Question

Post-lab: Neutralization Power of an Antacid

1) This experiment used the technique of back titration. Describe the process of back titration in 2-3 sentences. Why is back titration used instead of directly titrating a solution of antacid with 0.15 M HCl?

2) Phenolphthalein is an indicator which turns a pink color at a pH of 8.3. In this experiment, phenolphthalein is added to the antacid solution to indicate the equivalence point of the back titration. Phenolphthalein solution by itself is weakly acidic. If instead of adding one drop, a student adds several milliliters of the phenolphthalein solution, will this cause the student to under-estimate or over-estimate the neutralizing power of the antacid?

3) The active ingredient in Philips Milk of Magnesia is Mg(OH)2, in Amphojel it is Al(OH)3, and in Alka-Seltzer it is NaHCO3. Write balanced equations showing how each neutralizes stomach acid.

Answer 1

1. Back titration is a quantitative chemical analysis, used to determine the concentration of an analyte. It is reverse of titration i.e., the analyte, (which is an insoluble salt) is first reacted with an excess of reagent (to dissolve it), a titration is then performed on the remaining known solution. This helps in measuring the quantity consumed by the analyte and to determine how much is in excess.

The neutralization of an antacid is back titration instead of directly titrating with a solution of antacid with 0.15 M HCl because CaCO₃ is not completely soluble in water and an excess of acid is added to completely dissolve it, which is then neutralized with more base.