aspirin12 - SUSB-013 Colorimetric Determination of Aspirin...

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S U S B - 0 1 3 Colorimetric Determination of Aspirin prepared by R. C. Kerber and M. J. Akhtar, SUNY at Stony Brook (Rev 5/04, RFS) Purpose of this Exercise: To analyze your synthesized aspirin samples spectrophotometrically. To compare the results of titrimetric and spectrophotometric analyses of your synthesized aspirin samples. Concepts & Techniques: Color and light absorption: Absorbance/Percent Transmission; Analytical Wavelength; Blank; Beer’s Law; relationship between concentration and absorbance; absorptivity, path length, Beer’s Law Plot; slope of linear graph; complexation of SA vs ASA; hydrolysis; preparing precise dilutions; using Spectrophotometer Background Information You should have already read substantial background information on aspirin in earlier modules and that information will not be repeated here. The module SUPL-005 , which reviews the principles of spectrophotometric analysis should be read, or re-read before reading further in this one. Review: The relationship between absorbance and concentration is expressed in the Beer- Lambert Law which states that absorbance at a given wavelength is directly related to concentration of the colored species which absorbs at that wavelength. We can readily measure absorbance using Spectronic 20 or 21 spectrophotometers. If we first make up a series of solutions of accurately known concentrations and measure their absorbances, then we can relate concentration to absorbance by preparing a calibration plot, as shown in SUPL-005 . We can then use that calibration plot to determine the concentration of unknown samples. In the present case, it is important to remember that the purple color to be used for analysis comes from salicylic acid (SA), not from acetylsalicylic acid (ASA). The existence of salicylic acid in your synthesized samples may be the result of an incomplete reaction in the synthesis of aspirin or subsequent hydrolysis of the product. The equations for the hydrolysis reaction and the formation of the colored species are given below. O CH 3 C OH + C O O 3 C HO O C O + H 2 O Acetyl salicylic Acid Salicylic Acid (Asprin) A S A S A
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Fe OH 2 O O OH 2 O O H 2 H 2 H 2 H 2 + OO H 2 H 2 H 2 Fe O 2 O O O H O O 3+ 1- 2+ YELLOW SALICYLIC ACID PURPLE Procedures CAUTION This exercise involves working with hydrochloric acid solutions. This is a highly corrosive material, capable of causing severe and painful burns upon contact with the skin or eyes. It will also damage clothing with which it comes into contact. Avoid all contact. If contact occurs, immediately flush any skin or clothing with copious amounts of cold water. Before coming to class, it will be helpful if you prepare a table in your laboratory notebook in the same format as that in Datasheet 1 for the Beer’s law data.
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This note was uploaded on 11/12/2011 for the course CHEM chem 2a taught by Professor Cabrera during the Spring '09 term at San Jose City College.

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aspirin12 - SUSB-013 Colorimetric Determination of Aspirin...

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