susb012 - SUSB 012 Aspirin Purity by pH Titration prepared...

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SUSB 012 Aspirin Purity by pH Titration prepared by R. C. Kerber, M. J. Akhtar, and R. F. Schneider, SUNY at Stony Brook (Rev 02/05) Purpose of this Exercise : To determine the purity of your synthesized aspirin sample by acid- base titration, and to compare the results with those from pure salicylic acid and pure acetylsali- cylic acid. Background Information Aspirin (originally a legally restricted brand name owned by Bayer for its brand of ace- tylsalicylic acid, but now a generic term) has been in use as an analgesic (pain-killer) and antipy- retic (fever-reducer) for over a century. During most of that time, little was known about its mode of action in the body, although this has become better understood in recent years. It has also been realized that, in addition to its mild short-term analgesic effect, aspirin has some potent long-range benefits, including reducing the probability of users' suffering heart attacks and colon cancer. On standing for prolonged periods of time in a humid atmosphere, aspirin tablets develop a vinegary smell that you may have noticed. This was the basis for one of the tests you performed to verify that your synthesis produced aspirin. It is due to the hydrolysis of acetylsalicylic acid (ASA) to form salicylic acid (SA). A hydrolysis is a reaction with water to break one or more bonds. In this case, the acetyl group comes off as acetic acid, which causes the vinegary smell. This is a reversal of the synthesis process you carried out previously in this course. In this exercise, we will determine by acid-base titration the amounts of ASA and SA in your synthesized sample of aspirin. [Incomplete reaction or inadequate washing of the product are likely to have left the ASA contaminated with some SA.] The same titration process can be used to determine ASA content in commercial aspirin tablets. Commercial tablets usually contain inert binders such as starch, so it is not possible to determine the ratio of SA and ASA by titration alone unless the binder is removed. We can, however, determine the overall percent of active ingredient. We will make use of the effective molar mass of the synthesized samples to assay their content, but we will also take note of the overall titration curve (see below) for confirmation of our conclusions. This can be done for both the synthesized sample and the tablets. Some data on SA and ASA relevant to this exercise are the following: Compound Salicylic Acid (SA) Acetylsalicylic Acid (ASA) F o r m u l a C 7 H 6 O 3 C 9 H 8 O 4 Molar Mass 138.12 180.15 K a 1.08 x 10 -3 2.72 x 10 -5 pK a 2.99 3.50 Solubility (g/100 mL) 0.18 0.25
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SA and ASA have significantly different molar masses. If we titrate the same weight of the two acids with base, it will take more base to reach the endpoint with salicylic acid, since there will be more mmoles of acid in the sample due to its lower molar mass.
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This note was uploaded on 11/12/2011 for the course CHEM CHEM 2B taught by Professor Cabrera during the Spring '09 term at San Jose City College.

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susb012 - SUSB 012 Aspirin Purity by pH Titration prepared...

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