other cation were of the analytical grade purchased from Merck Company. A 0.5% (w/v) all surfactant all from Merck Company including sodium dodecyl sulfonate (SDS), Triton X100, Brij 58, cetyltrimethylammonium bromide (CTMAB), n-dodecytrimethylammonium bromide (DTMAB) was prepared by dissolving 0.5 g of each surfactant in 100 mL volumetric flask with stirring. The ligand ␣-benzoin oxime was purchased from Merck Company. The chromium determinations were carried out on a Perklin–Elmer 603 atomic absorption spectrometer with a hallow cathode lamp and a deuterium background corrector at resonance line using an air–acetylene flame. 2.3. Procedure calibration curve for spectrophotometric in micellar media Standard Cr(III) ion solutions were prepared in the range of 0.01–25.0 g mL−1 . Several aliquots of Cr(III) ion were added to 10 mL volumetric flask and 0.35 mL 0.02 M ␣-BO and 0.7 mL of 7.8 mM of Triton X-100 were added to each flask and with phosphate buffer (pH 8.0) was filled to the mark and calibration curve of Cr(III) ion was constructed using a UV–vis spectrometer. According to IUPAC recommendation detection limit has been calculated. 2.4. Pretreatment of real samples Analysis waste water sample for determination of Cr(III) ions content was done as follow: 250 mL of water samples was poured in a beaker, while stirring, it heated to reach its volume to half. After adjustment of samples pH to desired
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value the spectrophotometric experiments were performed according to general described procedure. A synthetic sample was prepared. In all of real and synthetic sample amount of Cr(III) ion was found by standard addition method and a reference atomic absorption spectrometry.
sensitivity were examined. The composition of complex and their respective parameters including molar absorptivity and maximum wavelength were evaluated. At optimum conditions, time dependency of complex, repeatability, efficiency, effect of interference of other metal ions, accuracy and precision of developed methods for real sample analysis were investigated and evaluated.
3. Results and discussion 3.1. Optimizing condition for spectrophotometric determination of Cr(III) ion in micellar media in the presence of α-BO In preliminary experiments, it was shown that chromium interfere in copper determination with ␣-BO. It is occur to us that due to incorporation of nitrogen and oxygen atoms as donor and acceptor and low density of electrons in the ligand structure as hard base in comparison with chromium ion as hard acid, through the ion–dipole interaction between oxygen and chromium ion and donating accepting between them a rapid quantitative complexation occur. Due to these types of interaction, rate of complex formation is high and reversible. It seems that be useful for construction of calibration curve based on spectrophotometric determination of chromium ion. In preliminary experiments, complexation between Cr(III) ion and ␣-BO was examined using mole ratio method using UV–vis spectrophotometry and conductometry and results confirmed a stoichiometric relation Cr(III) ion to ␣-BO equal to 1:2. A sensitive and sharp peak at about 295 nm (where ligand and Cr(III) ion do not show any absorbance) indicates the strong interaction between Cr(III) ion and the ␣-BO which is shown in Fig. 1. In acidic media pH < 4, its oxime group (nitrogen atom) protonated and its capacity for complex formation with chromium reduced. The effect of various parameters such as pH, type and amount of surfactant, amount of ligand and ionic strength on
Fig. 1. Comparing spectra in the presence (A) and absence (B) of Triton X100 at optimum conditions of reagents and 12 g mL−1 Cr(III) ion, 7 mM BO, pH 8.0, 5.5 × 10−4 Triton X-100.
3.2. Absorption spectra of Cr (α-BO)2 in Triton X-100 media and composition of complex After Cr(III) ion, ␣-BO and Triton X-100 were added to a 10 mL volumetric flask so that their concentrations were 12 g mL−1 , 7 mM and 0.55 mM, respectively, the solution was diluted to the mark with phosphate buffer (pH 8.0). Then, the absorption spectrum of complex in the presence and absence of surfactant Triton X-100 was obtained, which is shown in Fig. 1. To study the composition of Cr(III)- ␣BO–Triton X-100, the mole ratio method for Cr(III) ion and ␣-BO in the presence and absence Triton X-100 were used. The mole ratio of ␣-BO and Cr(III) ion by continuous variation method has been calculated. The molar composition of ␣-BO-Cr(III)–Triton X-100 in the presence and absence of surfactant are as 2:1 and 2:1:1. A confirmation of this stoichiometric is inability of us for extraction of complex to any immiscible organic phase at optimum conditions in the presence and absence of Triton X-100. The molar absorptivity of binary and ternary complexes as slope of calibration curve found by least squares analysis of five analysis was 2.1 × 103 and 5.35 × 103 L mol−1 cm−1 . It can be concluded that application of surfactant media in analytical chemistry involves the beneficial alteration of metal ion–ligand complex spectral properties via surfactant association [29]. Experiment in micellar media has higher sensitivity and also no need to extraction of complex to organic phase and do not have disadvantages including time consuming, need to harmful organic solvent and labor intensive. The studies focused on investigation of complexation between Cr(III) ion and ␣-BO in surfactant media and evaluating optimum conditions for sensitized spectrophotometric determination of Cr(III) ion using ␣-BO. The stability and decomposition rate of complex seriously depend on pH and time. The low sensitivity and omitting spectra in acidic media is an indication. At optimum pH the time dependency and rate of complex formation was investigated. Investigation in surfactant media display that complex completely formed at less than 1 min and up to days is stable. 3.2.1. Effect of pH on sensitivity The influence of the test solution pH on the absorbance of complex (3.4 g mL−1 Cr(III) ion) in 5.5 × 10−4 M Triton X-100 media against a reagent blank was investigated. Results, which are shown in Fig. 2, display that complex showed the maximum absorption at pH 8.0 that 0.006 M phosphate buffer has maximum sensitivity and selected for further studies. From this experimental fact,
M. Ghaedi et al. / Spectrochimica Acta Part A 63 (2006) 182–188
Fig. 2. Effect of pH on sensitivity of Cr(III) ion in micellar media, 7 mM BO, 3.4 g mL−1 Cr(III) ion, 5.5 × 10−4 Triton X-100.
