Strength of Antacids

I include this exercise for two reasons: first, to fit weakly into the unit we do in lecture on medicine and drugs and second, to give them their only buret titration experience in the CHM 1101/1102 sequence. The principle of the exercise is to add commercial antacid to a known amount of excess HCl, determine residual acid by titration, and calculate the amount of acid neutralized by the antacid.

I use store brands of Tums, Alka-Seltzer, and Mylanta. Gaviscon gave problems because it contains aluminum which precipitates and confounds the endpoint. Mylanta is green in color, but that doesn't seriously interfere with visibility of the titration endpoint. Alka-Seltzer is interesting because it contains lots of citric acid, which has three endpoints from pH 3 to around 7. As a result, its neutralization efficiency depends on the indicator you select for the endpoint: alizarin with an endpoint of 6 gives a much higher value than bromphenol blue with its endpoint of 3.8. I used the latter indicator so students' titrations would be over before they ran into the pH range for neutralization of carbonic acid, because carbonate salts are common antacid ingredients.

Students put one antacid tablet into 25 mL of 1.00 M HCl and let it dissolve. Alka-Seltzer dissolves quickly, but Tums and Mylanta take about 20 minutes. They then warm the flask to remove excess carbon dioxide, wash down the sides of the flask, and add two drops of bromphenol blue (0.1% in water). They then carry out the titration using 50-mL burets containing 0.500 M NaOH (they need from 20 to 40 mL). After tabulating their data, they determine the amount of acid consumed by their antacid.

The exercise concludes with a question regarding an old TV ad that rankles me. In the ad, an antacid was shown neutralizing acid in a beaker compared with the effect (no effect) of a tablet of Zantac 75, and the announcer says "look at how much more effective our antacid is than Zantac 75". Having discussed the action of H-2 blockers in class, I ask them what is misleading about the ad. This is a good example of the utility of "chemical literacy" to consumers.

Revised 9/1/06