Aspirin Purity by pH Titration
prepared by R. C. Kerber, M. J. Akhtar, and R. F. Schneider, SUNY at Stony Brook (Rev 3/00)
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, acetylsalicylic acid, and with
commercial aspirin tablets.
Aspirin (originally a legally restricted brand name owned by Bayer for its brand of acetylsalicylic 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.
Chewing willow twigs was a traditional folk remedy in many parts of Europe for headache and other
minor pains. In the nineteenth century, chemists isolated salicylic acid from willow twigs and showed that
it was the effective ingredient.
The name derives from the Latin name
, the genus of willow trees.
Pure salicylic acid, however, is very damaging to the stomach, and the acetyl derivative was prepared in
order to reduce this undesirable side effect.
Acetylsalicylic acid has become the most extensively used
pharmaceutical product in the world.
On standing for prolonged periods of time in a humid atmosphere, aspirin tablets develop a vinegary
smell that you may have noticed.
This is due to the
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
(see below) for confirmation of our conclusions.
can be done for both the synthesized sample and the tablets.