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A novel spectrophotometric determination of trace copper based on charge transfer complex
Spectrochimica Acta Part A 61 (2005) 937–941
A novel spectrophotometric determination of trace copper based on charge transfer complex Junwei Di∗ , Ying Wu, Yun Ma Department of Chemistry, Suzhou University, Suzhou 215006, PR China Received 17 March 2004; accepted 27 April 2004
Abstract A new type of colored complex, the charge transfer complex, was used to develop the spectrophotometric determination of copper. The method was based on the formation of a colored product, the charge transfer complex of copper substituted tungstophosphate with 3,3 ,5,5 -tetramethybenzidine (TMB), which was stabilized and sensitized by the addition of polyvinyl alcohol (PVA) in aqueous solution. The structure of copper substituted tungstophosphate was Keggin-type according to the results of infrared (IR) spectra. The optimum reaction conditions and other important analytic parameters had been investigated. Beer’s law was obeyed in the copper(II) concentration range of 0.003–0.1 g mL−1 , and the molar absorptivity at 660 nm is 2.54 × 105 L mol−1 cm−1 . The proposed method was simple, selective, and sensitive. It was applied to the analytic samples with satisfactory results. © 2004 Elsevier B.V. All rights reserved. Keywords: Copper determination; Spectrophotometry; Charge transfer complex
1. Introduction Several methods are available for trace determination of copper including spectrophotometry. It involves lesser expensive instrumentation and provides high sensitivity when appropriate chromogenic reagents are available. Several kinds of complexes were developed for the determination of copper by spectrophotometric method. Many binary complexes were widely used. Some representative examples of the chromogenic reagents are dithizone (maximumwavelength(λmax ) = 550 nm, ε = 4.52 × 104 L mol−1 cm−1 ), sodium diethyldithicarbamate (λmax = 436 nm, ε = 1.4 × 104 L mol−1 cm−1 ), cuprizone or bis-cyclohexanone–oxalyldihydrazone (λmax = 595–600 nm, ε = 1.6 × 104 L mol−1 cm−1 ) [1], di-2-pyridyl ketone benzoylhydrazone(dPKBH) (λmax = 370 nm, ε = 3.92 × 104 L mol−1 cm−1 ) [2], Naphthazarin (λmax = 550 nm, ε = 1.84×104 L mol−1 cm−1 ) [3], 3,3 -(1,3-propanedyldiimine) bis-[3-methyl-2-butanone]dioxime (PnAO) (λmax = 525 nm, ε = 2.95 × 104 L mol−1 cm−1 ) [4], 3-{2-[2-(2-hydroxyimino-1-methyl-propylideneamino)-eth∗ Corresponding author. Tel.: +86 512 65112645; fax: +86 512 65108012. E-mail address: [email protected] (J. Di).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.04.021
ylamino]-ethyl-imino}-butan-2-one oxime (H2mdo) (λmax = 570 nm, ε = 0.16 × 104 L mol−1 cm−1 ) [5], S,S -bis (2-aminophenyl) oxalate (λmax = 504 nm, ε = 0.5365 × 104 L mol−1 cm−1 ) [6], 4,5-dimercapto-1,3-dithyol -2-thionate (DMIT) (λmax = 430 nm, ε = 9.06×104 L mol−1 cm−1 ) [7], eriochrome black T (λmax = 550 nm, ε = 2.61 × 104 L mol−1 cm−1 ) [8]. However, the molar absorption coefficients of these complexes are commonly low. Mixed-ligand complexes and ion-associate complexes were used to improve sensitivity in the determination of copper. In these systems, surface-active substances were often introduced to increase the sensitivity. The red colored Cu(II)-PAN-TX-100-N,N -diphenylbenzamidine complex in chloroform shows maximum absorbance at 520 nm with molar absorptivity value of 1.14 × 105 L mol−1 cm−1 [9]. The complex Cu(II)–9-phenyl-2,3,7-trihydroxy-6-fluorone (PF)–cetylpyridinium chloride (CP)–Triton X-100 shows maximum absorbance at 595 nm with a molar absorptivity value of 9.67 × 104 L mol−1 cm−1 [10]. Particularly, the molar absorption coefficients of the ion-associate complexes, which were obtained by the reaction of tungstocoppate heteropoly acid anion and dye cation in the presence of polyvinyl alcohol (PVA) [11–13], were as high as 106 . However, the difference of the maximum wavelength between the dye and complex was only
938
J. Di et al. / Spectrochimica Acta Part A 61 (2005) 937–941
