Biology Honors
Name ____________________________
Acids, Bases and pH PART 2
Stomach Acid and Antacids
The epithelial cells that line the inside of the stomach normally secrete hydrochloric acid (HCl). HCl is a very
strong acid with a pH of about 2 that functions in the digestion of proteins. The HCl helps to break apart the
proteins that you took in as part of your diet into smaller molecules. This is necessary so that they can be more
easily absorbed into your bloodstream and carried to all the cells of your body where they are utilized. HCl
dissociates in the water in the stomach to form H+ ions.
HCl –––––> H+ + ClHigh levels of acid in the stomach can lead to heartburn. Heartburn occurs when the stomach contents move
backwards into the esophagus. The esophagus is the tube that leads from the mouth to the stomach. The acid
burns delicate tissues in the esophagus. The stomach lining itself is usually protected from the burning effect of
the acid because some stomach cells secrete mucus, which protects the tissue. There are cases where too much
hydrochloric acid in the stomach can overwhelm the mucus barrier, and cause sores to appear in the stomach
lining. It has been found in recent years that these sores, called ulcers, usually are caused by bacterial infections,
and not HCl alone.
Antacids such as Alka Seltzer®, Milanta® and Maalox® help to neutralize stomach acid, and relieve the
burning sensation. The active ingredients in Maalox® are Magnesium hydroxide [Mg(OH)2] and aluminum
hydroxide [Al(OH3)].
1. What was the pH of Maalox®? ________
2. Look at the chemical formulas for these compounds, and explain how they neutralize stomach acid.
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In this section of the lab, you will test some of these antacids for their effectiveness in neutralizing acid in
the stomach
Materials:
• Goggles
• One small plastic cup (stimulates your stomach).
• Hydrochloric acid
• Different types of antacids (use 1 tablet or 1 teaspoon if liquid)
• Straw or stirrer, to mix the cup’s contents.
• pH paper
• Paper towels to keep your work area clean
Each group of students will receive one antacid from each of the types listed above. The antacids will be added
to three different cups each containing 10 mL of HCl, which will simulate the stomach contents. The pH of the
solution will be checked before the antacid is added, and three minutes after the antacid is added, to determine
how much neutralization took place. Once the antacid has been added, the contents of the cup should be stirred
continuously for two minutes, to simulate the normal churning action of the stomach.
Results
Initial pH of stomach acid
Type of Antacid
pH after 3 minutes
Buffers and pH
The body uses substances called buffers to insure that the pH of body fluids does not fluctuate very much.
Buffers prevents large sudden changes in pH. Buffers work by accepting H+ ions from solution when they are in
excess, and donating H+ ions to the solution when they have been depleted. Even small pH changes can
drastically alter body function. Each pH unit represents a tenfold difference, so a slight change in pH represents
a large change in the actual pH. A change in the pH of a solution from 4 to 2 indicates that the solution is
100 times more acidic. If the blood becomes too acidic, a serious condition called acidosis occurs. If the blood
becomes too basic (or alkaline), another serious condition called alkalosis occurs. Both acidosis and alkalosis
can be fatal if the body does not adjust the pH on its own, or proper medical treatment is not given.
1. What is a buffer, and why are buffers important?
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2. Would a change in blood pH from 7.4 to 8.4 be significant? Explain.
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3. What condition would result if the blood pH was changed to 8.4?
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4. If you were a physician, how might you treat this condition?
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An example of a buffer system in the human body is the carbonic acid/bicarbonate ion buffer system, which
exists in the blood, and helps keep the blood at a relatively neutral pH.The equation that follows explains how
this system works to either remove excess hydrogen ions if the body fluid becomes too acidic, or release
hydrogen ions if the body fluid becomes too basic. Note that this chemical reaction is reversible, which is to say
it can go in both directions, depending on the needs of the body. H2CO3 can be either a reactant or a product,
depending upon which way the reaction is going.
Note: Not all bases release hydroxide ions, but all bases remove hydrogen ions from the solution, thereby
raising the pH.
Use the equation below to answer the next 2 questions:
carbonic acid
bicarbonate
ion
1. How does this buffer system respond to a rise in pH?
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2. How does this buffer system respond to a drop in pH?
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pH, Shampoo and Hair
Many detergent products such as liquid soap and shampoo are “pH balanced, ” meaning that they contain
ingredients that help to keep hair or skin at the correct pH. The pH of hair is about 5.5. At this pH, hair proteins
retain their correct shape. When the pH changes, protein fibers may fray, causing “split ends.” The detergent in
shampoos is often a compound known as lauryl sulfate. Look at the pH of detergent on the first page, then look
at the ingredients of a typical shampoo bottle that are listed below. See if you can find the specific ingredient
that is used to make the shampoo “pH balanced.”
Shampoo ingredients: ammonium laurel sulfate, glycol distearate dimethicone, sodium chloride, cetyl
alcohol, cocamide MEA, citric acid, disodium EDTA, sodium benzoate, tricaprate, hydrogenated polydecene.
Ingredient that is supposed to balance the pH of shampoo: ____________________________________
How would this ingredient help to balance the pH? Make sure you mention something about the
detergent in your answer. ______________________________________________________________
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pH and Acid Rain
Read the article on the next page on acid rain, and answer the following questions:
1. How is acid rain formed?
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2. What is the pH of acid rain? __________________________
3. How does sulfuric acid (H2SO4) and nitric acid (HNO3) decrease the pH? (look at the chemical formula)
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4. Why is acid rain an ecological problem? Give an example.
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5. Is acid rain the result of human influence? ___________
Explain ___________________________________________________________________________
Common Acidic substances
Acids typically have a sour taste, while bases tend to be bitter. Lemon juice, vinegar, vitamin C,
yogurt and aspirin all have a sour taste. Name the compound in each of these substances that is
responsible for them being acidic and tasting sour. You will need to look on the internet to find the name
of these compounds.
lemon juice _________________________________________________________________________
vinegar ____________________________________________________________________________
Vitamin C _________________________________________________________________________
yogurt ____________________________________________________________________________
Aspirin ____________________________________________________________________________
Buffered Aspirin vs. Regular Aspirin
The word “buffer” is frequently used to describe how compounds are neutralized. Read the following
information and then answer the next question on buffered vs regular aspirin.
Your stomach normally produced HCl in order to help digest proteins in your diet. Some people tend to produce
too much HCl at various times, and the excess H+ can irritate the stomach lining and cause heartburn and ulcers.
Foods that contain acidic compounds like citrus fruits and tomatoes can make the condition worse. The main
ingredient in aspirin is salicylic acid, and so this can also exacerbate the problem.
1. You have a headache and you are deciding whether to take regular aspirin or buffered aspirin. Considering
that you also have a tendency to suffer from heartburn you decide to take the buffered aspirin instead. Why
is this a good idea? (Refer to to the pH chart for the pH of these two types of aspirin and include something
about the ingredient in aspirin that you mentioned above).
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