Alka-Seltzer titration exercise: pH titration exhibiting buffering, and ppt. of Aspirin at pH < 2

The Power point includes a video of an old Aspro Clear advert, which implies that soluble Aspirin is more effective than ordinary aspirin tablets due to it dissolving in the glass. I get participants to criticise this statement at the end of the titration.

This is an exercise that enables students to 'discover' buffering after initially trying to predict the form of the titration curve. after calculating the relative amounts of acidic and basic components in a single tablet.

Alka-Seltzer tablets contain citric acid (a tricarboxcylic acid). The pKa of these acids give a relatively linear pH titration curve, with overlapping buffering regions from pH 6 - 2). The presence of aspirin in the tablet, leads to the precipitation of the aspirin at the end of the titration (pH < 2, 22-23 mL 1 M HCl)

This aspirin could be further utilised in additional experiments via rapid isolation by vacuum filtration (Buchner) from the Alka-Seltzer solution at the end of the titration.

We use this as a convenient source of relatively pure aspirin ( ca 100-150 mg > 98% for subsequent analysis by NMR HPLC. The citric acid remains in solution along with a proportion of the original aspirin and Salicylic acid. The Salicylic acid being derived fro hydrolysis of the aspirin at the low pH encountered in this titration. Hence the need for rapid aspirin isolation from the low pH solution present at the end if the titration.