pH Lab
MATERIALS
 1 micro centrifuge tube rack
 10 micro centrifuge tubes
 250 mL HCI, 0.1 M
 175 mL NaOH, 0.1 M
 3 plastic scoopulas
 10 dropper pipets
 Ground antacid samples of Rolaids, Tums, and Alka-Mints
 Universal indicator
 Phenolphthalein
 Electronic Balance with a readability of 0.01 grams
 pH meter
BACKGROUND
What is the purpose of an antacid? As their name suggests, antacids work to
alleviate the discomfort caused by too much acid working in the stomach and not
enough protein for the acid to work on. In the absence of protein (from food),
stomach acid will digest stomach tissues, which are composed of protein. Over a
prolonged period, this digestive action in the absence of food protein results in a
peptic ulcer. A good antacid will maintain the functional pH of the stomach
environment, while reducing excess stomach acid. This is achieved as excess
stomach acid combines with a base, the active ingredient in antacids, producing
a neutralization reaction.
An acid is a chemical substance that increases the concentration of hydrogen ion
(H+) when dissolved in water. For example, when HCI dissolves in water the
following reaction occurs: HCI(aq) H+(aq) + CI-(aq)
A base is a chemical substance that increases the concentration of hydroxide ion
(OH-) when dissolved in water. For example, when NaOH dissolves in water the
following reaction occurs: NaOH(aq)Na+(aq) + OH-(aq)
Solutions with a pH less than 7 are acidic. Solutions with a pH greater than 7 are
alkaline or basic. The addition of an acid to a basic solution to produce a pH of 7
is called neutralization. Neutralization results in the production of a salt and
water, as represented by the following chemical equation:
HCI(aq)
Acid
+
+
NaOH(aq)
Base


NaCI(aq)
Salt
+
+
H2O(I)
Water
Pepsin is the primary digestive enzyme found in the stomach, the site of the
initiation of protein digestion. Pepsin is an activated form of pepsinogen, a
secretion of the chief cells located in the recesses of gastric glands. These
glands are widely distributed throughout the walls of the stomach. Inactive
pepsinogen is converted to its active form, pepsin, by the presence of
hydrochloric acid (HCI), secreted by the parietal cells (also located in the depths
of gastric glands). The secretions of these specialized cells, in addition to the
slightly alkaline secretions of the mucous cells located in the neck and at the
opening of gastric glands, combine to produce what is known as gastric juice,
with a pH of approximately 2.0.
Heartburn is a burning sensation experienced when stomach contents which
have come in contact with gastric juice enter the lower esophagus due to
incomplete closure of the lower esophageal sphincter. Acidic gastric juice
subsequently causes irritation of the tissues of the esophagus. Continued tissue
irritation may eventually lead to an ulcer in this region.
Antacids are composed of a variety of bases (brand dependent) designed to
neutralize the hydrochloric acid present in gastric juice and thus lessen the
burning sensation and discomfort associated with heartburn.
A homeostatic system, a feature of most biologic systems, must maintain a
specific pH, usually of very limited range, if these systems are to continue
operating. Proteins and enzymes (catalytic proteins) operate within specific pH
ranges. Outside of these ranges catalytic proteins cease to operate or may even
degrade. The optimum pH range for pepsin activity is from 1.5-2.0 depending on
the substrate. Therefore, a good antacid will neutralize excess stomach acid
while at the same time maintaining a functional environment for digestion. The
purpose of the following investigation is to test the neutralizing capabilities of
three samples of different antacid brands.
LAB PROCEDURE
PART I:
1. Label a clean micro centrifuge tube
“A”. Carefully pipet 1.0 mL of 0.1 M HCI
into this tube. Next, use a clean
pipet and add exactly one drop
of universal indicator to tube “A”.
After closing the lid on the tube,
gently agitate it. What happens?
Does the solution change color?
Place the micro centrifuge tube in
your rack as shown in Figure 2.
2. Test same HCl solution ( 3-4mL) with your pH probe
Record your observations in the
Appropriate space in Data Table 1.
3. Arrange micro centrifuge tubes 1-9 as shown in Figure 2. Using a clean pipet,
add 1.0 mL of 0.1 HCI to each of the nine tubes.
4. Add exactly one drop of universal indicator to micro centrifuge tubes 1-9,
and record the color of the solution in each tube in Data Table 1 in the column
labeled “Univ. Ind”.
5. Now test same HCl solutions (3-4mL) label 1-9 with pH probe and record the
your data in Data Table 1
pH
Color Transition
2…………………………….Red
3………………………..Red-Orange
4…………………………..Orange
5…………………….…Yellow-Orange
6…………………….……..Yellow
7…………………….……...Green
8…………………….…...Blue-Green
9…………………….……Blue-Gray
10…………………….…….Violet
6. Label a clean scoopula “#1”. Use this scoopula and add one level scoopula of
antacid #1 (Rolaids) to micro centrifuge tubes 1, 2, and 3. Close the lids tightly.