we determined that complex was quantitatively formed and well dissolved in Triton X-100 media at this pH. In more acidic media due to the ligand protonation and incomplete complex formation and in more alkaline media because of complex hydrolysis and hydroxide formation the absorbance reduced. 3.2.2. Effect of surfactant on sensitivity To investigate the effect of types of surfactants, Triton X100 and Brij 58 as non-ionic, sodium dodecylsulfate (SDS) as anionic surfactant and cetyltrimethyl ammonium bromide CTMAB) and n-dodecyltrimethyl ammonium bromide (DTMAB) as cationic surfactant on the above mention complex were examined. In 12 g mL−1 concentrations of Cr(III) ion and 8.0 × 10−3 M ␣-BO and 5.5 × 10−4 each surfactant effect of type of surfactant on spectra and sensitivity were examined and results displayed in Table 1. As can be seen from Table 1, for 5.5 × 10−4 Triton X-100 media, spectra with high sensitivity and red shift could be constructed and its slope was about two times more than in the absence of it, while the anionic or cationic surfactant showed no positive effect or diminished it. This suggests that above mention complex was dissolved in surfactant phase due to the hydrophobic solvation of the desired chelate. Therefore, Triton X-100 was selected for further studies. This observation that in the presence of non-ionic surfactant, method had high sensitivity is
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an indication that surfactant lead to aggregation of complexes and increase in sensitivity. Because a metal ion is a cation, the electrostatic attractive interaction between a metal ion and cationic surfactant is not present, and the complex-forming process is not affected. It seems that ␣-BO combines with Cr(III) ion to form a low polar complexes, and the complexes is extracted instantaneously into the local no polar environment of micelle of non-ionic surfactant. We saw that although the concentration of the Triton X-100 were somewhat less than micelle concentration, since further addition lead to precipitation, mention complexes was homogenously dissolved in surfactant media. The 5.5 × 10−4 of Triton X100 has been used for further studies. We saw that although the concentration of the Triton X-100 were somewhat less than critical micelle concentration (0.0082 M, 0.05%), mention a complex was homogenously dissolved in surfactant media. The 5.5 × 10−4 of Triton X-100 has been used for further studies. 3.2.3. Effect of Triton X-100 concentration In order to stabilize the colored complex formed and to increase the sensitivity, the various concentration of Triton X-100 was added to solutions at optimum and sensitivity was examined. Results are shown in Fig. 3. As shown in Fig. 3, the increasing the concentration of Triton X-100 up to 5.5 × 10−4 as non-ionic surfactant was most effective in improving absorbance and the absorbance development was stable and reproducible. With the concentration of Triton X-100 varying from 1 × 10−5 to 5.5 × 10−4 at pH 8.0 for ␣-BO the absorbance of desired complexes at concentration of 3.4 g mL−1 Cr(III) ion was investigated. The maximum absorbance was obtained when the concentration of Triton X-100 was 5.5 × 10−4 M. We assumed that although the concentration of the surfactant was somewhat less than critical micelles concentration complex was homogeneously dissolved in surfactant media.
Table 1 Effect of type of surfactant on spectra and sensitivity, 12 g mL−1 Cr(III) ion, 7 mM BO, pH 8.0, 5.5 × 10−4 surfactant Surfactant
Absorbance
Maximum wavelength (nm)
Brij CTAB DTAB SDS Triton X-100
1.12 0.64 0.66 0.89 1.39
293 284 284 287 295
Fig. 3. Effect of Triton X-100 concentration on sensitivity at optimum condition, 7 mM BO, 3.4 g mL−1 Cr(III) ion, pH 8.0.
