Question

Rather than titrating the active ingredient in an antacid directly with a standard solution of a strong acid in our buret, in this experiement we first dissolve the antacid tablet in a measured excess of standard acid, and then titrate (with standard sodium hydroxide solution) the portion of the acid that was not consumed by the tablet. Explain.

Answer 1

Antacids are bases that react stoichiometrically with acid. The number of moles of acid that can be neutralized by a single tablet of a commercial antacid will be determined by back titration. To do the experiment, an antacid tablet will be dissolved in a known excess amount of acid. The resulting solution will be acidic because the tablet did not provide enough moles of base to completely neutralize the acid. The solution will be titrated with base of known concentration to determine the amount of acid not neutralized by the tablet. To find the number of moles of acid neutralized by the tablet, the number of moles of acid neutralized in the titration is subtracted from the moles of acid in the initial solution.

An acid is a source of aqueous H⁺(aq). For example, HCl(aq) is the acid in your stomach: HCl(aq) → H⁺(aq) + Cl^–(aq). In a healthy stomach, pH is regulated naturally and digestion functions properly when the pH is around 3 (recall neutral is pH = 7). Excess stomach acid can be combated with bases, or "antacids". Bases are H⁺(aq) acceptors; in water, they provide species that can react with H⁺(aq). Common ingredients in antacids are metal hydroxide and metal carbonate salts. The hydroxides provide hydroxide ion, OH^–, which can react with H⁺(aq) to form H₂O. Carbonates provide the carbonate ion, CO₃^2–, which can react with H⁺(aq) to form H₂O and CO₂. The reactions of interest in this lab are neutralization reactions.

H⁺(aq) + OH^–(aq) → H₂O(l)
2 H⁺(aq) + CO₃^2–(aq) → H₂O(l) + CO₂(g)

The active ingredients in the antacid used in this experiment are listed on the label as 110 mg of Mg(OH)₂ and 550 mg of CaCO₃. The balanced equations for the neutralization of acid with these active ingredients are:

Mg(OH)₂ + 2 HCl Mg²⁺ + 2 Cl^– + 2 H₂O
CaCO₃ + 2 HCl Ca²⁺ + 2 Cl^– + CO₂(g) + H₂O