30–40 nm, which resulted in some interference in the copper determination. Charge transfer complex, the supermolecule, is of some particular characteristic on molecular recognition function and spectral property. Therefore, it has been applied to determine several metal ions [14,15]. The aim of this work was to develop a highly sensitive and efficient spectrophotometric method for copper determination, based on the formation of colored charge transfer complex which was formed by the reaction of copper substituted heteropolyanion with 3,3 ,5,5 -tetramethylbenzidine (TMB) and sensitized by polyvinyl alcohol. Various factors influence the sensitivity of the proposed method such as wavelength, pH, temperature, effect of foreign ions, and ranges of applicability of Beer’s law in the determination of copper are also included. The method was applied to some pharmaceutical and environmental water samples. 2. Experimental
Two moles per liter HAc, 0.01 mol L−1 WO4 2- , and 0.01 mol L−1 PO4 3− solutions were prepared with HAc, Na2 WO4 ·2H2 O, and KH2 PO4 , respectively. All the solutions were prepared with analytic reagent grade chemicals. 2.3. Standard procedure A solution containing 0–2.5 g of copper was transferred into a series of 25-mL calibrated flasks to which 1 mL of 2 mol L−1 HAc, 5 mL of 0.01 mol L−1 Na2 WO4 , 0.3 mL of 0.01 mol L−1 NaH2 PO4 and 1 mL of 0.02% PVA solutions were added, respectively; the mixture was shaken and 5 mL of TMB was added, this was made up near the mark with water. The solutions were kept in a boiling-water bath for about 15 min. After cooling, the mixtures were diluted with water to the mark (25 mL). Finally, the absorbance of the colored solution was measured at 660 nm in a 2 cm-optical pathlength cell against a reagent blank, and the calibration graph was constructed. 2.4. Procedure for samples
2.1. Instruments Absorbance measurements were carried out by means of a 7530G UV–Vis spectrophotometer (HP-Shanghai Analytical Instrument Ltd., China) and a 721 spectrophotometer (Shanghai, Chain). Infrared (IR) absorption spectra were recorded on an AVATAR 360 FT–IR (Nicolet) spectrophotometer with KBr disk. 2.2. Chemicals and reagents A 100 g mL−1 of Cu2+ stock solution was prepared with CuSO4 . The working solutions were prepared as needed. 3,3 ,5,5 -tetramethybenzidine was offered by Suzhou New District BEC Fine Chemicals Co. Ltd., Suzhou, China. A stock solution of 0.1 mg mL−1 was prepared by dissolving an appropriate amount of TMB in 0.1 mol L−1 HCl. Polyvinyl alcohol, of which the average degree of polymerization is 2400–2500, was purchased from Shanghai Chemical Reagent Plant, imported from Japan. A 0.1% PVA stock solution was prepared by dissolving 0.1 g of PVA in 100 mL of water under heating to near boiling point. A 0.02% PVA solution was obtained by diluting the stock solution as needed.
A multivitamin–multimineral tablet, obtained from Huishi-baigong Pharmaceutical Co. Led., was lixiviated by diluted HCl. The filtrate was transferred into a 250-mL calibrated flask. This was made up to the mark with water. River water and wastewater samples were filtrated and collected in polythene bottles. 1:1 HCl was added to neutralize the alkalinity. A known volume of the sample was analyzed by the standard procedure.
3. Results and discussion 3.1. Spectral characteristics The proposed method involved the formation a green product with a λmax of 660 nm, as shown in Fig. 1. The reagent blank had negligible absorption at this wavelength. Therefore, we selected 660 nm as the wavelength in the copper determination. In the absence of PVA in the system, the green precipitation was obtained. It was washed with water and then dried. Its IR spectrum was recorded. The IR spectrum (Table 1),
Table 1 IR spectral data of the compounds (cm−1 ) Compound 5− –TMB
PCuW11 O39 ␣-H3 PW12 O40 PW12 O40 − –Gly PMoW11 O40 3− –TMB PW12 O40 3− –NMP CuW12 O40 6 –NB CuW12 O40 6− –BRB
(P–Oa )
(W–Ob )
(W–Oc )
(W–Od )
Reference
1079.34 1080 1079 1080.0 1079 – –
895.69 890 894 895.0 895 884 895
819.12 810 804 811.9 808 816 797
977.88 990 979 978.2 979 960 966
This work [15] [16] [13] [17] [10] [11]
Gly, glycine; NMP, metylpyrrolidone; NB, nile blue; BRB, butyl rhodamine B.
