Analysis of Stomach Acid Tablets
Antacid tablets consist of weakly basic substances that are capable of reacting with the
hydrochloric acid (HCl) found in the stomach. In this experiment, you will determine the amount of
stomach acid that several commonly used antacids are capable of neutralizing and attempt to evaluate
their effectiveness. You will then compare the effectiveness of the various commercial antacid tablets to
that of simple baking soda.
The tablets will be dissolved in an excess of 0.10 M HCl (which will simulate stomach acid), and
then the remaining acid (i.e., the portion of the acid that did not react with the antacid) will be titrated
with standard sodium hydroxide (NaOH) solution. It is not possible to titrate antacid tablets directly for
several reasons. First, commercial antacid tablets frequently contain binders, filler, flavorings, and
coloring agents which may interfere with the titration. Second, the bases found in most antacids are
weak bases and become buffered as they are titrated, often leading to an indistinct indicator endpoint.
Procedure:
1. Use a 50 mL beaker to fill a buret to the 0.0 mark with the available standardized NaOH solution (record the
exact concentration).
2. Obtain an antacid tablet. Record the brand, main ingredient(s), cost and number of tablets per container.
Calculate the cost per tablet.
3. Weigh the tablet and record the mass in your data table. Then wrap it in a piece of plastic wrap and crush the
tablet with a heavy object without tearing the wrap.
4. Weigh a clean, dry 125 mL Erlenmeyer flask to the nearest 0.01 g. Transfer approximately one quarter (1/4)
of the crushed antacid tablet to the flask, and reweigh the flask.
5. Using a 25 mL graduate, add exactly 25.0 mL of 0.10 M HCl to the crushed tablet in the flask. Swirl the flask
to dissolve the soluble portions of the tablet. All of the tablet may not dissolve.
6. Add 2-5 drops of bromophenol blue indicator solution to the sample. The indicator should be bright yellow at
this point, indicating the solution is acidic (excess HCl).
7. If the indicator is blue, this means that not enough hydrochloric acid was added to completely neutralize the
antacid tablet. If the solution is blue, add additional 0.1 M HCl, in 5.0-mL increments, (record the total volume
used) until the sample is yellow.
8. Titrate the sample with the standard NaOH solution until the solution barely turns blue. Record the total
volume of NaOH added from the buret at the color change on your data table.
9. Using pH paper, measure the pH of the titrated solution. Record value.
10. Repeat the procedure using the same brand of antacid tablet using another portion for your sample. If the
results (in mL of base per grams of tablet) are different by more than 1%, repeat a third time.
11. Repeat the entire procedure with another commercial brand of antacid tablet.
12. Then repeat the procedure using approximately 0.25g of baking soda (record your exact mass).
Results:
1. To calculate HCl neutralized (mL): HCl added to quarter tablet – NaOH titrated
2. To calculate HCl neutralized (g):
™ Convert your answer from mL to L.
™ Convert from L to moles. (Multiply the amount of HCl neutralized by antacid (in L) by the molarity of HCl.)
™ Convert from moles to grams of HCl (this will give you the grams of HCl neutralized by the quarter tablet).
3. To calculate g HCl/g tablet: Divide the grams of HCl neutralized by the mass of your quarter tablet.
⎛ gHCl ⎞⎛ grams ⎞⎛ 1tablet ⎞⎛ $1.00 ⎞
⎟⎟⎜
4. To calculate g HCl/¢ : ⎜⎜
⎟⎟
⎟⎜
⎟⎜⎜
⎝ gtablet ⎠⎝ 1tablet ⎠⎝ $ cos t ⎠⎝ 100¢ ⎠
Analysis of Stomach Acid Tablets
Concentration of HCl __________M (simulated stomach acid)
Concentration of NaOH __________M (added from buret)
Brand antacid #1 ____________________ Main ingredient(s) _____________________
Price ____________ # of tablets per container ________ Cost/tablet ________ mass tablet ______g
Data
Trial #1
Trial #2
Average
Flask (g)
Flask + (~1/4)tablet (g)
(~1/4)Tablet (g)
HCl added to tablet (mL)
NaOH titrated (mL)
pH value
Results
HCl neutralized (mL)
HCl neutralized (g)
g HCl/g tablet
g HCl/ ¢
Brand antacid #2 _____________________ Main ingredient(s) __________________
Price ____________ # of tablets per container ________ Cost/tablet ________ mass tablet ______g
Data
Trial #1
Trial #2
Average
Flask (g)
Flask + (~1/4)tablet (g)
(~1/4)Tablet (g)
HCl added to tablet (mL)
NaOH titrated (mL)
pH value
Results
HCl neutralized (mL)
HCl neutralized (g)
g HCl/g tablet
g HCl/ ¢
Brand of Baking Soda ___________________ Main ingredient NaHCO3
Price ____________ Weight (g) ___________ Cost/gram __________ mass of sample ____g
Data
Trial #1
Flask (g)
Flask + soda (g)
Soda (g)
HCl added to soda (mL)
NaOH titrated (mL)
pH value
Results
HCl neutralized (mL)
HCl neutralized (g)
g HCl/g soda
g HCl/ ¢
Trial #2
Average
Analysis of Stomach Acid Tablets Lab
Write Up Instructions
THIS IS A SEMI‐FORMAL LAB REPORT WORTH 20‐30 POINTS! IT SHOULD BE TYPED AND ORGANIZED IN THE ORDER BELOW. DATA TABLES AND GRAPHS NEED TO BE INCLUDED IN THE CORRECT ORDER – NOT ATTACHED TO THE END OF THE REPORT. ONE REPORT MAY BE TURNED IN PER PAIR. HEADING: Group members, date, period TITLE: Give a descriptive title of your experiment ABSTRACT: 2‐3 sentences describing the purpose of the experiment PROCEDURE: An outline of the experimental procedure (in your own words). This should be a bulleted or numbered list written as directions that someone else could follow. DATA TABLE: Either use the one provided or make your own. Present all data in tabular form. ANALYSIS: 1. Write the chemical reactions with phase notations for (a) the titration of HCl with NaOH (b) the reaction of HCl with solid sodium hydrogen carbonate (c) neutralization of the main ingredients of the antacid tablets you used with stomach acid. 2. Show your work for the calculations for one trial. (Questions #1‐4 from the Results section on the procedure page.) Don’t forget units! Graph: Prepare a graph of gram acid consumed per gram tablet/soda and per cost for each brand using the average from the two calculations performed per antacid. The following is the format for preparing a graph in Excel: g acid/ g tablet or soda g acid/ cent Antacid #1 Antacid #2 Baking Soda Fill in your data in Excel and then select all of the cells. Choose “Chart” from the “Insert” menu. Choose a “Column” chart and then follow the directions to label the axes and title the graph. RESULTS: (2‐3 sentences) Summarize your results from the lab. Which antacid neutralized the most amount of stomach acid? Which one was the most effective for the price? QUESTIONS: Respond thoroughly and thoughtfully to each of the questions. It is intended that the response is in depth and may require some outside research. 1. Based on your calculations and graph of the grams of stomach acid neutralized per cent: What substance is the best buy? Is cost the only consideration you would use if purchasing these products. Explain. 2. Generally, the substances used as antacids are either weak bases, or very insoluble bases. Why is a strong soluble base like NaOH not used in antacid tablets? 3. Many people abuse the use of antacids. Use outside resources to find out what side effects occur if antacids are used too frequently. 4. Find the pH of a “normal” stomach and match this value with the pH of the neutralized solutions with each tablet. Has the tablet truly “neutralized” excess stomach acid? Explain.