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DETERMINATION OF COBALT IN FOODS BY -CYCLODEXTRIN POLYMER PHASE SPECTROPHOTOMETRY USING 2-(5-BROMO-2-PYRIDYLAZO)-5DIETHYLAMINOPHENOL Zi-Tao Jiang a; Rong Li a; Jimmy C. Yu b a Department of Food Science and Engineering, Tianjin University of Commerce, Tianjin, P.R. China b Department of Chemistry and Environmental Science Programme, The Chinese University of Hong Kong, New Territories, Hong Kong Online Publication Date: 28 May 2002

To cite this Article Jiang, Zi-Tao, Li, Rong and Yu, Jimmy C.(2002)'DETERMINATION OF COBALT IN FOODS BY -CYCLODEXTRIN

POLYMER PHASE SPECTROPHOTOMETRY USING 2-(5-BROMO-2-PYRIDYLAZO)-5-DIETHYLAMINOPHENOL',Analytical Letters,35:5,825 — 835 To link to this Article: DOI: 10.1081/AL-120004072 URL: http://dx.doi.org/10.1081/AL-120004072

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ANALYTICAL LETTERS, 35(5), 825–835 (2002)

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CHEMICAL AND BIO-SENSORS

DETERMINATION OF COBALT IN FOODS BY b-CYCLODEXTRIN POLYMER PHASE SPECTROPHOTOMETRY USING 2-(5-BROMO-2-PYRIDYLAZO)-5DIETHYLAMINOPHENOL Zi-Tao Jiang,1,* Rong Li,1 and Jimmy C. Yu2 1

Department of Food Science and Engineering, Tianjin University of Commerce, Tianjin 300400, P.R. China 2 Department of Chemistry and Environmental Science Programme, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong

ABSTRACT The chromogenic agent, 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP) was included/adsorbed on b-cyclodextrin polymer (b-CDP) to form the modified polymer of inclusion of 5-Br-PADAP (b-CDP-5-Br-PADAP). The modified polymer can adsorb cobalt to form a colored complex and the maximum absorbance of 5-Br-PADAP-cobalt complex on b-CDP was measured at 588 nm. The optimum conditions for the adsorption of cobalt on b-CDP-5-Br-PADAP were: pH of 8.0, temperature of 25 C, and shaking time of 40 min for 25 ml of sample solution and 60 min for 200 ml of *Corresponding author. Fax: þ852-26035057; E-mail: [email protected] 825 Copyright & 2002 by Marcel Dekker, Inc.

www.dekker.com

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sample solution. The working range of the calibration graph was 0.11.6 mg of cobalt. The interferences from nickel, zinc, copper, chromium, manganese, lead and iron, which form colored species with 5-Br-PADAP in the polymer phase, were investigated. The method was applied satisfactorily for the determination of cobalt in laver (a kind of edible alga).


Key Words: b-Cyclodextrin polymer; Polymer spectrophotometry; Cobalt; 5-Br-PADAP

phase

INTRODUCTION Supramolecular chemistry, with b-(or a-, g-)cyclodextrin as one of the representative compounds, has been a very active research field in recent years (1–10). The b-Cyclodextrin polymer (b-CDP) that was synthesized by use of epoxy chloropropane as a cross-linking agent still retains the adsorption and inclusion property of b-cyclodextrin (b-CD) (11). Comparing it with ion-exchanger resins, b-CDP does not have a double bond in its molecule and only has less absorbance of background. The b-CDP used in our work is a transparent, colorless solid and unsoluble in aqueous solution. Its cavities as well as those of b-CDs are fairly hydrophobic; therefore b-CDP can separate and concentrate some organic compounds containing the hydrophobic aromatic group to form supramolecular complexes. Several possible or already realized applications of b-CDP have been reported (12–17). In the present paper, the chromogenic agent, 2-(5-bromo-2-pyridylazo)-5-diethylaminophenol (5-Br-PADAP), which may form complexes of very high absorptivity with many kinds of metal ions and has been used for spectrophotometric determination of these metals (18–22), is included/ adsorbed on b-CDP. Then, a colored complex with a microamount of cobalt is developed on the modified polymer and has directly been used for polymer phase spectrophotometric determination of cobalt. The method has shown several important advantages; sensitivity and selectivity that are much higher than those of the conventional spectrophotometry, low interference level, the use of conventional instrumentation and simultaneous occurence of concentration and color development for the metal ion. Polymer phase spectrophotometry used b-CDP seems to be an excellent and useful technique for determination of trace amounts of constituents. An outline of b-CDP phase spectrophotometry with 5-Br-PADAP and a 5-mm quartz cell is given and the microamounts of cobalt in laver (a kind of edible alga) are determined.

