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Composition-ratio determination of Chrome Azurol S (CAS) with Cu2+ and research on dynamic reaction process of CAS with EDTA
Spectrochimica Acta Part A 71 (2008) 1021–1026
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Composition-ratio determination of Chrome Azurol S (CAS) with Cu2+ and research on dynamic reaction process of CAS with EDTA Rutao Liu a,∗ , Ling Li a,b,1 , Yanmin Tian a,1 , Zhaohui Tan c , Canzhu Gao a , Xiaopeng Hao d a
School of Environmental Science and Engineering, Shandong University, Jinan, Shandong 250100, PR China State Key Laboratory of Pollution Control and Resources Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China c Chemistry group, Experimental Middle School of Zoucheng, Zoucheng, Shandong 273500, PR China d State Key laboratory of Crystal Material, Shandong University, Jinan 250100, PR China b
a r t i c l e
i n f o
Article history: Received 17 January 2008 Received in revised form 18 February 2008 Accepted 28 February 2008 Keywords: Cu2+ CAS EDTA Dynamic process
a b s t r a c t The composition-ratio of Chrome Azurol S (CAS) with Cu2+ and dynamic reaction process of CAS with EDTA were studied by spectrophotometry. The composition-ratio of Cu2+ with CAS (2:1) was successfully determined using the method of lines and EDTA complexing substitution. The effects of temperature, time, pH, concentration of cetyltrimethylammonium bromide (CTMAB), and concentration of Cu2+ on the absorption spectrum were also discussed. By optimizing experimental conditions, the dynamic process of displacement of CAS, which forms the ternary compound Cu–CAS–CTMAB, by EDTA were determined. The test results indicated that EDTA can replace 99.6% of the CAS quickly when heated. The system reached equilibrium finally. This research method can be applied to the qualitative and quantitative analysis of related Cu complexes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Chrome Azurol S (CAS) belongs to a class of triphenylmethane chromogenic reagents. It is the widest-used analytical reagent [1,2] (Support information Fig. 1a is its structure). CAS and surfactants can react with metallic ions and form ternary complexes. Quantitative and qualitative analysis of multi-metals can be done by the above reaction [3,4]. In recent years it has been demonstrated that the formation of mixed micelle of surfactant can improve the sensitivity and selectivity of determination [5]. In addition, the sensitivity and selectivity of determination can also be enhanced by the reaction of mixed coordination compounds with surfactant to form quaternary complex [6]. This has become a new field of coordination chemistry and has been widely used. Since the early 1850s, as a metal complexing agent, EDTA has been widely used in many fields [7]. The free acid and other forms of salt of the EDTA have been widely used in food and environmental science research [8–14]. The molecular weight of EDTA is 292.1 (Support information Fig. 1b is its structure). It has four acid protons and is slightly sol-
uble in water [15]. Its disodium salt is soluble in water and is used more. The real effect of EDTA is that it can form complex compounds with 62 different kinds of metal cations [16] (1:1). Owing to its powerful complex ability, it can displace other ligands from their metal complexes to form metal–EDTA complex [17]. But the composition-ratio of Cu with CAS and the mechanism research of displacement of CAS from Cu–CAS complex by EDTA have not been reported. A new strategy for composition-ratio of Cu with CAS determination has been developed with spectrophotometry. Also with spectrophotometry, the dynamic reactive process of EDTA with CAS in Cu–CAS–CTMAB complex has been determined. The instruments used in this experiment are simple and the reagents are reasonable. At the same time, the experiment reduces the production of pollutants coming from the analysis process. So it is an environmental-friendly analysis method. This research can provide thought and method for the qualitative and quantitative analysis of Cu complex. 2. Experiments 2.1. Instruments and materials
∗ Corresponding author. Tel.: +86 531 88364868; fax: +86 531 88364868. E-mail address: [email protected] (R. Liu). 1 Both authors made equal contributions to this work and share the second authorship. 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.02.044
Shimadzu UV–vis spectrophotometer 2450 (Shimadzu Company in Japan) with 1 cm quartz absorption cell, pHs-3C pH meter (Shanghai Pengshun scientific instruments Co., Ltd.), and Electronic
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Table 1a Effect of change of concentration of CAS on composition-ratio and absorbance CAS (×10−5 mol/L) Absorbance Maximum absorption wavelength (nm) Cu:CAS
2.4 0.030 632.4 2:1
4.8 0.716 597.4 1:1
9.6 2.310 596.6 1:2
14.4 3.250 597.6 1:3
19.2 3.551 598.2 1:4
24 3.588 600.6 1:5
28.8 3.305 609.2 1:6
a role of micelle sensitization enhancement. It makes the electron orbit of the chromogenic reagent appear like a dipole interaction, which narrows the energy gap between the ground states and excitation states [18,19]. The absorption spectrum of Cu–EDTA was far less than the Cu–CAS–CTMAB and the absorption spectrum of Cu–CAS–CTMAB was far less than the Cu–CAS–CTMAB under the same condition. This showed that a part of CAS has been displaced by EDTA.
3.2. Determination of composition-ratio of Cu with CAS
Fig. 1. Absorption spectra. (a) Cu–CAS, (b) Cu–CAS–CTMAB, (c) Cu–EDTA–CAS–CTMAB (heated in water bath for 4 min at 85 ◦ C), (d) Cu–EDTA. Con−4 −5 −4 ditions: CuSO4 : 6.25 × 10 mol/L; CAS: 4.0 × 10 mol/L; CTMAB: 3.0 × 10 mol/L; EDTA: 4.25 × 10−4 mol/L.
