Experiment 5-009: Analysis of Antacid Tablets

Report Sheets for Experiment 5-009: pdf format

 This week you will determine the antacid (base) content of some commercially available antacids. The analysis you will perform is called a back-titration and it is described briefly below:

The Chemistry:

Various commercial products are available for the relief of heartburn and indigestion. All of these remedies are little more than weak bases that react with and neutralize stomach acid, which is primarily hydrochloric acid, HCl.

Milk of magnesia (magnesium hydroxide, Mg(OH)₂) is a common antacid, normally sold as a suspension in water, that uses hydroxide ions as the base:

Mg(OH)₂(s) + 2 H⁺_(aq) -----> Mg²⁺_(aq) + 2 H₂O _(l) [1]

Other preparations, such as Tums (CaCO₃) and Bisodol (baking soda, NaHCO₃) neutralize acids as shown in reactions 2 and 3:

CO₃^2-_(aq) + H⁺_(aq) --------> HCO₃^- _(aq) [2]

HCO₃^-_(aq) + H⁺_(aq) ----> [ H₂CO_3(aq)] --------> CO_2(g) + H₂O _(l) [3]

A more elaborate base is found in Rolaids, which contains aluminum sodium dihydroxycarbonate, and uses both carbonate and hydroxide ions to neutralize "excess stomach acid":

NaAl(OH)₂CO₃ + 4 H⁺_(aq) --------> Na⁺_(aq) + Al³⁺_(aq) + 3 H₂O _(l) + CO_2(g) [4]

Not only do commercially available antacids vary in chemical content, they also vary in the amount of acid-neutralizing substance they contain. One brand of antacid may be inexpensive but contain very little base, but have lots of inert fillers or coloring and/or flavoring agents, while an expensive antacid may actually contain more acid-neutralizing substance per dollar. Determining the best buy among the various antacids requires a knowledgeable chemist. You will participate in a class project to determine the best antacid product per dollar from a group of selected products. You will be asked to analyze a product, report the base content, and calculate the relative effectiveness per dollar.

The Methodology: Back Titration

• An accurately weighed sample of pulverized antacid tablet is dissolved in excess HCl, so that all of the base in the antacid tablet is consumed. The amount of this acid consumed by the antacid is equal to the amount of base in the antacid sample:

moles HCl consumed = moles base in sample [5]

Remember: at this point, there is excess acid (HCl) remaining in the flask.

• The remaining HCl_(aq) is titrated with NaOH_(aq) to determine the amount of acid left over - the acid that was not consumed by the antacid:

moles NaOH used in titration = moles HCl not consumed by antacid [6]

One relationship must be understood to make sense of this lab:

• The remaining HCl_(aq) is titrated with NaOH_(aq) to determine the amount of acid left :

total moles HCl added = moles HCl consumed (by antacid) + moles HCl left over [7]

• The moles of HCl consumed by the antacid can then be calculated by rearranging [7]:

moles HCl consumed = total moles HCl moles HCl left over [8]

• If the concentrations of the HCl and NaOH solutions are accurately known, then it is possible to accurately determine the moles of base in the antacid sample by combining equations 5 and 8. Your results can be reported as:

a) moles of base per gram of tablet,

b) moles of base per tablet (or per recommended dose), or as

c) moles of base per dollar.

Conclusion and Error Analysis:

In your lab notebook, based on the data below, write a brief conclusion stating which brand of antacid you would chose (1 sentence) and the criteria you used to make this decision (1-2 sentences).  Turn in the yellow copy of this conclusion at the beginning of the next  lab period.

Possible criteria to consider are cost per unit (specify which unit), tablet size, active ingredients, dosage and recommended frequency.

Data  for Antacid Tablets

The mole of acid neutralized was calculated from the list of active ingredients on the container, not from titration data.  Because some active and inactive ingredients in the some brands affect the titration, your values may differ.  In the lab report, you should report values obtained from analysis (i.e. calculations) of your titration data.

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