What are the active ingredients in alka seltzer?

Answer

Alka seltzer contains a combination of aspirin, citric acid, and sodium bicarbonate (baking soda). The aspirin works to relieve headaches and muscle pain. The last two ingredients, citric acid and sodium bicarbonate, are simply a mixture of a weak acid and a weak base (when you add alka seltzer to water, the fizzing is the reaction of the two with each other). When you have a weak acid and a weak base, the result is a buffer solution, which works to stabilize the pH.

Aspirin (Acetylsalicylic acid) is also a weak acid which would be water soluble. However, in drinking water it will dissolve slowly. Adding a weak base (sodium bicarbonate) would help force the aspirin into it's conjugate base form, increasing the polarity, and thus increase solubility.

Other WikiAnswers pages indicate a pH of 9.2, a basic solution which is probably to increase the solubility of the Aspirin, as well as both buffering the effects of Aspirin's acidity on the stomach lining, as well as working as an antacid to buffer the stomach acid.

One notes, of course, that Sodium Bicarbonate (NaHCO3) dissociates in water (with the help of acetic acid) into Na+ OH- CO2, I.E. Sodium Hydroxide, a base, and Carbon Dioxide which would be the direct cause of the fizzing effect.

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