balance FA2004B (Shanghai Precise scientific instruments Co., Ltd.) are used. CuSO4 (Tianjin Kemiou Chemical Reagents Development Centre), CAS (Tianjin Institute of Chemical Reagents), Cetyltrimethylammonium Bromide (CTMAB) (Guoyao Group Chemical Reagents Co., Ltd.), EDTA solution: EDTA disodium (Tianjin Tianda Chemical Experiment Plant), acetic acid (Tianjin Standard Science and Technology Co., Ltd.), and sodium acetate (Tianjin Beihong Reagents Plant). The reagents mentioned above are all analytically pure. Water used in the experiment is deionized water. 3. Results and discussion 3.1. Absorption spectrum With the blank solution as reference, the absorption spectrum curves of the Cu–CAS, Cu–CAS–CTMAB, and Cu–CAS–CTMAB–EDTA were determined by scanning the systems between 400 nm and 700 nm at the speed of 200 nm/min. The results of the experiment are shown in Fig. 1. From Fig. 1, we know that blue-purple complex was formed by Cu and CAS. Its maximum absorption wavelength is located in 582.8 nm. The blue-green complex was formed by Cu, CAS, and CTMAB and its maximum absorption wavelength is 593.6 nm. After the surfactant was added, the wavelength moved toward infrared region. This might be because the cationic surfactant CTMAB plays
3.2.1. Determination using method of lines There are many methods to determine composition-ratio of complexes, mainly including mole ratio method, continuous variation method, slope ratio method, and method of lines. The first three are mainly used to determine the stable complex whose dissociation degree is smaller. Because of the stability of ternary complexes and the purity of the reagents used, mole ratio method cannot be used to determine composition-ratio of complexes. Continuous variation method and slope ratio method often have big errors. With 4.7 × 10−5 mol/L CuSO4 , 3.0 × 10−4 mol/L CTMAB, and HAc–NaAc buffer solution whose pH is 6.0, the concentration of CAS was changed according to Table 1. The maximum absorbance was determined, respectively, by scanning the system between 550 nm and 700 nm at the speed of 200 nm/min. The results are shown in Fig. 1a. Same as the experiment above, the concentration of CAS was changed with the concentration of Cu was 2.3 × 10−5 mol/L. The results of the experiment are shown in Table 1b. It is clear from Table 1a, that when the concentration of Cu ion was 4.7 × 10−5 mol/L and CAS was more than 9.6 × 10−5 mol/L, the absorbance A was much higher than 1. The results might deviate from Beer–Lambert law and were not suitable for data analysis because the error was large. So forsake it. Then we chose that the concentration of Cu ion was 2.3 × 10−5 mol/L and the curve (1/A to 1/Vb ) can be gained. The results are shown in Fig. 2a–f. When b = 0.5, 1, 2, 3, 4, and 5, respectively, the correlation coefficient which equals to 0.9909, 0.9806, 0.9249, 0.8767, 0.8482, and 0.8330 correspondingly could be gained using the least square method calculating the curves in Fig. 2.
3.2.2. Determination using chelated replacement by EDTA method EDTA has higher complex ability. It can displace CAS from Cu–CAS complexes to form ligand that is more stable. But when redundant Cu2+ exists in Cu–CAS–CTMAB system, EDTA will complex with redundant Cu2+ first. With the increasing of EDTA, the absorbance of the system increased. All free Cu2+ complex with EDTA when the absorbance of the system reaches its maximum.
Table 1b Effect of different composition-ratios of Cu with CAS on absorbance CAS (×10−5 mol/L) Absorbance Maximum absorption wavelength (nm) Cu:CAS
1.2 2.4 4.8 0.012 0.020 0.040 No peak, select points to determine, = 626 2:1 1:1 1:2
7.2 0.370 601.8 1:3
9.6 0.957 601.8 1:4
12 1.203 604.2 1:5
14.4 1.343 606.8 1:6
R. Liu et al. / Spectrochimica Acta Part A 71 (2008) 1021–1026
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Fig. 2. (a) Determination of Cu:CAS by straight-line method (b = 0.5, R2 = 0.9909), (b) Determination of Cu:CAS by straight-line method (b = 1, R2 = 0.9806), (c) Determination of Cu:CAS by straight-line method (b = 2, R2 = 0.9249), (d) Determination of Cu:CAS by straight-line method (b = 3, R2 = 0.8767), (e) Determination of Cu:CAS by straight-line method (b = 4, R2 = 0.8482), (f) Determination of Cu:CAS by straight-line method (b = 5, R2 = 0.8330).
That is:
The results indicated above: method of lines and method of chelated replacement by EDTA show good agreement when they determine composition-ratio of Cu with CAS.
Cu(free) + EDTA + Cu–CAS → Cu–EDTA + Cu–CAS The concentration of EDTA was changed in the solution, which contained 7.8 × 10−5 mol/L Cu2+ , 4.0 × 10−5 mol/L CAS, and 3.0 × 10−4 mol/L CTMAB. The pH of the solution was 6. The maximum attraction luminosities were determined by scanning the system between 550 nm and 700 nm at the speed of 200 nm/min. The results are shown in Table 2.As shown in Table 2, the absorbance of the system reached its maximum when the concentration of EDTA was 2 × 10−6 mol/L. Because the composition-ratio of Cu with EDTA is 1:1 [20], the concentration of free Cu2+ in the system was 2.0 × 10−6 mol/L, which equals 2.5 × 10−5 mol/L × 0.8 mL/10 mL. The concentration of Cu2+ that coordinated with CAS was 7.6 × 10−5 mol/L, which equals 7.8 × 10−5 mol/L–2.0 × 10−6 mol/L. So Cu:CAS = 7.6 × 10−5 mol/L:4 × 10−5 mol/L = 1.9:1 ≈ 2:1.
3.3. Optimization of experimental conditions of replacement of CAS from Cu–CAS–CTMAB by EDTA We come to know from the results above, that the compositionratio of Cu ion with CAS is 2:1. When Cu2+ was excessive, the complexation of free Cu and EDTA occurred first, which increased the absorbance of the system. And when all free Cu2+ complexed in the reaction, owing to its strong complex ability, EDTA could displace CAS out of Cu–CAS–CTMAB to form Cu–EDTA–CTMAB complex which is more stable. Then the system reached equilibrium state. The optimization and selection of the experiment are described below.
