In the Laboratory
A Redox Titration for a General-Organic-Biochemistry (GOB) Course Using Povidone Iodine Allan R. Pinhas Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172
[email protected] An iodine titration, or more broadly any redox titration, is a classic and important technique taught in most general chemistry laboratory courses because it requires students to learn about titrating, oxidizing and reducing agents, balancing equations, making solutions of known concentrations, and so forth (1). In an effort to generate a simplified and safer (2) iodine redox titration that is relevant to nursing students and other allied health students taking a general-organic-biochemistry (GOB) course, an experiment has been developed in which the quantity of iodine in povidone iodine is determined by titration with sodium thiosulfate. The net redox reaction for this titration is (3) 2Na2 S2 O3 þ I2 f Na2 S4 O6 þ 2NaI Advantages of Povidone Iodine Povidone iodine, also known as betadine, is a substance that nurses and other health care professionals use fairly often. For example, it is the reddish-brown liquid put on an arm prior to blood donation. Povidone iodine, a complex between polyvinylpyrrolidone and iodine that contains about 10% available iodine, is used because it is an effective antiseptic agent that kills bacteria and viruses, but has minimal toxicity to mammalian cells. Another attractive feature is that povidone iodine is more stable and more easily handled than pure iodine (4, 5). Povidone iodine is readily available from many sources. When purchased, the aqueous solution is approximately 10% povidone iodine by weight, which means it is approximately 1% free iodine by weight. This value is significantly higher than the quantity of I2 that can be dissolved in water. The higher solubility makes the titration easier for the students because there are no precipitates and there is no need to use organic solvents to increase the iodine solubility. The use of povidone iodine has other advantages as well. The solution is mildly acidic (pH is approximately 4.5), so disproportionation of iodine (I2) to hypoiodite (IO-) plus iodide (I-) is not a problem. Second, in a typical iodine titration, a dilute starch solution must be used as an indicator near the end of the titration. However, for the povidone iodine titration, the color change from reddish-brown to almost colorless works well as an indicator, and thus, reproducible results may be obtained without the addition of starch. Povidone Iodine Titration In a typical titration, 5.0 g of povidone iodine solution was dissolved in about 25 mL of water and titrated with a 0.10 M sodium thiosulfate solution. Using these quantities, it will take
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approximately 3 mL of the thiosulfate solution per titration. From the balanced equation given above, the concentration and volume of thiosulfate solution used and the initial mass of povidone iodine, the mass percent of iodine in povidone iodine may be calculated. Hazards Povidone iodine is used daily in hospitals and other medical facilities, and so, there is no hazard. In addition, because of the water solubility of the povidone iodine and the other solution used in this experiment, all glassware may be cleaned in a sink with water. A material safety data sheet for povidone iodine may be found in ref 6. Experimental Section A full experimental section including calculations and a table of results may be found in the supporting information. Acknowledgment The author thanks the University Research Council of the University of Cincinnati for financial support for this research and the faculty and students of the Department of Chemistry and the Department of Pharmacology of the University of Louisville for their hospitality during a sabbatical leave, when this communication was written. Literature Cited 1. See for example: (a) Sigmann, S. B.; Wheeler, D. E. J. Chem. Educ. 2004, 81, 1479. (b) Copper, C. L.; Koubek, E. J. Chem. Educ. 2001, 78, 652. (c) Rogers, C. U. J. Chem. Educ. 1969, 46, A34. 2. The toxicity of elemental iodine is discussed in the following: Young, J. A. J. Chem. Educ. 2006, 83, 1285. 3. See for example: (a) Denniston, K. J.; Topping, J. J.; Caret, R. L. General, Organic, and Biochemistry, 6th ed.; McGraw-Hill Higher Education: New York, 2007. (b) Moore, J. W.; Stanitski, C. L.; Jurs, P. C. Chemistry, The Molecular Science, 2nd ed.; Thomson, Brooks Cole: Belmont, CA, 2005. (c) Silberberg, M. S. Principles of General Chemistry; McGraw-Hill Higher Education: New York, 2007. 4. See for example: (a) National Cancer Institute: Povidone Iodine Solution, http://www.cancer.gov/Templates/drugdictionary.aspx? CdrID=539730 (accessed Jun 2010). (b) PVP-Iodine: Povidone Iodine Antiseptic Agent, http://online1.ispcorp.com/Brochures/Pharma/ pvpiodine.pdf (accessed Jun 2010). (c) PVP-iodine grades, http:// www.makeni.com.br/Portals/Makeni/prod/boletim/PVP%20iodo. pdf (accessed Jun 2010).
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 9 September 2010 10.1021/ed1002948 Published on Web 07/08/2010
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5. Atemnkeng, M. A.; Plaizier-Vercammena, J.; Schuermans, A. Int. J. Pharm. 2006, 317, 161. 6. Material Safety Data Sheet: Povidone-Iodine Solution 10%, http:// msds.farnam.com/m000359.htm (accessed Jun 2010).
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Supporting Information Available Experimental details. This material is available via the Internet at http://pubs.acs.org.
pubs.acs.org/jchemeduc
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r 2010 American Chemical Society and Division of Chemical Education, Inc.