J. Di et al. / Spectrochimica Acta Part A 61 (2005) 937–941
939
little absorbance of reagent blank resulted in TMB oxidized by oxygen in the solution. In the absence of PVA, the solutions became turbid when they were cooled down. The determinations of the absorbance of the complexes were carried out in turbid solutions. When PVA was added, the solution remained clear
Fig. 1. Absorption spectrum of the colored product. Final concentration of copper was 0.06 g mL−1 .
characterized by four prominent bands, was quite similar to the spectra for the Keggin-type complex [PW12 O40 ]n − or its derivate complexes [11,12,14,16–18]. It is reported that the redox potential of the copper substitute tungstophosphate anion was more positive than that of tungstophosphate [19,20]. Therefore, the copper substitute tungstophosphate anion is a better electron acceptor. The charge transfer complex, [PW11 CuO39 ]5− –TMB, was formed, since TMB is a good neutral organic electron donor [14]. The charge transfer complex anion can associate with H+ resulting in precipitation, which can be avoided by adding PVA in aqueous solution. 3.2. Optimization of experimental conditions Generally, acidity is important in the formation of heteropolyacid. The effect of pH in the range of 0.5–4 was studied using the recommended procedure. Experiment showed that the sensitivity was the highest at pH of about 2.5 in HAc solution against the reagent blank; meanwhile the absorbance of reagent blank was the lowest against water. The
Fig. 2. Effect of the volume of 0.02% PVA on the absorbance of the colored charge transfer complex.
Fig. 3. Absorbance changes with the volume of 0.01 mol L−1 Na2 WO4 (A), 0.01 mol L−1 KH2 PO4 (B) and 0.1 mg mL−1 TMB (C).
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J. Di et al. / Spectrochimica Acta Part A 61 (2005) 937–941
Table 2 Determination of copper in samples (n = 3) Samples
Multivitamin–multimineral tableta Rival water Waste water
The present method
ICP–AES method (g mL−1 )
Detected (g mL-1 )
Added (g mL−1 )
Found (g mL−1 )
Recovery (%)
R.S.D. (%)
0.0318
0.050
0.0827
102
0.3
0.032b
0.0021 0.0106
0.010 0.010
0.0125 0.0208
104 102
1.6 1.3
0.002 0.011
a A multivitamin–multimineral tablet formula: VA, 5000 U; VD, 400 U; VE, 30 U; VB1, 1.5 mg; VB2, 1.5 mg; VB6, 2 mg; VC, 60 mg; VB12, 6 g; VK1, 25 g; biotin, 30 g; folic acid, 400 g; niacinamide, 20 mg; pantothenic acid, 10 mg; Ca, 162 mg; P, 125 mg; K, 40 mg; Cl, 36.3 mg; Fe, 18 mg; Cu, 2 mg; Zn, 15 mg; Mn, 25.5 mg; I, 150 g; Mo, 25 g; Se, 25 g; Ni, 5 g; Si, 10 g; Sn, 10 g; V, 10 g. b Value calculated from the formula.
