Question

Experiment: Antacid tablets will be analyzed for their ability to neutralize acids. Specifically the quantity of 'simulated stomach acid' neutralized by the antacid.

Question: Why was methyl red used as an indicator in the experiment rether than phenolphthalein? (endpoint between pH 8 and 10)

Answer 1

You asked if you could use the other two in the neutralization   of acids. It depends on what acid you are standardizing with. If it is a strong acid such as HCl or HClO4, then you can probably use either phenolphthalein or methyl orange, although there might be a very slight but measurable difference. Methyl red changes color way on the acid side, so your results will be off using this indicator. We usually use KHP, or potassium hydrogen phthalate to standardize NaOH. This is a weak acid, and the equivalence point occurs in the alkaline pH range, right about at the transition range for phenolphthalein, that is, about pH 8.5-9.5.

The calculations will be the same, but if you use an indicator that changes color far from the equivalence point, then your results will be in error. For lactic acid, still a weak acid,. I'd use phenolphthalein. The pH transition range is the pH change to go from one color to the other. The usual assumption we make in the calculations is that 90% of the color of the titrated indicator will cover up the color of 10 % of the untitrated indicator.