1
Comparison of Antacids
Introduction
Antacids and Neutralization Reactions
An antacid is a substance that acts to neutralize excess stomach acid in order to relieve acid
indigestion and heartburn. Stomach acid is a component of gastric juice, a digestive fluid
secreted by glands located in the mucous membrane lining the stomach. This acid functions to
activate certain enzymes necessary for digestion and regulates other aspects of the digestive
process. In the case of acid indigestion and heartburn, excess gastric juices are produced,
causing burning and discomfort due to the increased acid concentration.
Acids (such as the acid in gastric juice) are a source of the H+ ion. The higher the concentration
of an acid, the more H+ present. Acids have a pH below 7. The higher the concentration of H+
from the acid, the lower will be the pH. Antacids react with H+ and effectively reduce its
concentration. Antacids are bases, which have a pH above 7. This reaction between equal
amounts (in moles) of a strong acid and an antacid (a strong base) is called a neutralization
reaction because the acid and antacid ―cancel‖ or ―balance‖ each other, resulting in a neutral
solution with a pH of 7. Here is an example of a neutralization reaction:
HCl + NaOH  NaCl + H2O
acid
base
salt
water
Many bases can be recognized by the presence of OH- (the hydroxide ion) in their formula. For
example, Mg(OH)2 is the base present in many commercial antacids. As in the reaction above,
the products of any neutralization reaction are some type of salt and water.
When you consume an antacid, such as Tums or Rolaids, the base in the antacid tablet reacts
with stomach acid in a neutralization reaction. One way to compare one antacid with another is
by cost effectiveness, that is, which product neutralizes more acid per dollar spent. In this
experiment you will determine the capacity of two different antacids to neutralize stomach acid
per unit cost, in order to determine which is the better buy.
Titration
To determine the capacity of the antacids to neutralize stomach acid, you will use a method
called a titration. A titration is an experimental procedure which allows you to determine the
exact amount of one substance that will react with a specific amount of another substance. In
this experiment, you will determine the exact amount of acid that will react with 0.5 grams of an
antacid. Although the antacid sample has a mass of 0.5 grams, the sample contains ―fillers‖
(such as flavorings) in addition to the base which acts to neutralize stomach acid. Therefore,
you won’t know how much of the 0.5 gram sample is the useful base, and how much is just filler.
The more filler present, the less effective will be a given amount of the antacid in combating
heartburn.
In this experiment the titration will consist simply of adding an acid solution to the 0.5 gram
antacid sample—as acid is added to the sample, a neutralization reaction will occur between the
acid being added and the base in the antacid sample. The acid will be added to the antacid
slowly by delivering it from an instrument called a buret. The buret has volume markings so that
2
it will be easy to determine the exact amount of acid needed to react with the antacid sample in
the neutralization reaction. As explained above, neutralization occurs when equal amounts (in
moles) of an acid and base react to ―balance‖ or ―cancel‖ each other. Therefore, you will add
acid to the antacid sample just until the amount of acid added (in moles) is exactly equal to the
amount of base (in moles) in the 0.5 gram antacid sample—at this point neutralization occurs.
The volume markings on the buret will enable you to easily determine the total amount of acid
that was needed for neutralization to occur. Since moles of acid are equal to moles of base at
the point of neutralization, this amount of acid added from the buret will be an indication of the
amount of base (in moles) that must be present in the 0.5 gram antacid sample.
The point at which neutralization occurs, when moles of acid added are equal to moles of base,
is called the endpoint of the titration. When the endpoint is reached, you will stop adding acid
since neutralization has occurred. If you add extra acid past the endpoint, there will be no base
for it to react with (since all the base has already reacted with acid and has been ―used up‖).
The problem with adding extra acid is that the volume reading on the buret will reflect the total
amount of acid added—you won’t know how much of this total volume was needed to reach
neutralization, and how much was ―extra.‖ Since the amount of acid added is meant to be used
as an indication of the amount of base in the antacid sample, an inaccurate total acid volume will
give an incorrect indication of the amount of base.
One more detail of the titration procedure needs to be explained: How will you know when you
have added an amount of acid that is exactly equal to the amount of base (in moles)? In other
words, how will you know when the endpoint of the titration has been reached and you should
stop adding acid? You will add a chemical called an indicator to the solution, and the indicator
will change color when the endpoint has been reached. The solution containing the antacid
sample will be green initially, due to the presence of the indicator.
As you add acid from the buret, the green color
will begin to change to yellow and eventually to
peach/orange. When the solution becomes bright
Buret
peach/orange and remains that color for about 10
seconds, the endpoint has been reached and you
should stop adding acid. The titration procedure is
then complete, and the amount of base in the
antacid sample can be determined.
