Preparing simulated stomach acid. Freshly secreted gastric juice is 0.4 to 0.5% hydrochloric acid (HCl) by mass, or about 0.1 M HCl, with a pH of around 1.
See if your school's science teacher can provide you with standardized 0.1 M HCl. If not,
you can buy hydrochloric acid at the hardware
store as "muriatic acid"; it's used for cleaning bricks. Muriatic acid is anywhere from 28% to 36% HCl, so you'll have to dilute it
by roughly 1:60 to get a solution that is an acceptable simulation of gastric juice for your purposes.
An adult should do the dilution:
hydrochloric acid is corrosive and poisonous.
Wear goggles while handling the concentrated acid and have a box of baking soda handy to neutralize spilled acid.
Measuring acid neutralizing capability.
You really want to measure the amount of stomach acid neutralized, not the change in pH.
Prepare a solution of a base that is similar in concentration to the acid (0.1 M NaOH would be ideal).
Determine what volume of base solution is necessary to neutralize 1) a measured volume of fresh acid,
and 2) the same volume of acid which has a weighed, crushed antacid tablet dissolved in it.
The difference between volumes of base in 1) and 2) is a good, reproducible measure of how much acid was actually neutralized by the antacid.
You might find the Utah State Office of Education's page on comparative studies of common household solutions helpful as you design this part of your experiment.
Also see S. Phillips' Chemistry in the Modern World Laboratory pages for ideas.
Detecting complete neutralization.
You'll need some way of determining when the acid has been completely neutralized.
Add an indicator to the acid before adding any base or antacid. An indicator is a dye that changes color when the pH of the solution changes. Bromothymol blue is a good choice for this experiment. It changes from yellow in acidic solution to blue in basic solution:
Author: Fred Senese firstname.lastname@example.org