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M. Ghaedi et al. / Spectrochimica Acta Part A 63 (2006) 182–188 Table 2 Investigation of tolerance limit of interfering ions on proposed methods at 1 g mL−1 Cr(III) ion, 7 mM BO, pH 8.0, 5.5 × 10−4 Triton X-100
Fig. 4. Effect of ␣-BO concentration on sensitivity at optimum conditions, 3.4 g mL−1 Cr(III) ion, 5.5 × 10−4 Triton X-100, pH 8.0.
3.2.4. Effect of α-BO concentration on sensitivity The analytical sensitivity and the reproducibility in the complex spectra were good in micellar media. It is known that Cr(III) ion interact stoichiometrically with ␣-BO to form a 1: 2 complexes and confirmed with conductometrically and spectrophotometry. For evaluating and investigating the effect of the chelating agent concentration various amount of ligand was added to similar solution at optimum condition. Results which are shown in Fig. 4 display 0.007 M ligand concentration lead to high sensitivity. For metal complexes to be formed quantitatively, one must added mor chelating agent to the sample solution. Therefore, sorbent ␣-BO was added more than 50 times Cr(III) ion concentration to reduce fluctuation in measurement of absorbance. 3.2.5. Validity of beer’s law, molar absorptivity, sandell s sensitivity, correlation coefficient and detection limit The calibration curves were prepared for the determination of Cr(III) ion by using sample solutions at the optimum conditions of method. A calibration curve was constructed at optimum conditions according to calibration curve procedure in Experimental Section. Beers law is obeyed in the dynamic range of Cr(III) was 0.1–13.7 g mL−1 with equation of A = 0.1464CCr(III) + 0.0023 where C is Cr(III) concentration as g mL−1 . The correlation coefficient (R2 ) was 0.9991, showing good linearity of calibration curve. Based on the signals of ten blank solutions and the slope of calibration curves, it was found that the detection limits was 0.8 ng mL−1 . 3.2.6. Interference effect The capability of ligand for complex formation at optimum conditions were investigated. Interference of various cation and anions were examined by applying the method to a fixed amount of Cr(III) ion in the presence of various increments of the ion being used. The tolerance limit was taken
Interfering Ion
Concentration ratio of interfering ion to Cr(III) ion
Co2+ Ni2+ Pb2+ Ag+ Hg2+ Zn2+ Cd2+ Mg2+ K+ Ba2+ Fe2+ Fe3+ Cu2+ Al3+ Br− SCN− I− EDTA Cr2 O7 2−
550 550 1000 850 800 1000 1000 1000 1000 100 1000 800 800 1000 1000 1000 1000 1000 500
as the interfering ions amounts that cause an error of less than ± 5% in the absence of interference. Therefore various amount of interfering ion was added to 1 g mL−1 of Cr(III) ion and results are shown in Table 2. Results display that method has good selectivity even in the presence of copper, cobalt and nickel, that this unique selectivity if experiment were performed in alkali media could not be achieved. The results of Table 2 indicate high selectivity and sensitivity of method that may be contributed to incorporation of nitrogen and oxygen atom as donating accepting compound that act as hard acid in coincide to oxime group as known substituted with high tendency to Cr(III) ion which lead to high selectivity. The R.S.D. of method at 5 g mL−1 of Cr(III) ion is 0.3%. 3.3. Analytical application in real samples We have explored the feasibility of the methodology using it for the determination of Cr(III) ion in different matrices. The procedure were applied for the determination of Cr(III) ion in different samples, including river water, tap water and synthetic mixture sample. Reliability was checked by spiking experiments and independent analysis. To ensure that the method is valid and has reasonable accuracy and precision, recovery of the Cr(III) ions in the river water, the tab water and synthetic sample were determined by these proposed techniques and the results are shown in Table 3 has a good agree with reference AAS method. The low relative standard deviations represent the high reproducibility in these measurements. Therefore, this proposed technique could be applied to the determination of ng mL−1 level of Cr(III) in real samples.
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Table 3 Investigation of accuracy and precision of proposed methods for real sample analysis, 7 mM BO, pH 8.0, 5.5 × 10−4 Triton X-100 Cr(III) ion added (g mL−1 )
Sample
Cr(III) ion founded (g mL−1 ) ␣-BO
AAS
200 mL River water (A) A + 500 g
0.0 2.5
2.380 ± 0.003 5.894 ± 0.001
2.388 ± 0.003 5.898 ± 0.001
Tab water (B) B + 40 g
0.0 0.2
0.078 ± 0.005 0.282 ± 0.003
0.0.085 ± 0.004 0.289 ± 0.003
2.05 ± 0.002
1.98 ± 0.002
Cu(II) 2, Cr(III) 2, Hg(II) 2, Pb(II) 2, Co(II) 2, Ni(II) 2,Cd(II) 2a a
Numbers are ions concentration
0
(g mL−1 ).