and the absorbance of the complex was markedly increased (Fig. 2). In this sense, PVA is a stabilizing as well as a sensitizing agent. Hence, 1.0 mL of 0.02% PVA solution was used in the experiments. The colored solution was stable for at least 10 h. Fig. 3 shows the effects of the volume of Na2 WO4 , KH2 PO4 , and TMB solutions on the absorbance of the system. According to experiments, 5 mL of 0.01 mol L−1 WO4 2− , 0.3 mL of 0.01 mol L−1 PO4 3− , and 5 mL of 0.1 mg mL−1 TMB were recommended at optimum conditions. The temperature of the reaction was very important in the color development. The green product was not observed after keeping at room temperature for about 24 h. Its absorbance reached maximum value after keeping in boiling-water bath for about 12 min and was stable for 12–40 min. Therefore, it is recommended that the color developing time in boiling-water bath was 15 min. 3.3. Calibration curve and sensitivity The calibration curve for determination of copper was obtained under the optimum conditions. The Beer’s law was obeyed over the range 0.003–0.1 g mL−1 copper(II) concentration. The linear regression equation was: A = 0.014 + 7.993CCu (g mL−1 ). The correlation coefficient was 0.998. The apparent molar absorption coefficient (ε) was 2.54 × 105 L mol−1 cm−1 calculated from the slope of calibration curve. The relative standard deviation was 2.2% under nine determinations for 0.04 g mL−1 of copper. Compared with other methods mentioned in previous section, the sensitivity of this proposed method was very high. In addition, all the reagents used in experiments were colorless. 3.4. Interference studies The effect of various cations and anions on the determination of copper(II) was investigated. The tolerance limit as taken as the amount that caused ±5% absorbance error in the determination of 0.04 mL−1 of copper(II). Large amounts of Na+ , K+ , NH4 + , Mg2+ , Cd2+ , ClO4 − , SO4 2− ,
CO3 2− , NO3 − , 200-fold excess of Ca2+ , Ba2+ , F− , Br− , 100-fold excess of Pb2+ , Zn2+ , 50-fold excess of Mn2+ , Ni2+ , SiO3 2− , citrate, EDTA, 20-fold excess of Al3+ and 10-fold excess of Fe3+ did not interfere in the determination of copper. 3.5. Analytic application For the validation of the proposed method, it is applied to the determination of copper(II) in a multivitamin– multimineral tablet, river water, and wastewater samples. The determination results are presented in Table 2. The results obtained with the present method are more satisfactory than those obtained by the formula or ICP–AES method.
4. Conclusions A new type of complex, a charge transfer complex of the copper substituted tungstophosphate–TMB, was developed to determine copper using spectrophotometry. Application of PVA, which plays the role of a protective colloid, gives the possibility to increase solubility of the charge transfer complex in analytic solution and sensitizes the determination of copper. The advantages of this method were a direct spectrophotometric measurement of absorbance of the analytic solution and all colorless reagents used in the color development. The elaborated method was simple, selective, and sensitive.
Acknowledgements This project is supported by the Science Foundations of the Bducation Bureau of Jiangsu province (02KJB150003).
References [1] Z. Marczenko, Spectrophotometric determination of elements (Y Zheng et al., Trans.). Geology Press, Beijing, 1983, p. 211.
J. Di et al. / Spectrochimica Acta Part A 61 (2005) 937–941 [2] J.J. Pinto, C. Moreno, M. Garc´ıa-Vargas, Anal. Bioanal. Chem. 373 (2002) 844. [3] R. Chaisuksant, W. Palkawong-na-ayuthaya, K. Grudpan, Talanta 53 (2000) 579. [4] S. Karaböcek, S. Nohut, Ö. Dalman, S. Güner, Anal. Chim. Acta 408 (2000) 163. [5] Ö. Dalman, M. Tüfekci, S. Nohut, S. Güner, S. Karaböcek, J. Pharm. Biomed. Anal. 27 (2002) 183. [6] S. Nohut, S. Karaböcek, S. Güner, Y. Gök, J. Pharm. Biomed. Anal. 20 (1999) 309. [7] S.R.G. Barreto, J. Nozaki, W.J. Barreto, Microchemical J. 62 (1999) 223. [8] H. Gao, Spectrochim. Acta, Part A 51 (1995) 1379. [9] M. Thakur, M.K. Deb, Talanta 49 (1999) 561. [10] W. Winkler, A. Arenhövel-Pacuła, Talanta 53 (2000) 277.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
941
Z. Li, J. Wang, Q. Xu, L. Li, Fenxi Huaxue 25 (1997) 835. Z. Li, J. Wang, Q. Xu, Fenxi Shiyanshi 16 (6) (1997) 6. Z. Li, J. Wang, Q. Xu, Yejin Fenxi 17 (5) (1997) 5. J. Di, Y. Tu, Talanta 55 (2001) 783. J. Di, Y. Tu, Y. Wu, D. Liu, Spectrosc. Spectral Anal. 22 (2002) 800. C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R. Thouvenot, Inorg. Chem. 22 (1983) 207. S. Guo, W. You, E. Wang, J. Liu, R. Huang, Fenzi Kexue Xuebao 16 (2000) 1. J. Niu, J. Wang, Y. Chen, Wuji Huaxue 15 (1999) 401. B. Keita, E. Abdeljalil, L. Nadjo, B. Avisse, R. Contant, J. Canny, M. Richet, Electrochem. Commun. 2 (2000) 145. E. Wang, C. Hu, L. Xu, Introduction of Polyacid Chemistry. Chemical Industry Press, Beijing, 1998.