Table 2 Effect of different concentrations of EDTA on maximum absorbance of the system of Cu–CAS–CTMAB Concentration of EDTA (×10−7 mol/L) Absorbance (A) Maximum absorption wavelength (nm)
0.0 0.610 597.4
2.5 0.632 597.1
12.5 0.633 598.6
20 0.642 598.1
22.5 0.506 594.2
25.0 0.499 599.1
3.0 0.468 611.7
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Table 3 Effect of reaction time on the absorbance and absorption wavelength of Cu–EDTA–CAS–CTMAB system Time (min) Absorbance Maximum absorption wavelength (nm)
0 0.659 601.4
3.3.1. Reaction time and temperature of the system to reach equilibrium at room temperature It requires some time for the replacement of CAS from Cu–CAS–CTMAB by EDTA reaching equilibrium at room temperature. When the concentrations of Cu2+ , CAS, CTMAB and EDTA were 7.8 × 10−4 mol/L, 2.0 × 10−5 mol/L, 1.0 × 10−4 mol/L, and 5.0 × 10−4 mol/L, respectively, and pH was 5.29 after different reaction time, the maximum attraction luminosities of the system were determined by scanning the system between 550 nm and 700 nm at the speed of 200 nm/min. The results are shown in Table 3. From Table 3, we know that when the system was placed at room temperature for 94 min, it was far from equilibrium. Because the time for the system to reach equilibrium is long, heating is needed for shortening the equilibrium time. Six colorimetric tubes, which contain the same solution, were heated at different temperatures for 4 min. The maximum attraction luminosity and absorption wavelength of the system were determined by scanning them between 550 nm and 700 m. The results are shown in Fig. 3a. From Fig. 3a, we know that heating temperature influences the absorbance of reaction system much. The absorbance decreased
Fig. 3. (a) Effect of temperature on absorbance and wavelength. Conditions: Cu2+ = 7.8 × 10−4 mol/L; CAS = 2.0 × 10−5 mol/L; CTMAB = 1.0 × 10−4 mol/L; EDTA = 5.0 × 10−4 mol/L; pH 5.29, Scan rate = 200 nm/min. (b) Effect of different heating time on absorbance and wavelength under 85 ◦ C. Conditions: Cu2+ = 7.8 × 10−4 mol/L; CAS = 2.0 × 10−5 mol/L; CTMAB = 1.0 × 10−4 mol/L; EDTA = 5.0 × 10−4 mol/L; pH 5.29.
12 0.632 601.2
24 0.619 601.6
48 0.582 601.6
94 0.543 601.2
rapidly with the increase of the temperature. When heating temperature reached 85 ◦ C, the system reached balance basically. From perspective of energy conservation, we chose 85 ◦ C as the heating temperature. Solutions, which contained the same substance were heated at 85 ◦ C for 0 min, 1 min, 2 min, 4 min, and 5 min, respectively, and then the maximum attraction luminosities were determined by scanning the system between 550 nm and 700 nm at the speed of 200 nm/min. The results are shown in Fig. 3b. From Fig. 3b, we know that the system did not reach equilibrium when not heated. Absorbance decreased rapidly with the increase of heating time. When the heating time was 4 min, the system reached equilibrium, and the maximum absorption wavelength of the system was about 598 nm under heated. So we determined the absorbance of the system at the wavelength of 598 nm under the condition of heating the system for 4 min at the temperature of 85 ◦ C.
3.3.2. Effect of pH on the absorbance of the Cu–CAS–CTMAB system The maximum absorption of the system was done at the wavelength of 589 nm by changing the pH, while all other factors remained fixed. Results are shown in Fig. 4. From Fig. 4, we know that with the increase of pH the absorbance increased gradually. The absorbance decreased greatly when the acidity of the solution was strong. Due to the increase of hydrogen ion concentration, less organic acids existed in anion state, which weakened their coordination balance of Cu2+ with negative charged organic acids. It is clear from Fig. 4 that the absorbance jumped when pH changed from 3.34 to 4.18. But the absorbance had a less change when the range of pH was 5.24–6.08. So we chose the optimal pH 5.24 as the optimal pH for determination. The absorbance decreased when pH reached 10.00. This accounts for that hydroxyl ions competed with complexing agent to coordinate with Cu2+ . When the alkalinity was too strong, a large number of hydroxyl ion combined with Cu2+ to form precipitation, which was not suitable for the research conditions of this technology [21].
Fig. 4. Effect of pH on absorbance of Cu–CAS–CTMAB system. Conditions: Cu2+ = 7.8 × 10−4 mol/L; CAS = 2 × 10−5 mol/L; CTMAB = 1 × 10−4 mol/L.
R. Liu et al. / Spectrochimica Acta Part A 71 (2008) 1021–1026
Fig. 5. Effect of concentration of CTMAB on absorbance of Cu–CAS–CTMAB system. Conditions: Cu2+ = 7.8 × 10−4 mol/L; CAS = 4 × 10−5 mol/L; pH 5.24.
3.3.3. Effect of the concentration of CTMAB on the absorbance of the Cu–CAS–CTMAB system The maximum attraction luminosities of the system were determined, respectively, at the wavelength of 598 nm while the concentration of CTMAB changed. The results are shown in Fig. 5. It is clear from Fig. 5 that the absorbance of the system was large and stable when the concentration of CTMAB was between 2 × 10−4 mol/L and 4 × 10−4 mol/L. The volume added was between 0.4 mL and 0.8 mL. When the concentration was lower than 2 × 10−4 mol/L, CTMAB could not play the role of micellar solubilization and sensitization enhancement and the solution was turbid. The absorbance of the binary system of Cu–CAS without CTMAB was low. When the concentration was higher than 4 × 10−4 mol/L, the absorbance decreased sharply with the increase in concentration. This might be because of the pseudo homogeneous extraction of CAS by cation surfactant CTMAB micelle. This reaction decreased the concentration of CAS in water and destroyed the complex equilibria of Cu–CAS–CTMAB. So the absorbance decreased sharply [19]. We chose 3 × 10−4 mol/L as the concentration of CTMAB.