Table 4 Specification of proposed method
References
Figures of merit
Optimum values
Ligand concentration (M) PH Linear range (g mL−1 ) Regression equation Surfactant and its concentration (M) Equilibration time and stability time of complex Solvent Selectivity Detection limit (ng mL−1 ) Accuracy and precision Advantages
0.007 8.0 0.1–13.7 A = 0.1446CCr3+ + 0.002 0.00055
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Disadvantages
20 days at least Water High 0.8 High High repeatability, sensitivity, selectivity, wide linear range and no need to organic solvent, its suitable for chromium speciation and cloud point extraction Do not preconcentrate
4. Conclusion Most of the spectrophotometric method for Cr(III) ion as its colored complex with the reagents, suffer from drawbacks including reagent cost, instability and impossibility of regeneration of the reagent fro re-use, small dynamic linear range, low sensitivity and selectivity. In order to cope with these difficulty, the proposed methods due to advantages such as high reliability, reproducibility, sensitivity, selectivity and high tolerance limit of common ions in the case of surfactant based method is a powerful tool for rapid and sensitive determination of Cr(III) ion in various media. Due to the mention advantages displayed in Table 4, these proposed methods have been successfully applied to the determination of Cr(III) ion at trace level. The low R.S.D. of real sample analysis is an indication of method versatility for real samples analysis. The characteristics performances of the method are shown in Table 4 indicate that each method has unique advantage over another method that complement for each other. Acknowledgement The authors gratefully acknowledge the support of this work by the University of Yasouj Research Council.
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Sensitized spectrophotometric determination of Cr(III) ion for speciation of chromium ion in surfactant media using ␣-benzoin oxime Mehrorang Ghaedi ∗ , Enayat Asadpour, Azam Vafaie Chemistry Department, Yasouj University, Yasouj 75914-353, Iran Received 27 January 2005; accepted 22 April 2005
Abstract A simple and accurate micellanized spectrophotometric method for determination of trace amounts of Cr(III) ion in tab and top water and a synthetic mixture has been described. The micellar method is based on effect of organized molecular assemblies such as micelles in spectrophotometric measurement due to their effect on the systems of interest. The ability of micellar system in solubilizing of sparingly soluble ligand or complexes has been used for increasing figures of merit of an analytical method. Due to solubility increasing in aqueous media requirement for a primary extraction can be eliminated. Using the ␣-benzoin oxime (␣-BO) spectrophotometric determination of Cr(III) ion has been performed and results are compared. The spectrophotometric determination of Cr(III) ion using ␣-BO in the presence of non-ionic surfactant Triton X-100 has been performed. The influence of type and amount of surfactant, pH, complexation time and amount of ligand were examined. Finally, the repeatability, accuracy and the effect of interfering ions on the determination of Cr(III) ion was evaluated. The proposed methods successfully with recovery yield of almost 100% have been applied to the rapid and simple determination of Cr(III) ion in the real samples. There is a good agreement between methods and atomic absorption spectrometry. The Beers law is obeyed over the concentration range of 0.1–13.7 g mL−1 for micellar media. The detection limit is 0.8 ng mL−1 . The molar absorptivity of complex is 5350 L mol−1 cm−1 . © 2005 Published by Elsevier B.V. Keywords: ␣-Benzoin oxime; Spectrophotometric method; Surfactant media; Triton X-100; Cr(III) ion
1. Introduction The importance of the determination of trace metal concentration in natural water samples is increasing in contamination monitoring studies. Chromium compounds are widely used by modern industries, resulting in large quantities of this element being discharged into the environment. Some of the main uses for chromium compounds are as follows: (1) plastic coating of surfaces for water and oil resistance, (2) electroplating of metal for corrosion resistance, (3) leather tanning and finishing and (4) in pigments and for wood preservative. Until a few years ago, it also was the favored corrosion control agent in cooling towers whose water blow down (waste) was dumped into rivers, pits, lakes ∗
Corresponding author. Tel.: +98 741 2223048; fax: +98 741 2223048. E-mail address: m [email protected] (M. Ghaedi).