A novel spectrophotometric determination of trace copper based on charge transfer complex Junwei Di∗ , Ying Wu, Yun Ma Department of Chemistry, Suzhou University, Suzhou 215006, PR China Received 17 March 2004; accepted 27 April 2004
Abstract A new type of colored complex, the charge transfer complex, was used to develop the spectrophotometric determination of copper. The method was based on the formation of a colored product, the charge transfer complex of copper substituted tungstophosphate with 3,3 ,5,5 -tetramethybenzidine (TMB), which was stabilized and sensitized by the addition of polyvinyl alcohol (PVA) in aqueous solution. The structure of copper substituted tungstophosphate was Keggin-type according to the results of infrared (IR) spectra. The optimum reaction conditions and other important analytic parameters had been investigated. Beer’s law was obeyed in the copper(II) concentration range of 0.003–0.1 g mL−1 , and the molar absorptivity at 660 nm is 2.54 × 105 L mol−1 cm−1 . The proposed method was simple, selective, and sensitive. It was applied to the analytic samples with satisfactory results. © 2004 Elsevier B.V. All rights reserved. Keywords: Copper determination; Spectrophotometry; Charge transfer complex
1. Introduction Several methods are available for trace determination of copper including spectrophotometry. It involves lesser expensive instrumentation and provides high sensitivity when appropriate chromogenic reagents are available. Several kinds of complexes were developed for the determination of copper by spectrophotometric method. Many binary complexes were widely used. Some representative examples of the chromogenic reagents are dithizone (maximumwavelength(λmax ) = 550 nm, ε = 4.52 × 104 L mol−1 cm−1 ), sodium diethyldithicarbamate (λmax = 436 nm, ε = 1.4 × 104 L mol−1 cm−1 ), cuprizone or bis-cyclohexanone–oxalyldihydrazone (λmax = 595–600 nm, ε = 1.6 × 104 L mol−1 cm−1 ) [1], di-2-pyridyl ketone benzoylhydrazone(dPKBH) (λmax = 370 nm, ε = 3.92 × 104 L mol−1 cm−1 ) [2], Naphthazarin (λmax = 550 nm, ε = 1.84×104 L mol−1 cm−1 ) [3], 3,3 -(1,3-propanedyldiimine) bis-[3-methyl-2-butanone]dioxime (PnAO) (λmax = 525 nm, ε = 2.95 × 104 L mol−1 cm−1 ) [4], 3-{2-[2-(2-hydroxyimino-1-methyl-propylideneamino)-eth∗ Corresponding author. Tel.: +86 512 65112645; fax: +86 512 65108012. E-mail address: [email protected] (J. Di).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.04.021
ylamino]-ethyl-imino}-butan-2-one oxime (H2mdo) (λmax = 570 nm, ε = 0.16 × 104 L mol−1 cm−1 ) [5], S,S -bis (2-aminophenyl) oxalate (λmax = 504 nm, ε = 0.5365 × 104 L mol−1 cm−1 ) [6], 4,5-dimercapto-1,3-dithyol -2-thionate (DMIT) (λmax = 430 nm, ε = 9.06×104 L mol−1 cm−1 ) [7], eriochrome black T (λmax = 550 nm, ε = 2.61 × 104 L mol−1 cm−1 ) [8]. However, the molar absorption coefficients of these complexes are commonly low. Mixed-ligand complexes and ion-associate complexes were used to improve sensitivity in the determination of copper. In these systems, surface-active substances were often introduced to increase the sensitivity. The red colored Cu(II)-PAN-TX-100-N,N -diphenylbenzamidine complex in chloroform shows maximum absorbance at 520 nm with molar absorptivity value of 1.14 × 105 L mol−1 cm−1 [9]. The complex Cu(II)–9-phenyl-2,3,7-trihydroxy-6-fluorone (PF)–cetylpyridinium chloride (CP)–Triton X-100 shows maximum absorbance at 595 nm with a molar absorptivity value of 9.67 × 104 L mol−1 cm−1 [10]. Particularly, the molar absorption coefficients of the ion-associate complexes, which were obtained by the reaction of tungstocoppate heteropoly acid anion and dye cation in the presence of polyvinyl alcohol (PVA) [11–13], were as high as 106 . However, the difference of the maximum wavelength between the dye and complex was only
938
J. Di et al. / Spectrochimica Acta Part A 61 (2005) 937–941
