Chemistry 104: Analysis of Commercial Antacid Tablets

Hydrochloric acid (HCl) is one of the substances found in gastric juices secreted by the lining of the stomach. HCl is needed by the enzyme pepsin to catalyze the digestion of proteins in the food we eat. Heartburn is a symptom that results when the stomach produces too much acid (hyperacidity).

Antacids are bases used to neutralize the acid that causes heartburn. Despite the many commercial brand, almost all antacids act on excess stomach acid by neutralizing it with weak bases. The most common of these bases are hydroxides, carbonates, or bicarbonates. The following table contains a list of the active ingredients found in several common commercial antacids, and the reactions by which these antacids neutralize the HCl in stomach acid.
Compound Chemical Formula Chemical Reaction
Aluminum hydroxide Al(OH)3 Al(OH)3(s) + 3 HCl(aq) -----> AlCl3(aq) + 3 H2O(l)
Calcium carbonate CaCO3 CaCO3(s) + 2 HCl(aq) -----> CaCl2(aq) + H2O(l) + CO2(g)
Magnesium carbonate MgCO3 MgCO3(s) + 2 HCl(aq) -----> MgCl2(aq) + H2O(l) + CO2(g)
Magnesium hydroxide Mg(OH)2 Mg(OH)2(s) + 2 HCl(aq) -----> MgCl2(aq) + 2 H2O(l)
Sodium bicarbonate NaHCO3 NaHCO3(aq) + HCl(aq) -----> NaCl(aq) + H2O(l) + CO2(g)
In this experiment, several brands of antacids will be analyzed to determine the number of moles of acid neutralized per tablet and the cost analysis of each tablet. The analytical procedure used is known as back titration. In this procedure, a known amount of HCl, which is in excess, will be reacted with a weighed portion of a ground antacid tablet. The HCl remaining after the antacid neutralization reaction occurs will be determined by titration with a standardized NaOH solution to a phenolphthalein endpoint. The number of moles of HCl neutralized by the antacid (HClneutralized) is the difference between the moles of HCl initially present in the excess (HClinitial) and the moles of HCl titrated by the NaOH (HCltitrated).

HClinitial HCltitrated = HClneutralized


Analytical Procedure
1.    Clean, rinse, and fill a buret with NaOH solution just as you did in last week's experiment. Record the molarity of the NaOH and the initial NaOH volume reading from the buret (+0.02 mL).

2.    Weigh an antacid table in a weighing boat and record the mass (+0.01 g). Transfer the tablet to a clean mortar and crush the tablet into a fine powder using the pestle.

3.    Weigh about 0.2 grams (+0.01 g) of the ground up tablet powder and transfer it to a clean 250 mL Erlenmeyer flask.

4.    Transfer about 20 mL (+0.02 mL) of the HCl to the flask from the acid buret located on the reagent bench. Record the molarity of the HCl and the initial and final volume readings from the buret (+0.02 mL).

5.    Swirl the flask to help dissolve the antacid. Since the antacid tablet may contain inert ingredients, much of the tablet may not dissolve and the liquid will be cloudy. The active antacid ingredient will dissolve and react with the acid very quickly while the liquid will remain cloudy.

6.    Add 3 - 4 drops of phenolphthalein indicator to the flask, and titrate the acid solution to a phenolphthalein endpoint. Record the final NaOH volume (+0.02 mL). Since the liquid is cloudy, the color change at the endpoint may be particularly hard to detect.

7.    Repeat this analysis on two other brands of antacid tablets.

8.    Before returning the buret to the lab bench, please rinse it out with a couple of water rinses.

Chemistry 104: Analysis of Commercial Antacid Tablets

Name________________________________________Hood No.______ Date_____________

Show all calculations on an attached second sheet.

Molarity of NaOH _______M

Molarity of HCl _______M
  Tablet 1 Tablet 2 Tablet 3
Antacid brand name ____________________ ____________________ ____________________
antacid tablet wt. ______________g ______________g ______________g
crushed antacid sample wt.  ______________g ______________g ______________g
initial HCl buret reading


final HCl buret reading ______________mL ______________mL ______________mL
mL HCl delivered ______________mL ______________mL ______________mL
moles of HCl  ______________moles ______________moles ______________moles
initial NaOH buret reading ______________mL ______________mL ______________mL
final NaOH buret reading ______________mL ______________mL ______________mL
mL NaOH delivered ______________mL ______________mL ______________mL
moles of NaOH  ______________moles ______________moles ______________moles
moles of HCl neutralized

by antacid sample

______________moles ______________moles ______________moles
moles of HCl neutralized per antacid tablet ___________moles/tablet ___________moles/tablet ___________moles/tablet
cost per tablet _____________$/tablet _____________$/tablet _____________$/tablet
cost per mol of HCl neutralized ________________$/mol ________________$/mol ________________$/mol