1386-1425/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.saa.2005.04.049
and oceans. Chromium occurs in wastewaters resulting from these operations in both trivalent [Cr(III)] and hexavalent [Cr(VI)] forms. There is a rapidly increasing demand for fast and reliable analytical methods for the determination of chemical forms of elements, especially chromium ion, in environmental samples. The interest in chromium is governed by the fact that its toxicity depends critically on its oxidation state. While chromium(III) is considered essential for mammals for the maintenance of glucose, lipid and protein metabolism, chromium(VI) is known to be toxic to humans [1,2]. The rationale for this approach is based on the assumption that inorganic Cr(III) solids are very insoluble and Cr(III) is not considered to be toxic, whereas Cr(VI) is known to be toxic, mutagenic and carcinogenic [3]. However, the possibility that the presence of organic ligands and/or acidic conditions in the environment
M. Ghaedi et al. / Spectrochimica Acta Part A 63 (2006) 182–188
increases Cr(III) mobility, and also MnO2 , present in soil, could oxidize Cr(III) to the more toxic and mobile Cr(VI) forms. Therefore, it is of major concern to understand the behavior of chromium in natural aquatic systems. In view of the difference between the oxidation states, and in order to follow the pathways for inter conversion in the environment, it is increasingly important to monitor the concentration of the individual chemical species as well as the total concentration of chromium in the environment. In view of the marked differences in the chemical behavior and toxicity of chromium in its different oxidation states, numerous methods have been tried in the continuing effort to quantify the chromium species in natural waters. In recent years, the determination and speciation of chromium due to the different properties and toxicities of the chemical form of chromium have become very important [4]. The direct determination of chromium in water not is possible with sufficient sensitivity by also using expensive analytical method such as inductive coupled plasma emission spectroscopy or electro thermal atomic absorption spectrometry [5], because of low concentration and matrix interference. Due to the presence of chromium in environmental samples at low levels and matrix effects with notation to detection limit of lead with traditional method spectrophotometry, in addition to separation from matrix elements. Extractive and micellar sensitized spectrophotometric with ability to separate and preconcentrate of chromium solve these problems and lead to a higher confidence level and easy determination of the trace elements by less sensitive, but more accessible instrumentation such as spectrophotometer. For this purpose various preconcentration method of chromium in micellar media [6,7] and spectrophotometric and extraction method based on different ligands [8–20] has been described for chromium determination. Many of these methods are time consuming or require complicated and expensive instrument and some of them has low repeatability. Due to low concentration of Cr(III) ion content above mention problems, development of new sensitive and selective methods for selective, sensitive, rapid and convenient determination of this ion in sub-micron levels is still a challenging requirement. In recent years extractive spectrophotometric determination [21,22] and organic micellar media as powerful tools are very useful in analytical application, especially in UV–vis spectrophotometry [23–26] and fluorescence process, because they are stable in aqueous solution and transparent optically, enhance sensitivity and readily available [27]. The utility of surfactants in sensitized spectrophotometry emerged from the various pathway that among them formation of ternary complex with concomitant shift in the analytical wavelength [28,29] and increasing the solubility of insoluble complexes and/or ligands [30,31] has been benefited. In this view, the selectivity and sensitivity of numerous analytical spectrophotometric-based reactions can be improved using certain surfactants. In surfactant media complexes of metal ions with complexing agent are most stable and formed
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aggregates that cause an improvement in sensitivity and detection limits [27]. For determination of Cr(III) ion at trace level with UV–vis spectrometry various chelating agent have been used. Recently, sensitized spectrophotometric determination of Cr(III) ion based on surfactant media have been reported. In the present work, a simple and highly selective and sensitive spectrophotometric method for determination of Cr(III) ion using ␣-BO in surfactant media (Triton X-100) was established.
2. Experimental 2.1. Instrumentation A Shimadzu UV–vis 160 spectrophotometer was used to measure the absorbance of complex in Triton X-100 media. To adjust the pH and prepare the buffer solution a 691 pH/ion meter with a combined glass and calomel electrode has been used. 2.2. Reagent and solution All chemical such as nitrate of Cr(III) ion and other cation were of the analytical grade purchased from Merck Company. A 0.5% (w/v) all surfactant all from Merck Company including sodium dodecyl sulfonate (SDS), Triton X100, Brij 58, cetyltrimethylammonium bromide (CTMAB), n-dodecytrimethylammonium bromide (DTMAB) was prepared by dissolving 0.5 g of each surfactant in 100 mL volumetric flask with stirring. The ligand ␣-benzoin oxime was purchased from Merck Company. The chromium determinations were carried out on a Perklin–Elmer 603 atomic absorption spectrometer with a hallow cathode lamp and a deuterium background corrector at resonance line using an air–acetylene flame. 2.3. Procedure calibration curve for spectrophotometric in micellar media Standard Cr(III) ion solutions were prepared in the range of 0.01–25.0 g mL−1 . Several aliquots of Cr(III) ion were added to 10 mL volumetric flask and 0.35 mL 0.02 M ␣-BO and 0.7 mL of 7.8 mM of Triton X-100 were added to each flask and with phosphate buffer (pH 8.0) was filled to the mark and calibration curve of Cr(III) ion was constructed using a UV–vis spectrometer. According to IUPAC recommendation detection limit has been calculated. 2.4. Pretreatment of real samples Analysis waste water sample for determination of Cr(III) ions content was done as follow: 250 mL of water samples was poured in a beaker, while stirring, it heated to reach its volume to half. After adjustment of samples pH to desired
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value the spectrophotometric experiments were performed according to general described procedure. A synthetic sample was prepared. In all of real and synthetic sample amount of Cr(III) ion was found by standard addition method and a reference atomic absorption spectrometry.
sensitivity were examined. The composition of complex and their respective parameters including molar absorptivity and maximum wavelength were evaluated. At optimum conditions, time dependency of complex, repeatability, efficiency, effect of interference of other metal ions, accuracy and precision of developed methods for real sample analysis were investigated and evaluated.
