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:
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