Buret
The acid used during the titration will be contained
in a buret (pictured at the right). The acid will be
delivered out of the bottom of the buret by opening
and closing the stopcock near the bottom.
Volume markings along the entire length of the
buret will enable you to determine how much acid
was delivered.
Acid will be delivered by draining it
from bottom of buret
stopcock
3
Reading a Buret
A buret is an instrument designed to deliver very precise volumes of a liquid reagent. Reading a
buret is very much like reading a graduated cylinder in that you should read from the bottom of
the meniscus and you should always estimate ONE more digit (between the lines) than you can
read from the scale.
The difference between a buret and a graduated cylinder is that 0.00 mL is at the TOP of the
buret—just the opposite of how a graduated cylinder is marked. The purpose of this is so that
you can fill the buret completely (to the 0.00 mL mark), deliver a certain volume of liquid out of
the buret by opening the stopcock at the bottom, and then read the scale to determine what
volume was delivered. In other words, a buret reading shows the volume that was delivered
(drained out), rather than the volume that is contained in the buret (a graduated cylinder shows
the volume contained in the cylinder). Therefore, you should read the scale on a buret in a
downward direction. This is exactly the opposite of reading a graduated cylinder where you read
the scale in an upward direction. Take a look at the example below before attempting to do the 3
buret questions on the prelab.
4
Comparison of Antacids
Prelab
Name_______________
1. How does an antacid, such as Rolaids or Tums, work to relieve acid indigestion and
heartburn?
2. What are the products of the neutralization reaction below between an acid and a base found
in an antacid? Be sure your reaction is balanced.
2HCl + Mg(OH)2 
acid
base (from
antacid)
Which of the compounds is the salt? __________
Explain why this type of reaction is called a ―neutralization‖ reaction.
3. What does ―cost effectiveness‖ of an antacid refer to?
4. What is the one-word term for the experimental method you will use to determine how much
acid can be neutralized by a small sample of an antacid?
What is the name of the piece of laboratory glassware that you will use to measure the volume
of acid that is neutralized by the antacid sample?
5. What two quantities will be equal at the endpoint of the titration carried out in this
experiment?
6. What type of chemical will be used to signal the endpoint of the titration?
Prelab continued on next page
5
Comparison of Antacids
Prelab
7. Assume that the buret at the right was previously
filled to the 0.00 mL mark.
What volume of reagent has been delivered?
(Remember to estimate between the lines to get one
more digit than can be read from the scale.)
8. Write down a reading for each
buret pictured below.
9. Assume that the buret below was
previously filled to the 5.00 mL mark.
What volume of reagent has been
delivered?
Name_______________
6
Comparison of Antacids
Procedure
Preparation of the Buret.
1. To ensure that your buret is clean and does not contain any chemical residue that will
interfere with the titration, rinse it thoroughly with tap water. It can be awkward to try to rinse a
buret at the sink, so you may want to ask the instructor for suggestions.
Be sure to open the stopcock at some point to allow the rinse water to clean the tip of the buret
as well. After thorough rinsing with tap water, rinse once with distilled water and allow all liquid
to drain out of the buret.
2. Pour approximately 5 mL of 0.3 M HCl (the acid that will be used in the titration) into the
buret. With the stopcock closed, hold the buret in a nearly horizonatal position and roll the
solution around to wet the entire inner surface of the buret. Then drain the HCl out of the buret
by opening the stopcock. The purpose of this step was to use a little HCl to rinse out any
remaining water which would otherwise dilute the HCl that will next be added to the buret.
3. Clamp the buret to a ring stand on your lab bench using a special buret clamp. Be sure the
stopcock is closed! Then use a funnel to fill the buret with 0.3 M HCl so that the liquid level is
just a little above the 0.00 mL mark.
4. It is necessary to fill the lower tip of the buret with HCl in order to end up with accurate
volume readings. To fill the tip with acid, place a clean beaker under the tip of the buret to catch
any liquid, and then open the stopcock all the way for just a second or two. After closing the
stopcock again, carefully examine the tip of the buret to see if it is entirely filled with liquid or if
there are some air bubbles trapped inside. If there are air bubbles present, open the stopcock
―full blast‖ for a few seconds until the rapid flow of liquid causes the air bubbles to be pushed
out. If you are not able to eliminate a stubborn air bubble, ask the instructor for help. Air
bubbles in the tip will lead to inaccurate volume readings.