DETERMINATION OF COBALT IN FOODS BY b-CDP

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MATERIALS AND METHODS


Reagents All chemicals used were of analytical reagent grade. Deionized water was distilled before it was used. A cobalt(II) standard solution was prepared by dissolving 4.04 g of cobalt dichloride hexahydrate (CoCl26H2O) in 5 ml of concentrated hydrochloric acid and diluting to 1000 ml with distilled water to give a 1 mg/ml standard stock solution. The solution was standardized gravimetrically by mercuric thiocyanate method (23). A working standard solution containing 1mg/ml cobalt(II) was prepared from the stock solution by an appropriate dilution. The 5-Br-PADAP solution was prepared by dissolving an appropriate amount of 5-Br-PADAP (Tianjin Research Institute of Chemical Reagents, Tianjin, China) in 95% ethanol to give a 5.0 104 mol/l solution. Buffer solutions used for pH values in the range of 1.04.0 were sodium citrate–hydrochloric acid system, and in the range of 4.06.5 were acetic acid–sodium acetate system. The solutions used for pH values in the range of 6.512.0 were sodium hydroxide–potassium dihydrogen phosphate system. The masking solution for chromium, lead, and copper used was 4.0 104 mol/l of EDTA solution, and for manganese and iron used was 8.0 103 mol/l of sodium pyrophosphate solution. 0.1 mol/l of nitric acid solution or 3.0 mol/l of hydrochloric acid solution was used for eliminating the interference from nickel, zinc, chromium, manganese and lead by treatment of the colored polymer. b-CDP was synthesized by use of epoxy chloropropane as a crosslinking agent as described earlier (11). Before use, the polymer was ground and sieved first into 4060, 6080, 80100 and over 100 mesh fractions. Each of these was immersed in distilled water, then washed with 4.0 103 mol/l of EDTA solution and distilled water so as to remove the remaining metal ions. Finally they were dried under vacuum and kept in a desiccator. The 6080 mesh fraction was used in our experiment.

Apparatus A Shimadzu UV-240 spectrophotometer with matched 5- and 10-mm quartz cells was used to measure the absorbance and spectra. At the bottom of the 5-mm quartz cell, a small hole was made by an emery wheel before use, in order to release the solution that existed in the colored polymer when the colored polymer was packed into a sample cell. A Shanghai pHs-2 pH-

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meter was employed for pH measurements. A thermostatic shaker, model Peking SHZ-2, was used to perform all inclusion procedures.

Procedures


Preparation of the Modified Polymer of Inclusion of 5-Br-PADAP in b-CDP (b-CDP-5-Br-PADAP) To a 100-ml stoppered conical flask, 5.0 g of b-CDP, 10 ml of buffer solution (pH, 8.0) and 25.0 ml of 5-Br-PADAP solution (5.0 104 mol/l) were added, respectively. The mixture was diluted to 50 ml with distilled water and shaken mechanically for 1 h at room temperature (25 C). The polymer was filtered off and washed with distilled water until the filtrate was colorless and clear. After being dried under vacuum at 85 C, the polymer was kept out of light in a desiccator. In order to determine the quantity of 5-Br-PADAP fixed on the polymer, the amount of 5-Br-PADAP in the supernatant solution was determined by measurement of its absorbance at 444 nm.

Determination of Cobalt To a 50-ml stoppered conical flask, 0.5 g of b-CDP-5-Br-PADAP and 10.0 ml of buffer solution (pH, 8.0) were added. After the mixture stood for approximately 15 min so that the polymer could be swollen sufficiently, 1.0 ml of cobalt(II) solution (1 mg/ml) was added and diluted to 25 ml with distilled water. After the mixture was shaken mechanically for 40 min at room temperature, the colored polymer was transferred into a 5-mm quartz cell. The absorbance was measured at 588 nm (the absorption maximum of 5-Br-PADAP-cobalt complex in the polymer phase) against a b-CDP-5-Br-PADAP blank as a reference.