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Fig. 7. The metathesis curve of CAS in Cu–CAS–CTMAB system by EDTA. (a) EDTA = 0∼1.5 × 10−4 mol/L; (b) EDTA = 1.5 × 10−4 ∼5 × 10−4 mol/L; Conditions: Cu = 6.25 × 10− 4 mol/L; (c) EDTA = 5 × 10−4 ∼7.5 × 10−4 mol/L. CAS = 4 × 10−5 mol/L; CTMAB = 3 × 10−4 mol/L; pH 5.24; t = 85 ◦ C.
3.3.4. Effect of the concentration of Cu2+ on the absorbance of Cu–CAS–CTMAB system The maximum attraction luminosities of the system were determined at the wavelength of 598 nm while the concentration of Cu2+ changed. The results are shown in Fig. 6. It is clear from Fig. 6 that the absorbance of the Cu–CAS–CTMAB system increased with the increase of Cu2+ . The absorbance reached its maximum when the concentration of Cu2+ was 6.25 × 10−4 mol/L. So we chose 6.25 × 10−4 mol/L as the concentration of Cu2+ . 3.4. The experiment of displacing CAS in Cu–CAS–CTMAB system by EDTA The maximum attraction luminosities of the system were determined, at the wavelength of 598 nm while the concentration of EDTA changed. The results are shown in Fig. 7. Fig. 7 shows the whole process of displacement of CAS in Cu–CAS–CTMAB by EDTA. In segment a, the absorbance of the system increased little when the concentration of EDTA ranged from 0 mol/L to 1.5 × 10−4 mol/L. The reason was that EDTA complexes with Cu2+ first, if free Cu2+ exists in the system. In segment b, when EDTA complexed with all free Cu2+ , its strong complexing capacity made it to displace CAS from Cu–CAS. Because the absorbance of Cu–EDTA complex is much less than Cu–CAS complex, so the absorbance of the system decreased continuously. The absorbance of the system decreased significantly when the concentration of EDTA was more than 3.75 × 10−4 mol/L, and the majority of CAS has already been displaced by EDTA. As shown in segment c, when the concentration of EDTA was 5.00 × 10−4 mol/L, the system almost reached balance and the absorbance tended to be stable. The system might be a mixture of Cu–CAS and Cu–CAS–CTMAB or the system might contain Cu–CAS complex and free CAS, and CTMAB. In the following experiment we determined whether Cu–CAS–CTMAB ternary complex exists in the system after reaction. 3.5. Composition of the system after reaction
Fig. 6. Effect of the concentration of Cu on the absorbance of Cu–CAS–CTMAB system. Conditions: CAS = 4 × 10−5 mol/L; CTMAB = 3 × 10−4 mol/L; pH 5.24.
3.5.1. Determination of the existence of Cu–CAS–CTMAB ternary complex The concentration of EDTA was changed with the concentration of Cu2+ being 6.2 × 10−4 mol/L and pH was 5.24. Then the maxi-
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Table 4 Effect of the concentration of EDTA on absorbance of the system Concentration of EDTA (×10−4 mol/L) Absorbance
5.50 0.011
6.00 0.010
6.75 0.009
7.00 0.008
7.25 0.008
7.50 0.008
mum attraction luminosities were determined at the wavelength of 598 nm. The results are shown in Table 4. From the comparison between Table 4 and Fig. 7, we know that the absorbance of the Cu–EDTA 0.008 was much less than 0.085, which was the absorbance value of original system. This showed that EDTA has not replaced all CAS in Cu–CAS–CTMAB in reaction process, and there was a certain amount of Cu–CAS–CTMAB in the system, which has already reached equilibrium.
method. This can provide evidence for study of the complex of Cu with CAS in the future. When free Cu2+ exists in Cu–CAS–CTMAB system, EDTA complexes with free Cu2+ first. With increased EDTA, it can displace 99.6% of CAS from Cu–CAS–CTMAB. The system reaches equilibrium finally and contains complex of Cu–EDTA and Cu–CAS–CTMAB.
3.5.2. Determination of replacement amount of CAS after reaction The equation of this reaction is:
This work is supported by Natural Science Foundation of China (20607011), Program for New Century Excellent Talents in University (NCET-06-0582), SRF for ROCS, SEM (Liu RT), and Mid-young Scientists Awarding Foundation in Shandong Province (2007BS08005).