30–40 nm, which resulted in some interference in the copper determination. Charge transfer complex, the supermolecule, is of some particular characteristic on molecular recognition function and spectral property. Therefore, it has been applied to determine several metal ions [14,15]. The aim of this work was to develop a highly sensitive and efficient spectrophotometric method for copper determination, based on the formation of colored charge transfer complex which was formed by the reaction of copper substituted heteropolyanion with 3,3 ,5,5 -tetramethylbenzidine (TMB) and sensitized by polyvinyl alcohol. Various factors influence the sensitivity of the proposed method such as wavelength, pH, temperature, effect of foreign ions, and ranges of applicability of Beer’s law in the determination of copper are also included. The method was applied to some pharmaceutical and environmental water samples. 2. Experimental
Two moles per liter HAc, 0.01 mol L−1 WO4 2- , and 0.01 mol L−1 PO4 3− solutions were prepared with HAc, Na2 WO4 ·2H2 O, and KH2 PO4 , respectively. All the solutions were prepared with analytic reagent grade chemicals. 2.3. Standard procedure A solution containing 0–2.5 g of copper was transferred into a series of 25-mL calibrated flasks to which 1 mL of 2 mol L−1 HAc, 5 mL of 0.01 mol L−1 Na2 WO4 , 0.3 mL of 0.01 mol L−1 NaH2 PO4 and 1 mL of 0.02% PVA solutions were added, respectively; the mixture was shaken and 5 mL of TMB was added, this was made up near the mark with water. The solutions were kept in a boiling-water bath for about 15 min. After cooling, the mixtures were diluted with water to the mark (25 mL). Finally, the absorbance of the colored solution was measured at 660 nm in a 2 cm-optical pathlength cell against a reagent blank, and the calibration graph was constructed. 2.4. Procedure for samples
2.1. Instruments Absorbance measurements were carried out by means of a 7530G UV–Vis spectrophotometer (HP-Shanghai Analytical Instrument Ltd., China) and a 721 spectrophotometer (Shanghai, Chain). Infrared (IR) absorption spectra were recorded on an AVATAR 360 FT–IR (Nicolet) spectrophotometer with KBr disk. 2.2. Chemicals and reagents A 100 g mL−1 of Cu2+ stock solution was prepared with CuSO4 . The working solutions were prepared as needed. 3,3 ,5,5 -tetramethybenzidine was offered by Suzhou New District BEC Fine Chemicals Co. Ltd., Suzhou, China. A stock solution of 0.1 mg mL−1 was prepared by dissolving an appropriate amount of TMB in 0.1 mol L−1 HCl. Polyvinyl alcohol, of which the average degree of polymerization is 2400–2500, was purchased from Shanghai Chemical Reagent Plant, imported from Japan. A 0.1% PVA stock solution was prepared by dissolving 0.1 g of PVA in 100 mL of water under heating to near boiling point. A 0.02% PVA solution was obtained by diluting the stock solution as needed.
A multivitamin–multimineral tablet, obtained from Huishi-baigong Pharmaceutical Co. Led., was lixiviated by diluted HCl. The filtrate was transferred into a 250-mL calibrated flask. This was made up to the mark with water. River water and wastewater samples were filtrated and collected in polythene bottles. 1:1 HCl was added to neutralize the alkalinity. A known volume of the sample was analyzed by the standard procedure.
3. Results and discussion 3.1. Spectral characteristics The proposed method involved the formation a green product with a λmax of 660 nm, as shown in Fig. 1. The reagent blank had negligible absorption at this wavelength. Therefore, we selected 660 nm as the wavelength in the copper determination. In the absence of PVA in the system, the green precipitation was obtained. It was washed with water and then dried. Its IR spectrum was recorded. The IR spectrum (Table 1),
Table 1 IR spectral data of the compounds (cm−1 ) Compound 5− –TMB
PCuW11 O39 ␣-H3 PW12 O40 PW12 O40 − –Gly PMoW11 O40 3− –TMB PW12 O40 3− –NMP CuW12 O40 6 –NB CuW12 O40 6− –BRB
(P–Oa )
(W–Ob )
(W–Oc )
(W–Od )
Reference
1079.34 1080 1079 1080.0 1079 – –
895.69 890 894 895.0 895 884 895
819.12 810 804 811.9 808 816 797
977.88 990 979 978.2 979 960 966
This work [15] [16] [13] [17] [10] [11]
Gly, glycine; NMP, metylpyrrolidone; NB, nile blue; BRB, butyl rhodamine B.