7. Label a clean scoopula “#2”. Use this scoopula to add one level scoop of
antacid #2 (Tums) to micro centrifuge tubes 4, 5, and 6. Close the lids tightly.
8. Label a clean scoopula “#3”. Use this scoopula to add one level scoop of
antacid #3 (Alka-Mints) to micro centrifuge tubes 7, 8, and 9. Close the lids
tightly.
9. Agitate each of the tubes to dissolve the antacids.
10. Observe the color changes in each tube (1-9). These changes are directly
related to the antacid’s ability to neutralize acid.
11. Now repeat steps 6-10 using your pH probe for beakers 1-9 (3-4mL HCl)
adding 3-4 level scoopula of the three antacids and record your findings (in the
column labeled “pH+ Antacid”)
12. Record the color of the resulting solution in the column labeled “Univ. Ind. +
Antacid” and their approximate pH values (based on a comparison to the pH
color chart) in Data Table 1.
PART II:
CONTROL:
1. Use a measuring pipet to deliver 20 mL of 0.1 HCI into a clean 150-mL
beaker.
2. Add three drops of phenolphthalein indicator to the acid.
3. Fill a 25- or 50-mL buret with 0.1 M NaOH. Remove any air bubbles from the
tip of the buret by allowing a small volume of solution to flow through the
stopcock into an empty beaker.
4. Read the volume at the bottom of the meniscus to the nearest 0.01 mL and
record it in Data Table 2 as the initial buret reading.
5. Titrate the 0.1 M HCI with the 0.1 M NaOH until a faint pink color persists for
more than 30 seconds. The acid solution should be mixed during the titration by
carefully swirling the beaker.
6. Read the volume at the bottom of the meniscus to the nearest 0.01 mL and
record it in Data Table 2 as the final buret reading.
7. Repeat steps 1-6 for additional trial, as directed by your teacher.
ANTACID
1. Weigh out 0.1 grams of antacid powder and place it in a clean, dry 150-mL
beaker. Record the exact mass in the appropriate Data Table.
2. Use a measuring pipet to deliver 20 mL of 0.1 M HCI into the 150-mL beaker.
3. Stir the solution with a glass stirring rod, and then rinse any material that
sticks to the stirring rod back into the solution with a small amount of distilled
water.
4. Add three drops of phenolphthalein indicator to the acid solution.
5. Fill a 25- or 50-mL buret with 0.1 M NaOH. Remove any air bubbles from the
tip of the buret by allowing a small volume of solution to flow through the
stopcock into an empty beaker.
6. Read the volume at the bottom of the meniscus to the nearest 0.01 mL.
Record this volume as the initial buret reading in the appropriate Data Table.
7. Titrate the acid soluation with the 0.1 M NaOH until a faint pink color persists
for more than 30 seconds. The acid solution should be mixed during the titration
by carefully swirling the beaker.
8. Read the volume at the bottom of the meniscus to the nearest 0.01 mL.
Record this volume as the final buret reading in the appropriate Data Table.
9. Repeat steps 1-8 for each additional trial, as directed by your teacher.
Data Table 1
Antacid
None
Rolaids
Tums
Alka-Mints
Tube
No.
A
1
2
3
4
5
6
7
8
9
Univ. Ind.
Univ. Ind. + Antacid
pH pH+Antacid
-----------------------------
PART II:
Data Table 2: Control Test Results
Trial #
Final buret reading
Initial buret reading
Volume of NaOH required
Ave. Volume of NaOH
1
2
3
Antacid Test Results
Data Table 3
Antacid #1 = Tums
1
2
3
Data Table 4
Antacid #2 = Rolaids
1
2
3
Data Table 5
Antacid #3 = Alka-Mints
1
2
3
Trial #
Mass of antacid
Final buret reading
Initial buret reading
Volume of NaOH required
Ave. Volume of NaOH
Trial #
Mass of antacid
Final buret reading
Initial buret reading
Volume of NaOH required
Ave. Volume of NaOH
Trial #
Mass of antacid
Final buret reading
Initial buret reading
Volume of NaOH required
Ave. Volume of NaOH
QUESTIONS
1. Is the component of an antacid which acts to relieve acid indigestion an acid
or a base? List the active ingredients of each of the brands of antacids tested.
2. Which chemical compound is found in all three antacid products?
3. Is carbonate acidic or basic? What is its role in the function of an antacid?
4. Based on the data you listed in Data Table 1, which antacid appears to
neutralize the most acid? Support your answer.
5. Using data from Data Tables 2 and 3, what can you conclude about Tums’
neutralizing capabilities?
6. Using data from Data Tables 2 and 4, what can you conclude about Rolaids’
neutralizing capabilities?
7. Using data from Data Tables 2 and 5, what can you conclude about the AlkaMints neutralizing capabilities?
This project is funded by a grant awarded under the President’s Community Based Job Training Grant as
implemented by the U.S. Department of Labor’s Employment and Training Administration (CB-15-162-06-60).
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
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