Ask A Scientist Chemistry Archive

name         Nathan
status       student

Question -   We are doing an experiment on antacids in my chemistry class.  Why would an antacid 
containing calcium carbonate be more effective than an antacid containing aluminum hydroxide?
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Based solely upon the number of gm of antacid per mole of (H+) consumed, 50 and 26 gm for CaCO3 and 
Al(OH)3 respectively, aluminum hydroxide would be more effective. However, that may not be the only 
consideration. Cost, availability, and ease of formulation may also be factors. In addition, calcium 
is a mineral nutrient whereas aluminum [to my knowledge] is not. In fact, there have been some 
reports [I am not sure how reliable the studies are.] that aluminum has some undesired side effects. 
Another consideration is the pH at the equivalence point. A product like Alka Seltzer has a pH ~4-5 
at its equivalence point -- which is on the acid side -- nonetheless it is used widely. The reason is 
that "acid indigestion" involves a pH of ~1-2, so it makes little practical difference whether the 
ending pH is ~ 4 or 7.5. The "advantage" of the dissolved antacid is that the neutralization 
essentially instantaneous, where the "solid" antacids could become coated with food in the stomach 
and slow its neutralization of the gastric acid.

Vince Calder
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