J. Di et al. / Spectrochimica Acta Part A 61 (2005) 937–941
939
little absorbance of reagent blank resulted in TMB oxidized by oxygen in the solution. In the absence of PVA, the solutions became turbid when they were cooled down. The determinations of the absorbance of the complexes were carried out in turbid solutions. When PVA was added, the solution remained clear
Fig. 1. Absorption spectrum of the colored product. Final concentration of copper was 0.06 g mL−1 .
characterized by four prominent bands, was quite similar to the spectra for the Keggin-type complex [PW12 O40 ]n − or its derivate complexes [11,12,14,16–18]. It is reported that the redox potential of the copper substitute tungstophosphate anion was more positive than that of tungstophosphate [19,20]. Therefore, the copper substitute tungstophosphate anion is a better electron acceptor. The charge transfer complex, [PW11 CuO39 ]5− –TMB, was formed, since TMB is a good neutral organic electron donor [14]. The charge transfer complex anion can associate with H+ resulting in precipitation, which can be avoided by adding PVA in aqueous solution. 3.2. Optimization of experimental conditions Generally, acidity is important in the formation of heteropolyacid. The effect of pH in the range of 0.5–4 was studied using the recommended procedure. Experiment showed that the sensitivity was the highest at pH of about 2.5 in HAc solution against the reagent blank; meanwhile the absorbance of reagent blank was the lowest against water. The
Fig. 2. Effect of the volume of 0.02% PVA on the absorbance of the colored charge transfer complex.
Fig. 3. Absorbance changes with the volume of 0.01 mol L−1 Na2 WO4 (A), 0.01 mol L−1 KH2 PO4 (B) and 0.1 mg mL−1 TMB (C).
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Table 2 Determination of copper in samples (n = 3) Samples
Multivitamin–multimineral tableta Rival water Waste water
The present method
ICP–AES method (g mL−1 )
Detected (g mL-1 )
Added (g mL−1 )
Found (g mL−1 )
Recovery (%)
R.S.D. (%)
0.0318
0.050
0.0827
102
0.3
0.032b
0.0021 0.0106
0.010 0.010
0.0125 0.0208
104 102
1.6 1.3
0.002 0.011
a A multivitamin–multimineral tablet formula: VA, 5000 U; VD, 400 U; VE, 30 U; VB1, 1.5 mg; VB2, 1.5 mg; VB6, 2 mg; VC, 60 mg; VB12, 6 g; VK1, 25 g; biotin, 30 g; folic acid, 400 g; niacinamide, 20 mg; pantothenic acid, 10 mg; Ca, 162 mg; P, 125 mg; K, 40 mg; Cl, 36.3 mg; Fe, 18 mg; Cu, 2 mg; Zn, 15 mg; Mn, 25.5 mg; I, 150 g; Mo, 25 g; Se, 25 g; Ni, 5 g; Si, 10 g; Sn, 10 g; V, 10 g. b Value calculated from the formula.
