When too much acid builds up in your stomach, it can feel upset. Alka-Seltzer is a "buffer" which keeps your stomach from being too acidic. This demonstration using bromphenol blue and vinegar, will show how Alka-Seltzer neutralizes stomach acid.
Original effervescent formula Alka-Seltzer tablet
100 mL/3.4 oz of water
Bromphenol blue indicator
To about 100 mL/3.4 oz of water, add 5-10 drops of bromphenol blue indicator. Water must turn blue.
Add a dropperful of vinegar. The water will turn yellow.
Add an Alka-Seltzer tablet. As it starts to fizz, the color changes back to blue.
The bromphenol blue is an acid/base indicator. Vinegar is acidic, as is your stomach. Bromphenol blue turns yellow in the presence of excess acid, as is the case when the vinegar is added to the water. This experiment demonstrates how Alka-Seltzer turns the solution back to blue – indicating the acid has been neutralized.