Determination of the Molar Ratio of Cobalt to 5-Br-PADAP in the Polymer Phase To a 50-ml stoppered conical flask, 0.1 g of b-CDP-5-Br-PADAP and 10.0 ml of buffer solution (pH, 8.0) were added. After the mixture stood for approximately 15 min, 5.0 ml of cobalt(II) solution (1 mg/ml) was added and diluted to 25 ml using distilled water. After the mixture was shaken mechanically for 40 min at room temperature, 5.0 ml of the supernatant solution


829

containing excess cobalt(II) was transferred into a 10-ml volumetric flask. Then, 2.0 ml of buffer solution and 1.0 ml of 5-Br-PADAP solution were added. After the solution was made up to 10 ml with distilled water and mixed well, the absorbance was measured with 10-mm cell at 584 nm (the absorption maximum of 5-Br-PADAP-cobalt complex in the solution) against a reference blank solution that did not contain cobalt. The molar ratio of cobalt to 5-Br-PADAP in the polymer phase was calculated.


Determination of Cobalt in Laver The sample of laver was decomposed to ash using a standard method (24). The ash sample was dissolved with dilute hydrochloric acid, then filtered into a 50-ml volumetric flask and made up to the mark with distilled water. An aliquot of 5 ml of a sample solution was transferred into a 50-ml stoppered conical flask and the pH value of the solution was adjusted to 8.0 with a dilute sodium hydroxide. Then 10 ml of buffer solution (pH, 8.0), 0.5 g of b-CDP-5-Br-PADAP and 3 ml of masking solutions were added successively. After standing for approximately 15 min, the mixture was made up to 25 ml with distilled water and shaken mechanically for 40 min at room temperature. The colored polymer was filtered off and washed with 0.1 mol/l nitric acid or 3.0 mol/l hydrochloric acid for 5 min and cobalt in the polymer phase was determined as described above.

RESULTS AND DISCUSSION Absorption Spectra The absorption spectra of cobalt complex and 5-Br-PADAP in the polymer and in solution are shown in Figure 1. Maximum absorbance of 5-Br-PADAP-cobalt complex is at 588 nm in the polymer phase where the absorbance of the control blank is fairly small, and at 584 nm in solution phase, maximum absorbance in the polymer phase moves 4 nm toward the longer wavelength region than that in solution phase. Moreover, maximum absorbance of 5-Br-PADAP is at 468 nm in polymer phase and at 444 nm in solution phase. Maximum absorbance of 5-Br-PADAP moves 24 nm toward the longer wavelength region than that in solution phase. Each spectrum obtained is similar to that observed in solution.


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JIANG, LI, AND YU

Figure 1. Absorption spectra of 5-Br-PADAP and its cobalt complex in b-CDP. Solution volume: 25 ml; 5-Br-PADAP: 1.0 104 mol/l; pH: 8.0; Co(II): 1 mg; b-CDP: 0.5 g; 1: 5-Br-PADAP in solution; 2 : 5-Br-PADAP in b-CDP; 3: 5-BrPADAP-Co(II) in solution; 4: 5-Br-PADAP-Co(II) in b-CDP.

Effect of Shaking Time on the Adsorption of Cobalt(II) on b-CDP-5-Br-PADAP In conventional solid–liquid separation procedures, the solution containing the analyte was often stirred with the solid adsorbent for a fixed time in order to adsorb the analyte on the solid adsorbent. Shaking is adopted instead of stirring in this work. In such conditions, no destruction of the polymer particles occurred, but destruction was often observed when stirred rapidly. The shaking time that is required for attaining the adsorption equilibrium depends on the volume of sample solutions concerned. All of the cobalt(II) in 25-ml sample solution can be adsorbed on b-CDP-5-BrPADAP within 40 min by shaking as shown in Figure 2 and in a 200-ml sample solution within 60 min.

Effect of pH on the Adsorption of Cobalt(II) on b-CDP-5-Br-PADAP In the polymer phase, 5-Br-PADAP-cobalt complex, if once formed, is very stable over a broad pH interval for a certain period. It does not decompose in the presence of EDTA, even in a strongly acidic solution such as 0.1 mol/l nitric acid or 3.0 mol/l hydrochloric acid, but many other



831

Figure 2. Effect of shaking time on the adsorption of cobalt(II) on b-CDP-5-BrPADAP. b-CDP-5-Br-PADAP (6080 mesh): 0.5 g; solution volume: 25 ml; pH: 8.0; Co(II): 1: 0.5 mg; 2: 1 mg; 3: 1.5 mg.

