For the antacid analysis, the chemical reactions that occur in the titration are the following ones:
First, the antacid (composed of weak bases and carbonates) is completely neutralized by the H⁺ ions in the HCl
2HCl + CaCO₃ → CO₂ + H₂O + 2CaCl₂
HCl + OH⁻ → H₂O + Cl⁻
The titration reaction consists in titrating the excess H⁺ ions that are left in the solution, by the following reaction:
H⁺ + NaOH → H₂O + Na⁺
So, when the equivalence point is reached, the solution will go from acid to basic. As bromophenol blue is yellow in acidic solution and blue in basic solution, you'll expect the indicator to change from yellow to blue.