If you did not drain much HCl out of the buret in the process of filling the tip, the liquid level will
probably be slightly below the 0.00 mL mark. You should begin the titration with a nearly full
buret since you don’t know how much HCl will be needed. Therefore, if the HCl level has
dropped by 5 mL or more, add more HCl to the buret so that the liquid level is just a little below
the 0.00 mL mark.
You are now ready to take a volume reading from the buret. Be sure that the liquid level in the
buret is slightly below the 0.00 mL mark (remember to read from the lowest part of the
meniscus). Take an initial volume reading and record it on the report sheet. Remember to read
and record the volume to two digits after the decimal. (For example: 0.16 mL)
When finished, ask the instructor to check your volume reading to be sure you read the buret
correctly.
7
Titration of the Base in an Antacid Sample.
1. You will titrate two different antacids in order to determine which is the better buy. Choose
the antacid you want to begin with and record its name on the report sheet. Weigh out
approximately 0.5 grams of this antacid and record the exact mass (all digits) on the report
sheet. Place the powder into a 250 mL flask and add 50 mL of distilled water. Then add 8 drops
of the indicator to this solution. Swirl the flask well in order to dissolve the antacid in the water.
The solution should be green due to the presence of the indicator.
2. Begin the titration by draining a few mL’s of HCl out of the buret into the flask containing the
antacid sample. Stop and swirl the flask vigorously to mix the solution. As you added HCl you
may have seen the appearance of yellow or peach/orange color. However, after swirling the
flask, the color should have reverted to green.
In the next step you will continue to slowly add HCl in this manner while swirling the flask. As
you add more and more HCl, the peach/orange color will appear brighter (less yellow) and last
longer. A bright peach/orange color signals the endpoint of the reaction, the point at which the
base in the antacid sample has been neutralized by the acid from the buret. It is a bit tricky to
identify the exact endpoint for this particular reaction because the peach/orange color appears
but then disappears after swirling the flask. Therefore, when the bright peach/orange color
remains for about 10 seconds before fading back to yellow or green, you should stop the titration
(the endpoint has been reached).
3. Complete the titration by slowly adding more HCl while swirling the flask vigorously. When
you near the endpoint, the HCl should be added very slowly—only a couple drops at a time—so
as not to go past the endpoint.
When the endpoint has been reached, take a final volume reading from the buret and record it
on the report sheet. Use the initial and final volume readings to calculate the total volume of
HCl used in the titration; record this total volume on the second page of the report sheet (p. 9).
Discard the contents of the 250 mL flask down the drain and rinse the flask clean.
4. Prepare for the titration of the other antacid by refilling the buret with 0.3 M HCl so that the
liquid level is just below, or exactly on, the 0.00 mL mark. Take an initial volume reading and
record it on the report sheet.
Record the name of the other antacid on the report sheet. Repeat steps 1 – 3 of the titration
procedure for this antacid.
5. When both titrations are complete, discard any unused HCl from the buret down the drain
and rinse the buret thoroughly with tap water followed by distilled water. Complete the
calculations on the report sheet to determine which antacid is the better buy. To do the
calculations, you will need to obtain information from the labels on the antacid containers.
8
Comparison of Antacids
Report Sheet
Name _________________
Data
First Titration
Second Titration
1. Name of antacid
_____________
______________
2. Exact mass of antacid sample
__________ g
__________ g
3. Initial volume reading of HCl in buret __________ mL
__________ mL
4. Final volume reading of HCl in buret __________ mL
__________ mL
9
Comparison of Antacids
Report Sheet
Name _________________
Calculations
First Titration
Second Titration
5. Total volume of HCl used
__________ mL
__________ mL
6. Volume of HCl neutralized per
dose of antacid
__________ mL/dose
__________ mL/dose
Formula to use:
(vol HCl used / mass antacid sample) x mass of dose
Show work below:
7. Number of doses per container
__________ doses
__________ doses
Formula to use:
number of tablets per container / tablets per dose
Show work below:
8. Cost per dose
$ __________
$ __________
Formula to use:
cost of container / number of doses per container
Show work below:
9. Cost effectiveness: Volume of
HCl neutralized per unit cost
__________ mL per $
Formula to use:
vol HCl neutralized per dose / cost per dose
Show work below:
__________ mL per $
10
Comparison of Antacids
Postlab
Name_______________
1. Which antacid is the better buy? In other words, which one neutralized more HCl per dollar
spent?
2. What two quantities will be equal at the endpoint of the titration carried out in this
experiment?
How did you know when you had reached the endpoint (what did you see)?
Why is it important not to add too much HCl and go past the endpoint?
3. Show the products of the neutralization reaction below:
Be sure your reaction is balanced.
H2SO4
+
Mg(OH)2

Label the acid, the base, and the salt in your reaction above.