3. Results and discussion 3.1. Optimizing condition for spectrophotometric determination of Cr(III) ion in micellar media in the presence of α-BO In preliminary experiments, it was shown that chromium interfere in copper determination with ␣-BO. It is occur to us that due to incorporation of nitrogen and oxygen atoms as donor and acceptor and low density of electrons in the ligand structure as hard base in comparison with chromium ion as hard acid, through the ion–dipole interaction between oxygen and chromium ion and donating accepting between them a rapid quantitative complexation occur. Due to these types of interaction, rate of complex formation is high and reversible. It seems that be useful for construction of calibration curve based on spectrophotometric determination of chromium ion. In preliminary experiments, complexation between Cr(III) ion and ␣-BO was examined using mole ratio method using UV–vis spectrophotometry and conductometry and results confirmed a stoichiometric relation Cr(III) ion to ␣-BO equal to 1:2. A sensitive and sharp peak at about 295 nm (where ligand and Cr(III) ion do not show any absorbance) indicates the strong interaction between Cr(III) ion and the ␣-BO which is shown in Fig. 1. In acidic media pH < 4, its oxime group (nitrogen atom) protonated and its capacity for complex formation with chromium reduced. The effect of various parameters such as pH, type and amount of surfactant, amount of ligand and ionic strength on
Fig. 1. Comparing spectra in the presence (A) and absence (B) of Triton X100 at optimum conditions of reagents and 12 g mL−1 Cr(III) ion, 7 mM BO, pH 8.0, 5.5 × 10−4 Triton X-100.
3.2. Absorption spectra of Cr (α-BO)2 in Triton X-100 media and composition of complex After Cr(III) ion, ␣-BO and Triton X-100 were added to a 10 mL volumetric flask so that their concentrations were 12 g mL−1 , 7 mM and 0.55 mM, respectively, the solution was diluted to the mark with phosphate buffer (pH 8.0). Then, the absorption spectrum of complex in the presence and absence of surfactant Triton X-100 was obtained, which is shown in Fig. 1. To study the composition of Cr(III)- ␣BO–Triton X-100, the mole ratio method for Cr(III) ion and ␣-BO in the presence and absence Triton X-100 were used. The mole ratio of ␣-BO and Cr(III) ion by continuous variation method has been calculated. The molar composition of ␣-BO-Cr(III)–Triton X-100 in the presence and absence of surfactant are as 2:1 and 2:1:1. A confirmation of this stoichiometric is inability of us for extraction of complex to any immiscible organic phase at optimum conditions in the presence and absence of Triton X-100. The molar absorptivity of binary and ternary complexes as slope of calibration curve found by least squares analysis of five analysis was 2.1 × 103 and 5.35 × 103 L mol−1 cm−1 . It can be concluded that application of surfactant media in analytical chemistry involves the beneficial alteration of metal ion–ligand complex spectral properties via surfactant association [29]. Experiment in micellar media has higher sensitivity and also no need to extraction of complex to organic phase and do not have disadvantages including time consuming, need to harmful organic solvent and labor intensive. The studies focused on investigation of complexation between Cr(III) ion and ␣-BO in surfactant media and evaluating optimum conditions for sensitized spectrophotometric determination of Cr(III) ion using ␣-BO. The stability and decomposition rate of complex seriously depend on pH and time. The low sensitivity and omitting spectra in acidic media is an indication. At optimum pH the time dependency and rate of complex formation was investigated. Investigation in surfactant media display that complex completely formed at less than 1 min and up to days is stable. 3.2.1. Effect of pH on sensitivity The influence of the test solution pH on the absorbance of complex (3.4 g mL−1 Cr(III) ion) in 5.5 × 10−4 M Triton X-100 media against a reagent blank was investigated. Results, which are shown in Fig. 2, display that complex showed the maximum absorption at pH 8.0 that 0.006 M phosphate buffer has maximum sensitivity and selected for further studies. From this experimental fact,
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Fig. 2. Effect of pH on sensitivity of Cr(III) ion in micellar media, 7 mM BO, 3.4 g mL−1 Cr(III) ion, 5.5 × 10−4 Triton X-100.
