Aspirin is a very weak acid and is only slightly soluble in water, but it is ver

Question

Aspirin is a very weak acid and is only slightly soluble in water, but it is very soluble in ethanol. An aspirin solution can be made by first dissolving aspirin in ethanol, then adding water. An ideal way to analyze the for an amount of dissolved solid in a sample is to use a spectrophotometer, but aspirin forms a colorless solution. It is also a very weak acid and dissociates very little. This can be remedied by complexing the dissociated aspirin with Fe3+ which gives a purple solution.

Some students and I were trying a new experiment where we were to dissolve some aspirin tablets with a bit of NaOH to speed up the dissociation, and analyze the amount of aspirin by complexing the dissociated ions with Fe3+. The experiment did not work well as the amount of aspirin that dissociated was very small and the amount that complexed with the iron was, therefore, also very small. Any purple color formed by the complex was so dilute that it was imperceptible by the spectrophotometer. We dumped all our solutions into a clean, empty waste beaker and finished for the day, disappointed at our failure. When we returned the next day, the solution was so purple, that it was no longer translucent and looked like a solution of black ink. A quick spectrum showed that the color was indeed due to the iron - aspirin complex. Based what we’ve learned in the course through this chapter, what is a reasonable explanation for what was observed? Base your answer solely on the information given in the problem, i.e. - there was no spilled grape Kool-Aid mix in the beaker. (Yes, that is a real answer from a fellow teacher in previous years.)

Explanation / Answer

The NaOH addition is not a good idea, note that the aspirin is an acid, therefore addition of NaOH + acidic aspirin will react this "molecular acid" and go to "aqeueous" solution.

Note that we require molecular samples for Fe+3 complex formation test.

Eventually, as time passes by, the NaOH will lose "strength" and the aspririn will bcome molecular in solution

molecule + Fe+3 = complex formation

So this is why, at the next day, you will see the cmplex formation...

This is a topic in liquid-liquid extraction.

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