Horace Mann School
Chemistry
Delanty/Dilley/Hatch
Mitchell/Rosenthal
2006-07
ANTACID ANALYSIS
BACKGROUND
In a television commercial, the Warner-Lambert Pharmaceutical Co. has
claimed “Rolaids consumes 47 times its own weight of stomach acid.” This
experiment is designed to test that claim and to compare Rolaids with some
competitive antacid products.
The experimental method involves a gravimetric titration technique, in
which quantities of acid and base are weighed rather than measured by volume.
The acid in the stomach is about 0.5% hydrochloric acid, which is equivalent to
0.14 M HCl (“”stomach acid”) to an antacid table; that is, you will add more acid
than is needed to neutralize the base in the tablet. You will then titrate the excess
acid with 0.14 M NaOH.
The density of the HCl and NaOH solutions are very close to 1.0 g/mL, so
that the number of mLs of acid or base can be assumed to be equal to its mass in
grams. The excess acid will be delivered from a graduated cylinder. Since the
molarities of the HCl and NaOH solutions are the same, the mass of the NaOH
solution needed to reach the endpoint of the titration equals the mass of excess
“stomach acid”. The difference between this mass and the mass of HCl added to
the tablet equals the mass of “stomach acid” neutralized by the tablet.
PRELABORATORY EXERCISE
Your instructor will provide Rolaids and one or two other brands of antacid
tablets for analysis. However, you may wish to bring in your own brand for
analysis. You should select for analysis only those tablets which are easily
crushed and dissolved; do not try to analyze the “chewy”kind. Also, antacids
which have coloration may contain dyes which will interfere with the indicator
that is used tin the titration. White tablets are most easily analyzed.
PROCEDURE
Record the ingredients in the tablet what are listed on the Rolaids package.
Also record the price on the Rolaids bottle and the number of tablets per bottle.
Obtain the mass of a Rolaids tablet to the nearest 0.01 g. Use a mortar and
pestle to crush the tablet.
Measure 100 mL of 0.14 M HCl (“stomach acid”) with a graduated
cylinder. Add about 50 mLs of the acid to the crushed tablet in the mortar.
Transfer the mixture of tablet plus acid to a 250-mL beaker, being careful
not to spill any material. Use the remaining acid to rinse the mortar so that the
entire tablet is transferred to the beaker. If necessary, use distilled water to
complete the transfer. There will be some inert ingredients in the tablet which
will not dissolve, so the mixture will be somewhat cloudy.
Add 5 drops of bromphenol blue indicator to the acid mixture. The color
should now be yellow, the acid color of the indicator. Now you are ready to
titrate the excess acid with base.
The plastic squeeze bottle, labeled 0.14 M NaOH, should be at least halffull. If necessary, fill the bottle from the stock solution that is provided. Obtain
the mass of the bottle to the nearest 0.01 g. After recording the mass of the
bottle, add the base slowly to the acid mixture, stirring constantly to insure
proper mixing. The color at the endpoint of this titration is blue-gray. As you
approach the endpoint, add the base just one drop at a time until the solution just
barely begins to turn gray. When you have reached the endpoint, obtain the mass
of the plastic bottle.
Do a second trial with another Rolaids tablet.
Repeat the experiment with a competitive brand of tablet. Do two trials for
this brand. Record the ingredients that are listed on the package, the cost of the
bottle, and the number of tablets per bottle.
If time permits, analyze a third brand.
ANTACID ANALYSIS DATA
Antacid
Cost per Tablet
Tablet
Mass
(g)
Mass
(vol)
base
Used
(mL)
Mass acid
Ratio
Mass
solution
HCl
Neutralized g acid soln neutraliz
by tablet
g tablet
ed
(g)
(.14M *
L*36.5g)
Ratio
g acid
g tablet
I
II
III
III/I
IV
IV/I
1. According to our results, is the Rolaids claim valid?
2. Based on our results and the cost of each of the tablets, which tablet is the best buy?
3. Identify the base(s) contained in each of the tablets we analyzed. Write an equation
for the reaction of each of these bases with HCl.