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Aspirin & Salicylic Acid – Ferric Chloride Complexes

The major hydrolysis product of aspirin is salicylic acid, which can form coloured complex with neutral FeCl3. Aspirin is incapable of forming this complex.  Explain this process.

First marketed under the trademark Aspirin® in 1899, acetylsalicylic acid quickly attained a worldwide reputation as a non-prescription pain relief drug.  Even today it is one of the most widely used drugs in the treatment of headaches, body and muscle pains, arthritic pain, and more.

Salicylic acid is the major hydrolysis product of acetylsalicylic acid, as shown below in Equation 1, and its detection and quantification in samples containing acetylsalicylic acid can be important for a number of reasons.  First, because among the biggest concerns relating to pharmaceuticals is the degradation of the active ingredient during the shelf life of the drug.  Second, the production of counterfeit drugs is a problem of growing concern.  Poor or unregulated synthesis of acetylsalicylic acid can result in contamination with significant amounts of unreacted salicylic acid, which can have adverse health effects.

In the case of quick qualitative detection of acetylsalicylic acid and salicylic acid a simple visual test will suffice.  By the addition of an excess of Fe+3 in some suitable form, such as ferric chloride, to a test solution complexes of Fe+3 will be formed.  If the solution turns a deep purple colour, then salicylic acid is present in the test solution.  If it turns a yellow-orange colour, then no salicylic acid is present, and it can be assumed that only acetylsalicylic acid is present in the test solution.

This allows a quick visual test of samples of aspirin - commercial pharmaceutical preparations of aspirin should not contain any significant quantity of salicylic acid, but preparations of acetylsalicylic acid made in a university lab will contain unreacted salicylic acid.

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