Cu–CAS–CTMAB + Cu(Free) + EDTA → Cu–CAS–CTMAB + Cu–EDTA + CAS + CTMAB It is clear from Fig. 7 that the absorbance of the system was 0.085 when the added amount of EDTA was 7.5 × 10−4 mol/L. We supposed that all Cu2+ complex with EDTA. It is clear from Table 4 that the absorbance of Cu–EDTA was 0.008 under the same condition and the absorbance of CAS–CTMAB was 0.022. So the absorbance of Cu–CAS–CTMAB was 0.055, which equals 0.085–0.008–0.022. According to the working cure of Cu2+ , the concentration of Cu2+ was 2.5 × 10−6 mol/L, that is to say the concentration of Cu–CAS was 2.5 × 10−6 mol/L. We have known that the concentration of Cu–CAS was 6.25 × 10−4 mol/L in the system before reaction, so the replacement ratio of EDTA was 99.6%, which equals 1–2.5 × 10−6 /6.25 × 10−4 . The result showed that EDTA has already replaced the majority of CAS. So the hypothesis is correct. 4. Conclusions The replacement of CAS from Cu–CAS–CTMAB by EDTA under the optimum test conditions and methods was studied in this paper. Conclusions are as follows: The ternary system that has much more superior performance can be determined by study on Cu–CAS–CTMAB ternary system and Cu–CAS ternary system. The system’s absorbance increased significantly and its maximum absorption wavelength was redshifted. The composition-ratio of Cu2+ with CAS, which is 2:1 was determined by method of lines and chelated replacement by EDTA
Acknowledgements
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Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Composition-ratio determination of Chrome Azurol S (CAS) with Cu2+ and research on dynamic reaction process of CAS with EDTA Rutao Liu a,∗ , Ling Li a,b,1 , Yanmin Tian a,1 , Zhaohui Tan c , Canzhu Gao a , Xiaopeng Hao d a
School of Environmental Science and Engineering, Shandong University, Jinan, Shandong 250100, PR China State Key Laboratory of Pollution Control and Resources Reuse, School of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China c Chemistry group, Experimental Middle School of Zoucheng, Zoucheng, Shandong 273500, PR China d State Key laboratory of Crystal Material, Shandong University, Jinan 250100, PR China b
a r t i c l e
i n f o
Article history: Received 17 January 2008 Received in revised form 18 February 2008 Accepted 28 February 2008 Keywords: Cu2+ CAS EDTA Dynamic process
a b s t r a c t The composition-ratio of Chrome Azurol S (CAS) with Cu2+ and dynamic reaction process of CAS with EDTA were studied by spectrophotometry. The composition-ratio of Cu2+ with CAS (2:1) was successfully determined using the method of lines and EDTA complexing substitution. The effects of temperature, time, pH, concentration of cetyltrimethylammonium bromide (CTMAB), and concentration of Cu2+ on the absorption spectrum were also discussed. By optimizing experimental conditions, the dynamic process of displacement of CAS, which forms the ternary compound Cu–CAS–CTMAB, by EDTA were determined. The test results indicated that EDTA can replace 99.6% of the CAS quickly when heated. The system reached equilibrium finally. This research method can be applied to the qualitative and quantitative analysis of related Cu complexes. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Chrome Azurol S (CAS) belongs to a class of triphenylmethane chromogenic reagents. It is the widest-used analytical reagent [1,2] (Support information Fig. 1a is its structure). CAS and surfactants can react with metallic ions and form ternary complexes. Quantitative and qualitative analysis of multi-metals can be done by the above reaction [3,4]. In recent years it has been demonstrated that the formation of mixed micelle of surfactant can improve the sensitivity and selectivity of determination [5]. In addition, the sensitivity and selectivity of determination can also be enhanced by the reaction of mixed coordination compounds with surfactant to form quaternary complex [6]. This has become a new field of coordination chemistry and has been widely used. Since the early 1850s, as a metal complexing agent, EDTA has been widely used in many fields [7]. The free acid and other forms of salt of the EDTA have been widely used in food and environmental science research [8–14]. The molecular weight of EDTA is 292.1 (Support information Fig. 1b is its structure). It has four acid protons and is slightly sol-
uble in water [15]. Its disodium salt is soluble in water and is used more. The real effect of EDTA is that it can form complex compounds with 62 different kinds of metal cations [16] (1:1). Owing to its powerful complex ability, it can displace other ligands from their metal complexes to form metal–EDTA complex [17]. But the composition-ratio of Cu with CAS and the mechanism research of displacement of CAS from Cu–CAS complex by EDTA have not been reported. A new strategy for composition-ratio of Cu with CAS determination has been developed with spectrophotometry. Also with spectrophotometry, the dynamic reactive process of EDTA with CAS in Cu–CAS–CTMAB complex has been determined. The instruments used in this experiment are simple and the reagents are reasonable. At the same time, the experiment reduces the production of pollutants coming from the analysis process. So it is an environmental-friendly analysis method. This research can provide thought and method for the qualitative and quantitative analysis of Cu complex. 2. Experiments 2.1. Instruments and materials
∗ Corresponding author. Tel.: +86 531 88364868; fax: +86 531 88364868. E-mail address: [email protected] (R. Liu). 1 Both authors made equal contributions to this work and share the second authorship. 1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2008.02.044
Shimadzu UV–vis spectrophotometer 2450 (Shimadzu Company in Japan) with 1 cm quartz absorption cell, pHs-3C pH meter (Shanghai Pengshun scientific instruments Co., Ltd.), and Electronic
1022
R. Liu et al. / Spectrochimica Acta Part A 71 (2008) 1021–1026
Table 1a Effect of change of concentration of CAS on composition-ratio and absorbance CAS (×10−5 mol/L) Absorbance Maximum absorption wavelength (nm) Cu:CAS
2.4 0.030 632.4 2:1
4.8 0.716 597.4 1:1
9.6 2.310 596.6 1:2
14.4 3.250 597.6 1:3
19.2 3.551 598.2 1:4
24 3.588 600.6 1:5
28.8 3.305 609.2 1:6
a role of micelle sensitization enhancement. It makes the electron orbit of the chromogenic reagent appear like a dipole interaction, which narrows the energy gap between the ground states and excitation states [18,19]. The absorption spectrum of Cu–EDTA was far less than the Cu–CAS–CTMAB and the absorption spectrum of Cu–CAS–CTMAB was far less than the Cu–CAS–CTMAB under the same condition. This showed that a part of CAS has been displaced by EDTA.
3.2. Determination of composition-ratio of Cu with CAS
Fig. 1. Absorption spectra. (a) Cu–CAS, (b) Cu–CAS–CTMAB, (c) Cu–EDTA–CAS–CTMAB (heated in water bath for 4 min at 85 ◦ C), (d) Cu–EDTA. Con−4 −5 −4 ditions: CuSO4 : 6.25 × 10 mol/L; CAS: 4.0 × 10 mol/L; CTMAB: 3.0 × 10 mol/L; EDTA: 4.25 × 10−4 mol/L.
