Antacid Testing
The stomach secretes approximately 2 to 3 liters of gastric fluid daily. This fluid consists of
mucus, hydrochloric acid, pepsinogen (a form of pepsin which degrades proteins for use in the
body) and regulatory substances. The primary function of the mucus is to lubricate and protect
the stomach wall from the damaging effect of the hydrochloric acid which acts a an
antimicrobial agent that kills microorganisms. In addition, nervous and hormonal mechanisms
regulate gastric secretions so that the 'acidity' of the stomach can fluctuate significantly based
on what has been eaten or on the emotional state of the individual. It is this hydrochloric acid
that is most often the cause of what is referred to as "heartburn” or acid indigestion.
To counteract this acidity, over-the-counter medications known as antacids are used. The
variation of ingredients is quite small considering the almost overwhelming number of products
on the shelf. The most common antacids contain one or more of the following: sodium
bicarbonate, calcium carbonate, magnesium carbonate, aluminum hydroxide, magnesium
hydroxide, and magnesium trisilicate. Each of these can neutralize stomach acid.
In a neutralization, an acid (the hydrochloric acid) and a base (the antacid) combine to produce
a salt and water. When the same quantities of acid and base have been added, we say the
endpoint has been reached.
In this activity you will allow samples of simulated stomach acid to be neutralized by various
antacid tablets, as well as the well-known remedy of baking soda. The samples of the acid will
be large enough initially that the antacid tablets will not completely neutralize the acid. To
complete the neutralization you will add drops of sodium hydroxide (a base, NaOH) until a
color change is noted. The color change is due to the phenolphthalein indicator that has been
added. The point at which the color change occurs is the endpoint.
You will first prepare a blank sample. This sample will provide you with the number of drops of
base needed to neutralize the acid without any antacid present. This will represent a baseline
by which you may compare the effectiveness of antacids added before the neutralization.
Procedure
1. Place 10.0 mL of 1.0 M hydrochloric acid (HCl) into a beaker. Add 90.0 mL of distilled
water (using graduated cylinder). Mix it up well.
2. Using a pipet, put 50 drops of this acid solution into a small plastic cup. Add two drops of
phenolphthalein indicator.
3. Start adding drops of sodium hydroxide (NaOH) solution to the cup, one at a time, until the
solution stays pink for 30 seconds. Count the number of drops you added, and write
this in the data table for the “Blank”.
4. Next, place 10.0 mL of 1.0 M hydrochloric acid (HCl) into another beaker. Add ½ teaspoon
(estimate) of baking soda, and mix it up until it stops fizzing. Then add 90 mL of
distilled water to it.
5. Using a pipet, put 50 drops of this acid solution into a small plastic cup. Add two drops of
phenolphthalein indicator.