we determined that complex was quantitatively formed and well dissolved in Triton X-100 media at this pH. In more acidic media due to the ligand protonation and incomplete complex formation and in more alkaline media because of complex hydrolysis and hydroxide formation the absorbance reduced. 3.2.2. Effect of surfactant on sensitivity To investigate the effect of types of surfactants, Triton X100 and Brij 58 as non-ionic, sodium dodecylsulfate (SDS) as anionic surfactant and cetyltrimethyl ammonium bromide CTMAB) and n-dodecyltrimethyl ammonium bromide (DTMAB) as cationic surfactant on the above mention complex were examined. In 12 g mL−1 concentrations of Cr(III) ion and 8.0 × 10−3 M ␣-BO and 5.5 × 10−4 each surfactant effect of type of surfactant on spectra and sensitivity were examined and results displayed in Table 1. As can be seen from Table 1, for 5.5 × 10−4 Triton X-100 media, spectra with high sensitivity and red shift could be constructed and its slope was about two times more than in the absence of it, while the anionic or cationic surfactant showed no positive effect or diminished it. This suggests that above mention complex was dissolved in surfactant phase due to the hydrophobic solvation of the desired chelate. Therefore, Triton X-100 was selected for further studies. This observation that in the presence of non-ionic surfactant, method had high sensitivity is
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an indication that surfactant lead to aggregation of complexes and increase in sensitivity. Because a metal ion is a cation, the electrostatic attractive interaction between a metal ion and cationic surfactant is not present, and the complex-forming process is not affected. It seems that ␣-BO combines with Cr(III) ion to form a low polar complexes, and the complexes is extracted instantaneously into the local no polar environment of micelle of non-ionic surfactant. We saw that although the concentration of the Triton X-100 were somewhat less than micelle concentration, since further addition lead to precipitation, mention complexes was homogenously dissolved in surfactant media. The 5.5 × 10−4 of Triton X100 has been used for further studies. We saw that although the concentration of the Triton X-100 were somewhat less than critical micelle concentration (0.0082 M, 0.05%), mention a complex was homogenously dissolved in surfactant media. The 5.5 × 10−4 of Triton X-100 has been used for further studies. 3.2.3. Effect of Triton X-100 concentration In order to stabilize the colored complex formed and to increase the sensitivity, the various concentration of Triton X-100 was added to solutions at optimum and sensitivity was examined. Results are shown in Fig. 3. As shown in Fig. 3, the increasing the concentration of Triton X-100 up to 5.5 × 10−4 as non-ionic surfactant was most effective in improving absorbance and the absorbance development was stable and reproducible. With the concentration of Triton X-100 varying from 1 × 10−5 to 5.5 × 10−4 at pH 8.0 for ␣-BO the absorbance of desired complexes at concentration of 3.4 g mL−1 Cr(III) ion was investigated. The maximum absorbance was obtained when the concentration of Triton X-100 was 5.5 × 10−4 M. We assumed that although the concentration of the surfactant was somewhat less than critical micelles concentration complex was homogeneously dissolved in surfactant media.
Table 1 Effect of type of surfactant on spectra and sensitivity, 12 g mL−1 Cr(III) ion, 7 mM BO, pH 8.0, 5.5 × 10−4 surfactant Surfactant
Absorbance
Maximum wavelength (nm)
Brij CTAB DTAB SDS Triton X-100
1.12 0.64 0.66 0.89 1.39
293 284 284 287 295
Fig. 3. Effect of Triton X-100 concentration on sensitivity at optimum condition, 7 mM BO, 3.4 g mL−1 Cr(III) ion, pH 8.0.
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M. Ghaedi et al. / Spectrochimica Acta Part A 63 (2006) 182–188 Table 2 Investigation of tolerance limit of interfering ions on proposed methods at 1 g mL−1 Cr(III) ion, 7 mM BO, pH 8.0, 5.5 × 10−4 Triton X-100
Fig. 4. Effect of ␣-BO concentration on sensitivity at optimum conditions, 3.4 g mL−1 Cr(III) ion, 5.5 × 10−4 Triton X-100, pH 8.0.
