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. Mylanta is green in color, but that doesn't seriously interfere with visibility of the titration endpoint. I avoid some common antacids which may contain aluminum (such as Maalox and Gaviscon), which precipitates and confounds the endpoint, or citric acid (such as Alka-Seltzer), because the need to tirate the citric acid probably misrepresents, and certainly confuses, determination of their neutralization efficiency.

Students put one antacid tablet into 25 mL of 1.00 M HCl and let it dissolve. Tums and Mylanta typically take about 20 minutes to dissolve. Students 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 1/19/11