balance FA2004B (Shanghai Precise scientific instruments Co., Ltd.) are used. CuSO4 (Tianjin Kemiou Chemical Reagents Development Centre), CAS (Tianjin Institute of Chemical Reagents), Cetyltrimethylammonium Bromide (CTMAB) (Guoyao Group Chemical Reagents Co., Ltd.), EDTA solution: EDTA disodium (Tianjin Tianda Chemical Experiment Plant), acetic acid (Tianjin Standard Science and Technology Co., Ltd.), and sodium acetate (Tianjin Beihong Reagents Plant). The reagents mentioned above are all analytically pure. Water used in the experiment is deionized water. 3. Results and discussion 3.1. Absorption spectrum With the blank solution as reference, the absorption spectrum curves of the Cu–CAS, Cu–CAS–CTMAB, and Cu–CAS–CTMAB–EDTA were determined by scanning the systems between 400 nm and 700 nm at the speed of 200 nm/min. The results of the experiment are shown in Fig. 1. From Fig. 1, we know that blue-purple complex was formed by Cu and CAS. Its maximum absorption wavelength is located in 582.8 nm. The blue-green complex was formed by Cu, CAS, and CTMAB and its maximum absorption wavelength is 593.6 nm. After the surfactant was added, the wavelength moved toward infrared region. This might be because the cationic surfactant CTMAB plays
3.2.1. Determination using method of lines There are many methods to determine composition-ratio of complexes, mainly including mole ratio method, continuous variation method, slope ratio method, and method of lines. The first three are mainly used to determine the stable complex whose dissociation degree is smaller. Because of the stability of ternary complexes and the purity of the reagents used, mole ratio method cannot be used to determine composition-ratio of complexes. Continuous variation method and slope ratio method often have big errors. With 4.7 × 10−5 mol/L CuSO4 , 3.0 × 10−4 mol/L CTMAB, and HAc–NaAc buffer solution whose pH is 6.0, the concentration of CAS was changed according to Table 1. The maximum absorbance was determined, respectively, by scanning the system between 550 nm and 700 nm at the speed of 200 nm/min. The results are shown in Fig. 1a. Same as the experiment above, the concentration of CAS was changed with the concentration of Cu was 2.3 × 10−5 mol/L. The results of the experiment are shown in Table 1b. It is clear from Table 1a, that when the concentration of Cu ion was 4.7 × 10−5 mol/L and CAS was more than 9.6 × 10−5 mol/L, the absorbance A was much higher than 1. The results might deviate from Beer–Lambert law and were not suitable for data analysis because the error was large. So forsake it. Then we chose that the concentration of Cu ion was 2.3 × 10−5 mol/L and the curve (1/A to 1/Vb ) can be gained. The results are shown in Fig. 2a–f. When b = 0.5, 1, 2, 3, 4, and 5, respectively, the correlation coefficient which equals to 0.9909, 0.9806, 0.9249, 0.8767, 0.8482, and 0.8330 correspondingly could be gained using the least square method calculating the curves in Fig. 2.
3.2.2. Determination using chelated replacement by EDTA method EDTA has higher complex ability. It can displace CAS from Cu–CAS complexes to form ligand that is more stable. But when redundant Cu2+ exists in Cu–CAS–CTMAB system, EDTA will complex with redundant Cu2+ first. With the increasing of EDTA, the absorbance of the system increased. All free Cu2+ complex with EDTA when the absorbance of the system reaches its maximum.
Table 1b Effect of different composition-ratios of Cu with CAS on absorbance CAS (×10−5 mol/L) Absorbance Maximum absorption wavelength (nm) Cu:CAS
1.2 2.4 4.8 0.012 0.020 0.040 No peak, select points to determine, = 626 2:1 1:1 1:2
7.2 0.370 601.8 1:3
9.6 0.957 601.8 1:4
12 1.203 604.2 1:5
14.4 1.343 606.8 1:6
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Fig. 2. (a) Determination of Cu:CAS by straight-line method (b = 0.5, R2 = 0.9909), (b) Determination of Cu:CAS by straight-line method (b = 1, R2 = 0.9806), (c) Determination of Cu:CAS by straight-line method (b = 2, R2 = 0.9249), (d) Determination of Cu:CAS by straight-line method (b = 3, R2 = 0.8767), (e) Determination of Cu:CAS by straight-line method (b = 4, R2 = 0.8482), (f) Determination of Cu:CAS by straight-line method (b = 5, R2 = 0.8330).
That is:
The results indicated above: method of lines and method of chelated replacement by EDTA show good agreement when they determine composition-ratio of Cu with CAS.
Cu(free) + EDTA + Cu–CAS → Cu–EDTA + Cu–CAS The concentration of EDTA was changed in the solution, which contained 7.8 × 10−5 mol/L Cu2+ , 4.0 × 10−5 mol/L CAS, and 3.0 × 10−4 mol/L CTMAB. The pH of the solution was 6. The maximum attraction luminosities were determined by scanning the system between 550 nm and 700 nm at the speed of 200 nm/min. The results are shown in Table 2.As shown in Table 2, the absorbance of the system reached its maximum when the concentration of EDTA was 2 × 10−6 mol/L. Because the composition-ratio of Cu with EDTA is 1:1 [20], the concentration of free Cu2+ in the system was 2.0 × 10−6 mol/L, which equals 2.5 × 10−5 mol/L × 0.8 mL/10 mL. The concentration of Cu2+ that coordinated with CAS was 7.6 × 10−5 mol/L, which equals 7.8 × 10−5 mol/L–2.0 × 10−6 mol/L. So Cu:CAS = 7.6 × 10−5 mol/L:4 × 10−5 mol/L = 1.9:1 ≈ 2:1.
3.3. Optimization of experimental conditions of replacement of CAS from Cu–CAS–CTMAB by EDTA We come to know from the results above, that the compositionratio of Cu ion with CAS is 2:1. When Cu2+ was excessive, the complexation of free Cu and EDTA occurred first, which increased the absorbance of the system. And when all free Cu2+ complexed in the reaction, owing to its strong complex ability, EDTA could displace CAS out of Cu–CAS–CTMAB to form Cu–EDTA–CTMAB complex which is more stable. Then the system reached equilibrium state. The optimization and selection of the experiment are described below.