and the absorbance of the complex was markedly increased (Fig. 2). In this sense, PVA is a stabilizing as well as a sensitizing agent. Hence, 1.0 mL of 0.02% PVA solution was used in the experiments. The colored solution was stable for at least 10 h. Fig. 3 shows the effects of the volume of Na2 WO4 , KH2 PO4 , and TMB solutions on the absorbance of the system. According to experiments, 5 mL of 0.01 mol L−1 WO4 2− , 0.3 mL of 0.01 mol L−1 PO4 3− , and 5 mL of 0.1 mg mL−1 TMB were recommended at optimum conditions. The temperature of the reaction was very important in the color development. The green product was not observed after keeping at room temperature for about 24 h. Its absorbance reached maximum value after keeping in boiling-water bath for about 12 min and was stable for 12–40 min. Therefore, it is recommended that the color developing time in boiling-water bath was 15 min. 3.3. Calibration curve and sensitivity The calibration curve for determination of copper was obtained under the optimum conditions. The Beer’s law was obeyed over the range 0.003–0.1 g mL−1 copper(II) concentration. The linear regression equation was: A = 0.014 + 7.993CCu (g mL−1 ). The correlation coefficient was 0.998. The apparent molar absorption coefficient (ε) was 2.54 × 105 L mol−1 cm−1 calculated from the slope of calibration curve. The relative standard deviation was 2.2% under nine determinations for 0.04 g mL−1 of copper. Compared with other methods mentioned in previous section, the sensitivity of this proposed method was very high. In addition, all the reagents used in experiments were colorless. 3.4. Interference studies The effect of various cations and anions on the determination of copper(II) was investigated. The tolerance limit as taken as the amount that caused ±5% absorbance error in the determination of 0.04 mL−1 of copper(II). Large amounts of Na+ , K+ , NH4 + , Mg2+ , Cd2+ , ClO4 − , SO4 2− ,
CO3 2− , NO3 − , 200-fold excess of Ca2+ , Ba2+ , F− , Br− , 100-fold excess of Pb2+ , Zn2+ , 50-fold excess of Mn2+ , Ni2+ , SiO3 2− , citrate, EDTA, 20-fold excess of Al3+ and 10-fold excess of Fe3+ did not interfere in the determination of copper. 3.5. Analytic application For the validation of the proposed method, it is applied to the determination of copper(II) in a multivitamin– multimineral tablet, river water, and wastewater samples. The determination results are presented in Table 2. The results obtained with the present method are more satisfactory than those obtained by the formula or ICP–AES method.
4. Conclusions A new type of complex, a charge transfer complex of the copper substituted tungstophosphate–TMB, was developed to determine copper using spectrophotometry. Application of PVA, which plays the role of a protective colloid, gives the possibility to increase solubility of the charge transfer complex in analytic solution and sensitizes the determination of copper. The advantages of this method were a direct spectrophotometric measurement of absorbance of the analytic solution and all colorless reagents used in the color development. The elaborated method was simple, selective, and sensitive.
Acknowledgements This project is supported by the Science Foundations of the Bducation Bureau of Jiangsu province (02KJB150003).
References [1] Z. Marczenko, Spectrophotometric determination of elements (Y Zheng et al., Trans.). Geology Press, Beijing, 1983, p. 211.
J. Di et al. / Spectrochimica Acta Part A 61 (2005) 937–941 [2] J.J. Pinto, C. Moreno, M. Garc´ıa-Vargas, Anal. Bioanal. Chem. 373 (2002) 844. [3] R. Chaisuksant, W. Palkawong-na-ayuthaya, K. Grudpan, Talanta 53 (2000) 579. [4] S. Karaböcek, S. Nohut, Ö. Dalman, S. Güner, Anal. Chim. Acta 408 (2000) 163. [5] Ö. Dalman, M. Tüfekci, S. Nohut, S. Güner, S. Karaböcek, J. Pharm. Biomed. Anal. 27 (2002) 183. [6] S. Nohut, S. Karaböcek, S. Güner, Y. Gök, J. Pharm. Biomed. Anal. 20 (1999) 309. [7] S.R.G. Barreto, J. Nozaki, W.J. Barreto, Microchemical J. 62 (1999) 223. [8] H. Gao, Spectrochim. Acta, Part A 51 (1995) 1379. [9] M. Thakur, M.K. Deb, Talanta 49 (1999) 561. [10] W. Winkler, A. Arenhövel-Pacuła, Talanta 53 (2000) 277.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
941
Z. Li, J. Wang, Q. Xu, L. Li, Fenxi Huaxue 25 (1997) 835. Z. Li, J. Wang, Q. Xu, Fenxi Shiyanshi 16 (6) (1997) 6. Z. Li, J. Wang, Q. Xu, Yejin Fenxi 17 (5) (1997) 5. J. Di, Y. Tu, Talanta 55 (2001) 783. J. Di, Y. Tu, Y. Wu, D. Liu, Spectrosc. Spectral Anal. 22 (2002) 800. C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R. Thouvenot, Inorg. Chem. 22 (1983) 207. S. Guo, W. You, E. Wang, J. Liu, R. Huang, Fenzi Kexue Xuebao 16 (2000) 1. J. Niu, J. Wang, Y. Chen, Wuji Huaxue 15 (1999) 401. B. Keita, E. Abdeljalil, L. Nadjo, B. Avisse, R. Contant, J. Canny, M. Richet, Electrochem. Commun. 2 (2000) 145. E. Wang, C. Hu, L. Xu, Introduction of Polyacid Chemistry. Chemical Industry Press, Beijing, 1998.