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 antipyretic (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. A titration process can be used to determine ASA content in commercial aspirin tablets.

Indicator: phenolphthalein

Part 1. Indicator Titration

2. Accurately (on the analytical balance) weigh out 0.2 g samples of SA, ASA, and each of your synthesized aspirin samples. You don’t have to do all the weighings at once, but label containers clearly so you won’t get confused.

3. Dissolve each sample in 10 mL of 95% ethanol stirring vigorously using a 250 mL

Erlenmeyer flask. After the sample is completely dissolved, add 90 mL of distilled water, and stir the solution again.

4. Add two or three drops of phenolphthalein indicator to each solution and titrate with the standardized sodium hydroxide solution provided. (The concentration is about 0.05 M. Record the precise value in your notebook and on the data sheet.) Determine the endpoint carefully by looking for formation of a persistent light pink color. Repeat as necessary to assure an accurate and reproducible result.

5. Calculate the effective molar mass (EMM) of each sample, using the data sheet as a guide. The EMM is the mass of sample in mg which produces one mmole of H + ions upon titration. For a pure compound with one acid group per molecule, this should be the actual molar mass of the compound. For a mixture, this will be a weighted average of the molar masses of the acid components.

6. If the sample consists only of ASA and SA, then the percent composition can be derived from the EMM according to the relationships:

%SA = 100 ( 180.15 – EMM ) / ( 180.15 – 138.12 ) = 2.379 ( 180.15 – EMM )

%ASA = 100 ( EMM – 138.12 ) / ( 180.15 – 138.12 ) = 2.379 ( EMM – 138.12 )

Calculate the % composition of your sample.

Note that the relationship between percent composition and EMM applies to mixtures containing only SA and ASA! If the measured EMM is greater than 180.15 or less than 138.12, the above formulas do not apply. Simply report that the EMM was out of the possible range and assume the sample consists of 100% SA or ASA, whichever is appropriate.

  • Weigh a 0.2 g sample of ASA and a separate 0.2 g sample of aspirin.
  • Pour each sample into separate 250 mL Erlenmeyer flask
  • Add 10 mL of 95% ethanol into each flask to dissolve the samples
  • Add 2 drops of phenolphthalein indicator into each solution
  • Titrate each sample with a buret filled with NaOH and record the volume of NaOH it took to reach an endpoint with each sample

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