Table 2 Effect of different concentrations of EDTA on maximum absorbance of the system of Cu–CAS–CTMAB Concentration of EDTA (×10−7 mol/L) Absorbance (A) Maximum absorption wavelength (nm)
0.0 0.610 597.4
2.5 0.632 597.1
12.5 0.633 598.6
20 0.642 598.1
22.5 0.506 594.2
25.0 0.499 599.1
3.0 0.468 611.7
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Table 3 Effect of reaction time on the absorbance and absorption wavelength of Cu–EDTA–CAS–CTMAB system Time (min) Absorbance Maximum absorption wavelength (nm)
0 0.659 601.4
3.3.1. Reaction time and temperature of the system to reach equilibrium at room temperature It requires some time for the replacement of CAS from Cu–CAS–CTMAB by EDTA reaching equilibrium at room temperature. When the concentrations of Cu2+ , CAS, CTMAB and EDTA were 7.8 × 10−4 mol/L, 2.0 × 10−5 mol/L, 1.0 × 10−4 mol/L, and 5.0 × 10−4 mol/L, respectively, and pH was 5.29 after different reaction time, the maximum attraction luminosities of the system were determined by scanning the system between 550 nm and 700 nm at the speed of 200 nm/min. The results are shown in Table 3. From Table 3, we know that when the system was placed at room temperature for 94 min, it was far from equilibrium. Because the time for the system to reach equilibrium is long, heating is needed for shortening the equilibrium time. Six colorimetric tubes, which contain the same solution, were heated at different temperatures for 4 min. The maximum attraction luminosity and absorption wavelength of the system were determined by scanning them between 550 nm and 700 m. The results are shown in Fig. 3a. From Fig. 3a, we know that heating temperature influences the absorbance of reaction system much. The absorbance decreased
Fig. 3. (a) Effect of temperature on absorbance and wavelength. Conditions: Cu2+ = 7.8 × 10−4 mol/L; CAS = 2.0 × 10−5 mol/L; CTMAB = 1.0 × 10−4 mol/L; EDTA = 5.0 × 10−4 mol/L; pH 5.29, Scan rate = 200 nm/min. (b) Effect of different heating time on absorbance and wavelength under 85 ◦ C. Conditions: Cu2+ = 7.8 × 10−4 mol/L; CAS = 2.0 × 10−5 mol/L; CTMAB = 1.0 × 10−4 mol/L; EDTA = 5.0 × 10−4 mol/L; pH 5.29.
12 0.632 601.2
24 0.619 601.6
48 0.582 601.6
94 0.543 601.2
rapidly with the increase of the temperature. When heating temperature reached 85 ◦ C, the system reached balance basically. From perspective of energy conservation, we chose 85 ◦ C as the heating temperature. Solutions, which contained the same substance were heated at 85 ◦ C for 0 min, 1 min, 2 min, 4 min, and 5 min, respectively, and then the maximum attraction luminosities were determined by scanning the system between 550 nm and 700 nm at the speed of 200 nm/min. The results are shown in Fig. 3b. From Fig. 3b, we know that the system did not reach equilibrium when not heated. Absorbance decreased rapidly with the increase of heating time. When the heating time was 4 min, the system reached equilibrium, and the maximum absorption wavelength of the system was about 598 nm under heated. So we determined the absorbance of the system at the wavelength of 598 nm under the condition of heating the system for 4 min at the temperature of 85 ◦ C.
3.3.2. Effect of pH on the absorbance of the Cu–CAS–CTMAB system The maximum absorption of the system was done at the wavelength of 589 nm by changing the pH, while all other factors remained fixed. Results are shown in Fig. 4. From Fig. 4, we know that with the increase of pH the absorbance increased gradually. The absorbance decreased greatly when the acidity of the solution was strong. Due to the increase of hydrogen ion concentration, less organic acids existed in anion state, which weakened their coordination balance of Cu2+ with negative charged organic acids. It is clear from Fig. 4 that the absorbance jumped when pH changed from 3.34 to 4.18. But the absorbance had a less change when the range of pH was 5.24–6.08. So we chose the optimal pH 5.24 as the optimal pH for determination. The absorbance decreased when pH reached 10.00. This accounts for that hydroxyl ions competed with complexing agent to coordinate with Cu2+ . When the alkalinity was too strong, a large number of hydroxyl ion combined with Cu2+ to form precipitation, which was not suitable for the research conditions of this technology [21].
Fig. 4. Effect of pH on absorbance of Cu–CAS–CTMAB system. Conditions: Cu2+ = 7.8 × 10−4 mol/L; CAS = 2 × 10−5 mol/L; CTMAB = 1 × 10−4 mol/L.
R. Liu et al. / Spectrochimica Acta Part A 71 (2008) 1021–1026
Fig. 5. Effect of concentration of CTMAB on absorbance of Cu–CAS–CTMAB system. Conditions: Cu2+ = 7.8 × 10−4 mol/L; CAS = 4 × 10−5 mol/L; pH 5.24.
3.3.3. Effect of the concentration of CTMAB on the absorbance of the Cu–CAS–CTMAB system The maximum attraction luminosities of the system were determined, respectively, at the wavelength of 598 nm while the concentration of CTMAB changed. The results are shown in Fig. 5. It is clear from Fig. 5 that the absorbance of the system was large and stable when the concentration of CTMAB was between 2 × 10−4 mol/L and 4 × 10−4 mol/L. The volume added was between 0.4 mL and 0.8 mL. When the concentration was lower than 2 × 10−4 mol/L, CTMAB could not play the role of micellar solubilization and sensitization enhancement and the solution was turbid. The absorbance of the binary system of Cu–CAS without CTMAB was low. When the concentration was higher than 4 × 10−4 mol/L, the absorbance decreased sharply with the increase in concentration. This might be because of the pseudo homogeneous extraction of CAS by cation surfactant CTMAB micelle. This reaction decreased the concentration of CAS in water and destroyed the complex equilibria of Cu–CAS–CTMAB. So the absorbance decreased sharply [19]. We chose 3 × 10−4 mol/L as the concentration of CTMAB.