5-Br-PADAP-metal complexes can be destroyed in such conditions. The color of the cobalt complex has a maximum intensity for pH values in the range of 1.09.0 (Figure 3). The results are the same as in solution (18,25). Furthermore, the quantity of b-CDP adsorbing 5-Br-PADAP is maximum at pH-value of 8.0 and the b-CDP-5-Br-PADAP has a maximum stability for pH-values in the range of 6.09.0. For the reasons given the pH value of 8.0 is selected for our analysis.

Calibration, Precision and Detection Limit On the basis of the optimum conditions developed above, the calibration curve for the determination of cobalt is constructed according to the standard procedure. Good linearity is obtained for up to 1.6 mg of cobalt. The regression line may be expressed by the equation: Að, 588 nmÞ ¼ 0:0110 þ 0:3627XðmgÞ,

R ¼ 0:9980,

N¼6

Where X is the weight of cobalt in the sample solutions and A is absorbance of 5-Br-PADAP-cobalt complex in the polymer phase. The relative standard deviation for six replicate determinations is 1.2% for 1 mg in a 25 ml sample solution. The limit of detection of the method is 0.03 mg, which was calculated as three times the standard deviation of the blank (3 s criterion) (26).


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JIANG, LI, AND YU

Figure 3. Effect of pH on the adsorption of cobalt (II) on b-CDP-5-Br-PADAP; bCDP-5-Br-PADAP (60-80 mesh): 0.5 g; pH: 8.0; Co(II): 1 mg; solution volume: 25 ml.

Sensitivity In proposed polymer phase spectrophotometry, the concentration, separation and color development of the metal ion take place simultaneously. The major advantage of this method is the potential increase in sensitivity with an increase in the volume of sample solutions. The sensitivity for the system has been compared with that of the conventional spectrophotometric method (18). The sensitivity is 7.0 times as high as that of the previous method (18) with a 25-ml sample solution and 153.0 times with a 200-ml sample solution containing 1 mg of cobalt(II).

Molar Ratio of Cobalt to 5-Br-PADAP in the Polymer Phase The polymer used for analysis contains 0.81 mmol of 5-Br-PADAP per gram. In the polymer phase, the molar ratio of cobalt to 5-Br-PADAP in the complex is found to be 1 : 2 that is identical with that observed in solution (18). From this result, we may hypothesize that whole molecule of 5-BrPADAP does not reside in the cavity of b-CD, only one group or part of the molecule.

Effect of Foreign Ions The effects of foreign ions were examined and the results obtained are shown in Table 1. Metal ions do not interfere except for nickel, zinc,



833

Table 1.

Effect of Foreign Ions on Determination of 1mg Cobalt (Solution: 25 ml)

Foreign Ions

Weight-Ratio of Foreign Ion to Co

Ni2þ

50 : 1

Zn2þ

200 : 1

Cd2þ Pb2þ Cu2þ Fe3þ Mn2þ

200 : 1 500 : 1 100 : 1 100 : 1 150 : 1

Masking Solution Washing the colored polymer with 0.1 mol/l HNO3 or 3.0 mol/l HCl Washing the colored polymer with 0.1 mol/l HNO3 or 3.0 mol/l HCl 0.01 mol/l EDTA 1 ml 0.01 mol/l EDTA 1 ml 0.01 mol/l EDTA 1 ml 0.1 mol/l Na4P2O7 2 ml 0.1 mol/l Na4P2O7 2 ml

Co Found (mg)

Relative Error (%)

0.952

4.6

0.972

2.8

1.017 0.985 0.981 1.048 0.969

1.7 1.5 1.9 4.8 3.1

chromium, lead, copper, manganese and iron. These metal ions can react with 5-Br-PADAP to form colored complexes that have a large absorbance at 588 nm. Chromium, lead and copper can be masked with EDTA solution (4.0 104 mol/l). The effects of manganese and iron can be masked with sodium pyrophosphate solution (8.0 103 mol/l). Moreover, the interference from nickel, zinc, chromium, lead and manganese can be eliminated easily, because their 5-Br-PADAP complexes can be decomposed by treatment of colored b-CDP-5-Br-PADAP polymer with 0.1 mol/l nitric acid or 3.0 mol/l hydrochloric acid for 5 min after the shaking period. In this condition, no release of 5-Br-PADAP and its cobalt complex from the polymer was observed.