3.2.4. Effect of α-BO concentration on sensitivity The analytical sensitivity and the reproducibility in the complex spectra were good in micellar media. It is known that Cr(III) ion interact stoichiometrically with ␣-BO to form a 1: 2 complexes and confirmed with conductometrically and spectrophotometry. For evaluating and investigating the effect of the chelating agent concentration various amount of ligand was added to similar solution at optimum condition. Results which are shown in Fig. 4 display 0.007 M ligand concentration lead to high sensitivity. For metal complexes to be formed quantitatively, one must added mor chelating agent to the sample solution. Therefore, sorbent ␣-BO was added more than 50 times Cr(III) ion concentration to reduce fluctuation in measurement of absorbance. 3.2.5. Validity of beer’s law, molar absorptivity, sandell s sensitivity, correlation coefficient and detection limit The calibration curves were prepared for the determination of Cr(III) ion by using sample solutions at the optimum conditions of method. A calibration curve was constructed at optimum conditions according to calibration curve procedure in Experimental Section. Beers law is obeyed in the dynamic range of Cr(III) was 0.1–13.7 g mL−1 with equation of A = 0.1464CCr(III) + 0.0023 where C is Cr(III) concentration as g mL−1 . The correlation coefficient (R2 ) was 0.9991, showing good linearity of calibration curve. Based on the signals of ten blank solutions and the slope of calibration curves, it was found that the detection limits was 0.8 ng mL−1 . 3.2.6. Interference effect The capability of ligand for complex formation at optimum conditions were investigated. Interference of various cation and anions were examined by applying the method to a fixed amount of Cr(III) ion in the presence of various increments of the ion being used. The tolerance limit was taken
Interfering Ion
Concentration ratio of interfering ion to Cr(III) ion
Co2+ Ni2+ Pb2+ Ag+ Hg2+ Zn2+ Cd2+ Mg2+ K+ Ba2+ Fe2+ Fe3+ Cu2+ Al3+ Br− SCN− I− EDTA Cr2 O7 2−
550 550 1000 850 800 1000 1000 1000 1000 100 1000 800 800 1000 1000 1000 1000 1000 500
as the interfering ions amounts that cause an error of less than ± 5% in the absence of interference. Therefore various amount of interfering ion was added to 1 g mL−1 of Cr(III) ion and results are shown in Table 2. Results display that method has good selectivity even in the presence of copper, cobalt and nickel, that this unique selectivity if experiment were performed in alkali media could not be achieved. The results of Table 2 indicate high selectivity and sensitivity of method that may be contributed to incorporation of nitrogen and oxygen atom as donating accepting compound that act as hard acid in coincide to oxime group as known substituted with high tendency to Cr(III) ion which lead to high selectivity. The R.S.D. of method at 5 g mL−1 of Cr(III) ion is 0.3%. 3.3. Analytical application in real samples We have explored the feasibility of the methodology using it for the determination of Cr(III) ion in different matrices. The procedure were applied for the determination of Cr(III) ion in different samples, including river water, tap water and synthetic mixture sample. Reliability was checked by spiking experiments and independent analysis. To ensure that the method is valid and has reasonable accuracy and precision, recovery of the Cr(III) ions in the river water, the tab water and synthetic sample were determined by these proposed techniques and the results are shown in Table 3 has a good agree with reference AAS method. The low relative standard deviations represent the high reproducibility in these measurements. Therefore, this proposed technique could be applied to the determination of ng mL−1 level of Cr(III) in real samples.
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Table 3 Investigation of accuracy and precision of proposed methods for real sample analysis, 7 mM BO, pH 8.0, 5.5 × 10−4 Triton X-100 Cr(III) ion added (g mL−1 )
Sample
Cr(III) ion founded (g mL−1 ) ␣-BO
AAS
200 mL River water (A) A + 500 g
0.0 2.5
2.380 ± 0.003 5.894 ± 0.001
2.388 ± 0.003 5.898 ± 0.001
Tab water (B) B + 40 g
0.0 0.2
0.078 ± 0.005 0.282 ± 0.003
0.0.085 ± 0.004 0.289 ± 0.003
2.05 ± 0.002
1.98 ± 0.002
Cu(II) 2, Cr(III) 2, Hg(II) 2, Pb(II) 2, Co(II) 2, Ni(II) 2,Cd(II) 2a a
Numbers are ions concentration
0
(g mL−1 ).
Table 4 Specification of proposed method
References
Figures of merit
Optimum values
Ligand concentration (M) PH Linear range (g mL−1 ) Regression equation Surfactant and its concentration (M) Equilibration time and stability time of complex Solvent Selectivity Detection limit (ng mL−1 ) Accuracy and precision Advantages
0.007 8.0 0.1–13.7 A = 0.1446CCr3+ + 0.002 0.00055
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Disadvantages
20 days at least Water High 0.8 High High repeatability, sensitivity, selectivity, wide linear range and no need to organic solvent, its suitable for chromium speciation and cloud point extraction Do not preconcentrate
4. Conclusion Most of the spectrophotometric method for Cr(III) ion as its colored complex with the reagents, suffer from drawbacks including reagent cost, instability and impossibility of regeneration of the reagent fro re-use, small dynamic linear range, low sensitivity and selectivity. In order to cope with these difficulty, the proposed methods due to advantages such as high reliability, reproducibility, sensitivity, selectivity and high tolerance limit of common ions in the case of surfactant based method is a powerful tool for rapid and sensitive determination of Cr(III) ion in various media. Due to the mention advantages displayed in Table 4, these proposed methods have been successfully applied to the determination of Cr(III) ion at trace level. The low R.S.D. of real sample analysis is an indication of method versatility for real samples analysis. The characteristics performances of the method are shown in Table 4 indicate that each method has unique advantage over another method that complement for each other. Acknowledgement The authors gratefully acknowledge the support of this work by the University of Yasouj Research Council.
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