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Fig. 7. The metathesis curve of CAS in Cu–CAS–CTMAB system by EDTA. (a) EDTA = 0∼1.5 × 10−4 mol/L; (b) EDTA = 1.5 × 10−4 ∼5 × 10−4 mol/L; Conditions: Cu = 6.25 × 10− 4 mol/L; (c) EDTA = 5 × 10−4 ∼7.5 × 10−4 mol/L. CAS = 4 × 10−5 mol/L; CTMAB = 3 × 10−4 mol/L; pH 5.24; t = 85 ◦ C.
3.3.4. Effect of the concentration of Cu2+ on the absorbance of Cu–CAS–CTMAB system The maximum attraction luminosities of the system were determined at the wavelength of 598 nm while the concentration of Cu2+ changed. The results are shown in Fig. 6. It is clear from Fig. 6 that the absorbance of the Cu–CAS–CTMAB system increased with the increase of Cu2+ . The absorbance reached its maximum when the concentration of Cu2+ was 6.25 × 10−4 mol/L. So we chose 6.25 × 10−4 mol/L as the concentration of Cu2+ . 3.4. The experiment of displacing CAS in Cu–CAS–CTMAB system by EDTA The maximum attraction luminosities of the system were determined, at the wavelength of 598 nm while the concentration of EDTA changed. The results are shown in Fig. 7. Fig. 7 shows the whole process of displacement of CAS in Cu–CAS–CTMAB by EDTA. In segment a, the absorbance of the system increased little when the concentration of EDTA ranged from 0 mol/L to 1.5 × 10−4 mol/L. The reason was that EDTA complexes with Cu2+ first, if free Cu2+ exists in the system. In segment b, when EDTA complexed with all free Cu2+ , its strong complexing capacity made it to displace CAS from Cu–CAS. Because the absorbance of Cu–EDTA complex is much less than Cu–CAS complex, so the absorbance of the system decreased continuously. The absorbance of the system decreased significantly when the concentration of EDTA was more than 3.75 × 10−4 mol/L, and the majority of CAS has already been displaced by EDTA. As shown in segment c, when the concentration of EDTA was 5.00 × 10−4 mol/L, the system almost reached balance and the absorbance tended to be stable. The system might be a mixture of Cu–CAS and Cu–CAS–CTMAB or the system might contain Cu–CAS complex and free CAS, and CTMAB. In the following experiment we determined whether Cu–CAS–CTMAB ternary complex exists in the system after reaction. 3.5. Composition of the system after reaction
Fig. 6. Effect of the concentration of Cu on the absorbance of Cu–CAS–CTMAB system. Conditions: CAS = 4 × 10−5 mol/L; CTMAB = 3 × 10−4 mol/L; pH 5.24.
3.5.1. Determination of the existence of Cu–CAS–CTMAB ternary complex The concentration of EDTA was changed with the concentration of Cu2+ being 6.2 × 10−4 mol/L and pH was 5.24. Then the maxi-
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Table 4 Effect of the concentration of EDTA on absorbance of the system Concentration of EDTA (×10−4 mol/L) Absorbance
5.50 0.011
6.00 0.010
6.75 0.009
7.00 0.008
7.25 0.008
7.50 0.008
mum attraction luminosities were determined at the wavelength of 598 nm. The results are shown in Table 4. From the comparison between Table 4 and Fig. 7, we know that the absorbance of the Cu–EDTA 0.008 was much less than 0.085, which was the absorbance value of original system. This showed that EDTA has not replaced all CAS in Cu–CAS–CTMAB in reaction process, and there was a certain amount of Cu–CAS–CTMAB in the system, which has already reached equilibrium.
method. This can provide evidence for study of the complex of Cu with CAS in the future. When free Cu2+ exists in Cu–CAS–CTMAB system, EDTA complexes with free Cu2+ first. With increased EDTA, it can displace 99.6% of CAS from Cu–CAS–CTMAB. The system reaches equilibrium finally and contains complex of Cu–EDTA and Cu–CAS–CTMAB.
3.5.2. Determination of replacement amount of CAS after reaction The equation of this reaction is:
This work is supported by Natural Science Foundation of China (20607011), Program for New Century Excellent Talents in University (NCET-06-0582), SRF for ROCS, SEM (Liu RT), and Mid-young Scientists Awarding Foundation in Shandong Province (2007BS08005).
Cu–CAS–CTMAB + Cu(Free) + EDTA → Cu–CAS–CTMAB + Cu–EDTA + CAS + CTMAB It is clear from Fig. 7 that the absorbance of the system was 0.085 when the added amount of EDTA was 7.5 × 10−4 mol/L. We supposed that all Cu2+ complex with EDTA. It is clear from Table 4 that the absorbance of Cu–EDTA was 0.008 under the same condition and the absorbance of CAS–CTMAB was 0.022. So the absorbance of Cu–CAS–CTMAB was 0.055, which equals 0.085–0.008–0.022. According to the working cure of Cu2+ , the concentration of Cu2+ was 2.5 × 10−6 mol/L, that is to say the concentration of Cu–CAS was 2.5 × 10−6 mol/L. We have known that the concentration of Cu–CAS was 6.25 × 10−4 mol/L in the system before reaction, so the replacement ratio of EDTA was 99.6%, which equals 1–2.5 × 10−6 /6.25 × 10−4 . The result showed that EDTA has already replaced the majority of CAS. So the hypothesis is correct. 4. Conclusions The replacement of CAS from Cu–CAS–CTMAB by EDTA under the optimum test conditions and methods was studied in this paper. Conclusions are as follows: The ternary system that has much more superior performance can be determined by study on Cu–CAS–CTMAB ternary system and Cu–CAS ternary system. The system’s absorbance increased significantly and its maximum absorption wavelength was redshifted. The composition-ratio of Cu2+ with CAS, which is 2:1 was determined by method of lines and chelated replacement by EDTA
Acknowledgements
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