Table 2.

Trace Amounts of Cobalt in Laver (n ¼ 3)

Co Found (mg/g) Samples Laver Laver Laver Laver

1 2 3 4

This Method

AAS*

Co Added (mg/g)

Co Recovered (mg/g)

Recovery (%)

0.458 0.499 0.446 0.532

0.465 0.511 0.431 0.550

0.4 0.4 0.4 0.4

0.826 0.890 0.856 0.902

92.0 97.8 102.5 92.5

*Atomic absorption spectrometry.

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Determination of Cobalt in Laver


The developed method was applied to the determination of cobalt in four laver samples purchased from the local market and grown in different sea areas. The results are given in Table 2. The cobalt contents in four different laver samples are in the range of 0.4460.532 mg/g. The recoveries of cobalt added are in range of 92.0102.5%. In order to further check the validation of the proposed method, cobalt contents in the samples were determined simultaneously by atomic absorption spectrometry and the results are also shown in Table 2. As can be seen, the proposed method is very sensitive and selective.

ACKNOWLEDGMENTS The authors wish to acknowledge the Educational Commission of Tianjin for financial support (grant number 990709).

REFERENCES 1. Szejtli, J. Cyclodextrin and Their Inclusion Complexes. Akademiai Kiado: Budapest, 1982. 2. Szejtli, J.; Budai, Z. Acta Chim. Acad. Sci. Hung. 1979, 99, 433. 3. Szejtli, J.; Szente, L.; Banky-Elod, E. Acta Chim. Acad. Sci. Hung. 1979, 101, 27. 4. Hinze, W.L. Separation and Purification Methods. Marcel Dekker: New York, 1981; Vol. 10, pp. 159. 5. Tardy-Lengyel, M.; Szejtli, J. J. Inclusion Phenom. 1982, 35, 127. 6. Hamai, S. Bull. Chem. Soc. Jpn. 1982, 55, 2721. 7. Harata, S. Bull. Chem. Soc. Jpn. 1975, 48, 2409. 8. Song, L.X.; Meng, Q.J.; You, X.Z. Chem. Res. App. 1994, 6, 99. 9. Lu, X.F.; Xia, Z.; Xu, F.P.; Xie, M.G. Chinese J. Anal. Chem. 1996, 24, 621. 10. Szejtli, J. Starch 1982, 34, 379. 11. Komiyama, M.; Hirai, H. Polym. J. 1987, 19, 773. 12. Johnson, R.L.; Chandler, B.V. J. Sci. Food Agric. 1982, 33, 287. 13. Shaw, P.E.; Wilson, C.W. J. Food Sci. 1983, 48, 646. 14. Shaw, P.E.; Wilson, C.W. J. Food Sci. 1985, 50, 1205. 15. Shaw, P.E.; Tatum, J.H.; Wilson, C.W. J. Agric. Food Chem. 1984, 32, 832. 16. Shaw, P.E.; Buslig, B.S. J. Agric. Food Chem. 1986, 34, 837.


17. 18. 19. 20. 21. 22.


23. 24. 25. 26.

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Yu, E.K.C. Appl. Microbiol. Biotechnol. 1988, 28, 546. Zbiral, J.; Sommer, L. Fresenius J. Anal. Chem. 1981, 306, 129. Ping, X.K. Yankuang Ceshi 1989, 8, 101. Wei, F.S.; Zhu, Y.R.; Yin, F. Anal. Lett. 1981, 14, 241. Wei, F.S.; Qu, P.H.; Shen, N.K.; Yin, F. Talanta 1981, 28, 189. Taguchi, S.; Kasahara, I.; Fukushima, Y.; Goto, K. Talanta 1981, 28, 616. Vogel, A.I. Textbook of Quantitative Inorganic Analysis. ELBS: London, 1962; 530. Chinese National Standard for Food and Hygiene, GB 5009.4-85, 1985. Ma, L.Y.; Huang, S.Y.; Zhang, Y.L. Yankuang Ceshi 1988, 7, 166. Currie, L.A. Nomenclature in Evaluation of Analytical Methods Including Detection and Quantification Capabilities (IUPAC Recommendations 1995). Anal. Chim. Acta 1999, 391, 105–126.

Received October 24, 2001 Accepted January 17, 2002


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