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Purpose:
In this experiment, students will measure the rate of change in
pH of a dilute hydrochloric acid (HCl) solution when antacid tablets
are added to it. Students will also compare the neutralization ability
of a solid antacid tablet to a crushed tablet.
Background Information:
Digestion in the stomach results from the
action of gastric fluid, which includes secretions of digestive
enzymes, mucous, and hydrochloric acid. The acidic environment of
the stomach makes it possible for inactive forms of digestive enzymes
to be converted into active forms (i.e. pepsinogen into pepsin),
and acid is also needed to dissolve minerals and kill bacteria that
may enter the stomach along with food. However, excessive acid production
(hyperacidity) results in the unpleasant symptoms of heartburn and
may contribute to ulcer formation in the stomach lining. Antacids
are weak bases (most commonly bicarbonates, hydroxides, and carbonates)
that neutralize excess stomach acid and thus alleviate symptoms
of heartburn. The general neutralization reaction is:
Antacid (weak base) +
HCl (stomach acid) —› salts + H2O
+ CO2
The hydrochloric acid solution used in this
experiment (0.1 M) approximates the acid conditions of the human
stomach, which is typically 0.4 to 0.5% HCl by mass (pH ~ 1).
Hypothesize: How does the ability of a solid antacid tablet
to neutralize excess acid compare to the ability of a crushed antacid
tablet to neutralize excess acid? Predict what difference there
will be, if any, in the time it takes for the reaction to occur.
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Equipment:
For each lab group:
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Computer Interface:
PASPORT: Xplorer GLX (PS-2002) | Xplorer (PS-2000) | USB Link (PS-2100A) | PowerLink (PS-2001)
ScienceWorkshop: 500 Interface (CI-6400)
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Glassware: 250-mL beakers (4) , 100-mL graduated
cylinder
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Lab stand & buret clamp for sensor
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Magnetic stir bar set-up
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Mallet and wax paper, mortar and pestle, or
similar set-up for crushing antacid tablet
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Wash bottle of distilled water
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Antacid tablets: 2 of the chosen brand to be
tested (1 to be crushed)
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Hydrochloric acid, 0.1 M solution
(100 mL needed per antacid test: 50 mL each for whole tablet
and for crushed tablet)
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Goggles, gloves, aprons, and/or other protective
gear
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Software Setup:
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Connect the pH sensor to the USB link or Xplorer
(PASPORT users), or to the 500 interface (ScienceWorkshop 500
users).
Additionally, for ScienceWorkshop 500 users -- associate the
pH sensor with the interface in the Experiment Setup window
(double-click or drag).
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The PASPORT pH sensor typically does not need
to be calibrated; its accuracy is approximately ±0.5
pH units. If better accuracy is needed, or to calibrate the
CI-6507A pH Sensor, refer to DataStudio's online help menu for
specific calibration instructions.
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Use the wash bottle to rinse the end of the
electrode. Fill one of the clean, dry beakers with approximately
100 mL of distilled water from the wash bottle, and soak the
pH electrode in the water for 5-10 minutes while setting up
the rest of the equipment.
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Equipment Procedure:
- Pour 50 mL of the 0.1 M HCl solution into the second of the
250-mL beakers.
- Carefully add a magnetic spin bar to the beaker, and then place
the beaker on the magnetic stirrer.
- Remove the pH electrode from the beaker containing the distilled
water. Use the lab stand and buret clamp to position the sensor
so that the end of the electrode is immersed in the acid solution
without the electrode tip touching the stir bar.
- Turn on the magnetic stirrer, checking to make sure the pH electrode
has contact with the solution but is not interfering with stirring.
(Note: if a magnetic stir bar set-up is not available, one student
should carefully stir the solution with a glass stirring rod.)
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Data
Collection & Recording:
- When the stirring set-up is ready, click the Start button (
 ) to
begin collecting data.
- After 10 seconds, or when the pH reading has stabilized, drop
one whole antacid tablet into the solution.
- Continue recording data until the antacid tablet is entirely
dissolved and the pH reading stabilizes (5-10 minutes, depending
on tablet size or antacid brand). Click the Stop button (
).
- Turn off the magnetic stirrer and carefully remove the pH Sensor
from the solution.
- Using your 3rd beaker as a waste container, rinse the pH Sensor
with distilled water, and then return the electrode to the beaker
of distilled water to soak while you prepare the next set-up.
- Crush the remaining antacid tablet, being careful not to lose
any of the tablet pieces.
- Repeat the equipment set-up procedure described above using
the 4th clean, dry beaker.
- Repeat the data collection procedure described above, this time
adding the crushed antacid tablet in step #2.
- After all data runs have been completed, dispose of acid solutions
as instructed and clean up all glassware and equipment.
Data Analysis:
- Scale the axes to fit the data using the Scale to Fit button
(
) in the Graph toolbar.
- Use the Graph display analysis tools to determine the rate of
change of pH for the whole antacid tablet versus the crushed antacid
tablet. Note that for this experiment, the slope of the graph
(
 y
/ x)
equals the rate of change (pH
/ t).
There are several ways to analyze the section of the graph of
interest:
- Click the Smart Tool button (
) to activate the Smart Tool functions and features. Hover
with the mouse until you see the hand symbol ( 
), which will let you pinpoint the location of interest on
the graph. After dragging the cross hairs of the Smart Tool
to a data point, hover over the edges of the small box around
the cross hairs until you see the Delta Tool ( 
). Now when you drag the triangle to a 2nd data point, a dashed
box will appear with the selected coordinates at the corners,
and the difference between coordinate values along the sides
of the box.
- Click and drag to select the region
of the graph of interest. Click on the Slope Tool button (
) to access the Slope Tool features. Position the Slope Cursor
( 
) over the area of interest to view the calculated slope of
the graph.
- Click and drag to select the
region of the graph of interest, then click on the Fit Button
(
) and select "Linear". The best-fit line will be drawn through
the selected region of the graph, and from the box of statistics
that appears you can read the slope of the line.
- Analyze each data run to compare the results from the whole
table to the crushed tablet, or between antacid brands if you
had time to test a variety of products.
Sample Data Table:
|
Antacid |
Whole or crushed? |
pH |
t |
Rate of change in pH: |
| 1. |
|
|
|
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| 2. |
|
|
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| 3. |
|
|
|
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| 4. |
|
|
|
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| 5. |
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Hint: Consider changing the graph display settings to display time
in minutes along the x-axis, or convert the time in seconds to minutes.
This will give units of pH/minute
for the rate of change in the last column.
Conclusions and Extensions:
- Does the rate of change of pH depend on whether the antacid
tablet is crushed or whole?
- Does the overall change in pH depend on whether the antacid
tablet is crushed or whole?
- Which type of antacid tested had the fastest neutralization
rate? The slowest?
- Which of the tested antacids do you predict would give the best
relief of heartburn symptoms? Why?
- What other factors might influence the effectiveness of an antacid?
(Hint: consider active ingredients, recommended dosage, inert
ingredients such as flavorings, etc.)
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