In the Classroom edited by
Erica K. Jacobsen The Dalles, Oregon 97058
JCE Classroom Activity #106. Sequestration of Divalent Metal Ion by Superabsorbent Polymer in Diapers Yueh-Huey Chen,* Jia-Ying Lin, Li-Pin Lin, and Han Liang Department of Applied Science, National Dong Hwa University (Meilun Campus), Hualien 970, Taiwan, ROC *
[email protected] Jing-Fun Yaung Department of Food Science, National Kinmen Institute of Technology, Kinmen 893, Taiwan, ROC
Instructor Information Superabsorbent polyacrylate, known as a highly waterabsorbing material, is frequently found in disposable diapers. When this hydrophilic polymer comes in contact with a solution containing multivalent metal ions, it swells and takes up the metal ions (1). In this activity, the students investigate the action of the polymer contained in disposable diapers using CuCl2 solution. The sequestration of Cu2þ ions by the polymer is verified by the generation of blue-colored polymer gels, color loss of the resulting solution, and better sudsing action of the solution with soap. (See Figure 1.) Background Superabsorbent polymers commonly used in disposable diapers and personal care products have a wide variety of applications based on their remarkable water-absorbing capability (2). Several activities involving these polymers can be found in this Journal (3-6). Some of these commercially important polymers are lightly cross-linked sodium salt of poly(acrylic acid). When exposed to a solution of Cu2þ, the polymer sequesters the Cu2þ ions through the ion exchange with the sodium ions of its -COONa groups. The generation of (-COO)2Cu groups results in additional cross-links in the polymer network, and colors the polymer blue. (See Scheme 1.) Consequently, Cu2þ ions are withdrawn from the solution by the insoluble polymer,
and the color of the solution is lost. Cu2þ ions in aqueous solution have an action similar to the Ca2þ and Mg2þ ions contained in hard water, and will precipitate with soap molecules when soap solution is added. This results in a poor sudsing action of the solution. Therefore, the solution treated with superabsorbent polymer can be differentiated from the original Cu2þ solution using soap solution. (See Figure 2.) Integrating the Activity into Your Curriculum This activity demonstrates an alterative usage of superabsorbent polymer commonly known as a water-absorbing material. It can be used in curriculum units related to polymer chemistry and water chemistry. Chemical concepts involved in this activity are chemical bonding, chemical reaction, ionic compound, and solution/concentration. It is also appropriate to extend this activity to a take-home exercise on water softening, replacing the Cu2þ solution with relatively hard tap water or CaCl2 solution. The CaCl2 solution can be made by dissolving CaCl2 or the CaCl2 deicer (not the NaCl deicer) in water. About the Activity This activity is adapted from experiments described in a prior publication in this Journal.(6) It had been performed by approximately 60 students, some of whom are science majors and some are nonscience majors. The entire experiment can be accomplished within 40 min. Photos of the stages of the activity are included in the online supporting information. Pampers Easy-Ups disposable training pants for girls XL 12þ kg and Pampers Newborn Extra Soft diapers, size S for 3-8 kg were the disposable diapers we used. The mesh cover of the unused diaper Scheme 1. The Exchange of the Naþ Ions in the Polymer with Divalent Metal Ions (M2þ) in the Solution
Figure 1. A dilute solution of Cu2þ is treated with a small square of unused disposable diaper containing superabsorbent polymers.
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Vol. 87 No. 9 September 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100415e Published on Web 07/15/2010
In the Classroom
companies; it can also be found at some stores where it is available as a moisture-absorber such as DampRid, or as a deicer. The soap solution is prepared by dissolving ∼1.0 g of Ivory bar soap in 100 mL of warm deionized water (∼50-60 °C) and used when it is warm. Bars of Ivory soap were purchased from a supermarket, and students grated this into flakes using a cheese/vegetable grater before using it in the activity. Although other bar soaps (possibly Camay) can also be used, we have found that Ivory bar soap has been a better choice for yielding a clear soap solution. Literature Cited
Figure 2. The treated (left) and the untreated (right) Cu2þ solutions are differentiated by the addition of soap solution.
is removed. The center pad (a rectangular area of ∼23 cm 6.5 cm for the Pampers Newborn diaper, see photos in the supporting information) of the diaper is then cut into small squares (∼3 cm 3 cm). The weight of each diaper piece used for the treatment of 100 mL of 0.005 M Cu2þ solution is in the range of 0.6-0.7 g. The solution of Cu2þ for 20 pairs of students is prepared by dissolving ∼1.7 g of CuCl2 3 2H2O (or ∼2.5 g of CuSO4 3 5H2O) in 2 L of deionized water. CuCl2 3 2H2O (CAS# 10125-13-0) can be purchased from chemical supply companies. CuSO4 3 5H2O (CAS# 7758-99-8) can be purchased from chemical supply companies; the compound available at hardware stores labeled for use as a root killer also works for this activity. The solution of 0.005 M Ca2þ is prepared by dissolving ∼1.1 g of CaCl2 in 2 L of deionized water. CaCl2 (CAS# 10043-52-4) can be purchased from chemical supply
r 2010 American Chemical Society and Division of Chemical Education, Inc.
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1. Mouginot, Y.; Morlay, C.; Cromer, M.; Vittori, O. Anal. Chim. Acta 2000, 407, 337–345. 2. Superabsorbent Polymers; Buchholz, F. L., Peppas, N. A., Eds.; ACS Symposium Series 573; American Chemical Society: Washington, DC, 1994. 3. Buchholz, F. L. Superabsorbent Polymers;An Idea Whose Time Has Come. J. Chem. Educ. 1996, 73, 512–515. 4. Criswell, B. Ions or Molecules? Polymer Gels Can Tell. J. Chem. Educ. 2006, 83, 576A–576B. 5. Criswell, B. A Diaper a Day and What's Going on with Gaviscon? Two Lab Activities Focusing on Chemical Bonding Concepts. J. Chem. Educ. 2006, 83, 574–576. 6. Yaung, J.-F.; Chen, Y.-H. Coloring a Superabsorbent Polymer with Metal Ions. An Undergraduate Chemistry Experiment. J. Chem. Educ. 2009, 86, 347–349.
Supporting Information Available Student activity worksheet; notes with photographs for instructors; answers to student questions. This material is available via the Internet at http://pubs.acs.org.
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Vol. 87 No. 9 September 2010
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Journal of Chemical Education
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