Communication pubs.acs.org/jchemeduc
Detection of Fe3+ and Al3+ by Test Paper Lili Li, Haifeng Xiang, Xiangge Zhou,* Menglong Li, and Di Wu* College of Chemistry, Sichuan University, Chengdu 610064, China S Supporting Information *
ABSTRACT: A porphyrin-based test paper has been designed and prepared. It can be used to analyze for Al3+ and Fe3+ in aqueous solution. An experiment employing the test paper can help students understand basic principles of spectrophotometry and how spectrophotometry is used in analyzing for metal ions.
KEYWORDS: First-Year Undergraduate/General, Second-Year Undergraduate, Analytical Chemistry, Demonstrations, Environmental Chemistry, Laboratory Instruction, Coordination Compounds, Spectroscopy, UV−Vis Spectroscopy, Water/Water Chemistry
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Scheme 1. Coordination Reactions of TPPS4 (middle) and Metal Ions (Al3+, Fe3+): Color Changes with the Formation of Complexes
ecause of its convenience, pH test paper is often used in assessing acidity in and out of the laboratory. Could the metal ions be detected similarly by test paper? The coordination reactions between metal ions and ligands and the resulting color changes could be used. Porphyrins, “pigments of life”, are often employed as candidate ligands in chemical sensors because of their various optical properties and abundant coordination abilities.1−3 Metal complexes of porphyrins usually exhibit characteristically sharp and intense absorption bands in the visible region4 that can be used as highly sensitive chromogenic sensors for detection of metal ions. Considering the application of ion sensors in pollution monitoring, food safety, and diagnosis of diseases, a simple and water-soluble porphyrin dye, 5,10,15,20-tetrakis(4sulfonatophenyl)porphyrin (TPPS4) (Scheme 1) is employed for a test paper. A porphyrin-based ion test paper is designed and prepared that can identify some transition-metal ions in aqueous solution.
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EXPERIMENT AND RESULTS Solutions of TPPS4 were prepared with a concentration of 3.0 × 10−5 mol/L. Solutions of the metal salts Al2(SO4)3, FeCl3, CaCl2, CuCl2, CoCl2, ZnCl2, and MnCl2 were prepared with concentrations of 3.0 × 10−2 mol/L. Distilled water was used to prepare all solutions. A solution of the porphyrin ligand was mixed with different metal ions individually and the color change noted. The color changed from lavender of the TPPS4 solution to chartreuse or goldenrod upon the addition of Al3+ or Fe3+, respectively. The Soret band5 of TPPS4 was red-shifted from 414 to 434 nm and new Q bands6 appeared at 497, 713 nm and 495, 648 nm when © 2012 American Chemical Society and Division of Chemical Education, Inc.
adding Al3+ and Fe3+, respectively (Figure 1). The detection limits of these two ions were found to be approximately 15 μM for Al3+ and 1.4 mM for Fe3+ (Supporting Information Figure SI2). Addition of other metal ions such as Mn2+, Co3+, Ca2+, Cu2+, and Zn2+ made the bands of TPPS4 in UV−vis spectra shift slightly, but the lavender solution color remained almost Published: February 2, 2012 559
dx.doi.org/10.1021/ed200114j | J. Chem. Educ. 2012, 89, 559−560
Journal of Chemical Education
Communication
Foundation (08ZQ026-041) and Ministry of Education (NCET-10-0581).
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REFERENCES
(1) Wu, D.; Descalzo, A. B.; Weik, F.; Emmerling, F.; Shen, Z.; You, X. Z.; Rurack, K. Angew. Chem., Int. Ed. 2008, 47, 193−197. (2) Michael, D. W. J. Chem. Educ. 2001, 78, 321−328. (3) Drain, C. M.; Hupp, J. T.; Suslick, K. S.; Wasielewski, M. R.; Chen, X. J. Porphyrins Phthalocyanines 2002, 6, 243−258. (4) Kadish, K. M., Smith, K. M., Guilard, R., Eds. The Porphyrin Handbook; Academic Press: New York, 2000. (5) Soret band is a very strong absorption band in the blue region of the optical absorption spectrum of a heme protein that was defined by Jacques-Louis Soret. It can be used to describe the maximum absorption of all porphyrins at about 400 nm. (6) Q bands are several weak absorption bands in the region of 500− 750 nm. They are also characteristic absorption peaks of porphyrins. (7) Battersby, A. R.; Fookes, C. J. R.; Matcham, G. W. J.; McDonald, E. Nature 1980, 285, 17−21.
Figure 1. Absorption spectra of TPPS4, Fe(III)-TPPS4, and Al(III)TPPS4.
unchanged, which proved these ions would not affect the recognition of TPPS4 to Al3+ or Fe3+.
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TEST PAPER Test paper for Al3+ and Fe3+ detection was implemented based on the above results. A strip of filter paper was immersed in TPPS4 solution for 2 min and then dried in air, resulting in a lavender ion test paper (Supporting Information Figure SI1, left). By adding a drop of Al3+ or Fe3+ solution to the test paper, the color changed from lavender to chartreuse (Supporting Information Figure SI1, middle) or goldenrod (Supporting InformationFigure SI1, right), respectively. Therefore, the color changes can indicate the existence of these two metal ions. Various metal ions can form complexes with porphyrin,7 and their influences on the HOMO−LUMO band gap of TPPS4 are different, which result in different shifts of the bands in absorption spectra, causing different color changes upon complexation. Compared with free ligand TPPS4, Al(III) and Fe(III) complexes have about 20 nm red shifts for the Soret band, and the Q bands have also changed. These remarkable shifts cause the color changes.
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CONCLUSION Porphyrin-based Al3+ and Fe3+ ion test paper is designed based on the reaction of TPPS4 and metal ions and the different color changes after coordination. The test paper works in aqueous solution and the color change can be easily observed by the eye. This portable, qualitative, and selective ion test paper possesses potential usage in various fields including environment, food industry, biology, and medicine.
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ASSOCIATED CONTENT
S Supporting Information *
Instructor notes; UV−vis spectra. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (X.Z.) and
[email protected]. cn (D.W.).
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ACKNOWLEDGMENTS This project was sponsored by the Natural Science Foundation of China (Nos. 20901052, 21072132), Sichuan Provincial 560
dx.doi.org/10.1021/ed200114j | J. Chem. Educ. 2012, 89, 559−560
