Lab4_Spectrophotometric_Indicator - Chem 223...

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1 Chem 223 Spectrophotometric Determination of an Indicator's pK a Version of 9/4/10 From 1990 to 2004, a variant of this lab was part of Chem 223 and its predecessors. Because of problems making the lab work reliably, it was dropped. With several changes, we re-introduced the experiment in 2009. It worked better, but it still wasn't all that we hoped. The changes included measuring the pH of buffers, not just hoping they were as marked. It also included improvements to the way we dealt with activity coefficients. Now, for 2010, we include two more improvements. First, the way that data are analyzed is being improved. Second, we include instructions on doing adequate signal averaging with the spectrophotometers to improve data precision. Will the lab finally work? If it fails, will it fail in a way that indicates how to change it for the better (as it did in 2009)? If it works, everyone will get a nice, neat result. If it fails, your insight can perhaps point the direction to future success. Think of this as a graded research project. References: 1. R. W. Ramette, Chemical Equilibria and Analysis , Addison-Wesley, Reading, MA, 1981, Chap. 13. 2. H. A. Laitinen and W. E. Harris, Chemical Analysis , 2 nd ed., McGraw-Hill, New York, 1975, pp. 48-51. 3. D. J. Harris, Quantitative Chemical Analysis , 5 th ed., W. H. Freeman, New York, 1998, Chap. 12. (in the 7 th edition, relevant material is in chapters 8-11). INTRODUCTION In this experiment, the Stellarnet Diode Array Spectrophotometers will be used to evaluate the pK a of an acid-base indicator. In the process, fundamental characteristics of indicators and acid-base equilibria in general should become apparent. As seen in a previous lab, titration is a common method of quantitative analysis. Titrations depend upon accurate determination of the equivalence point for a given stoichiometric reaction. (In fact, the "end point" rather than the "equivalence point" is measured.) Indicators make this quantitative analysis possible. Harris defines an indicator as "a compound having a physical property that changes abruptly near the equivalence point of a chemical reaction." For the acid-base indicators used in this lab, the "physical property" is a color that is dependent on the chemical state of the indicator. Thymol blue (thymolsulfonephthalein) is one of a member of a large class of acid-base indicators that are known as the "sulfone-phthaleins." The possible forms of the indicator and their colors are shown in the figure on the next page. The pH at which an indicator undergoes a color transition determines if it is acidic or basic, and corresponds to that indicator's pK a i.e. an acid indicator has a transition range at an acidic pH. Most indicators have only one transition range, but some indicators are polyprotic and can have multiple transitions as seen above for thymol blue.
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2 Figure 1. Different chemical forms of the acid-base indicator thymol blue. Regardless of an indicator's transition range, the compound itself behaves the same as a buffer and can
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This note was uploaded on 01/06/2011 for the course CHEM 223 taught by Professor Scheeline during the Fall '08 term at University of Illinois at Urbana–Champaign.

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Lab4_Spectrophotometric_Indicator - Chem 223...

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