The Neutralizing Ability of Antacid Tablets
Purpose: In this experiment you will measure the amount of stomach
acid consumed (or neutralized) by various antacid tablets. If you have
a favorite one, bring a package to the lab (one color only).
Procedure: This experiment involves several steps. First the
tablet is dissolved and an excess of acid of known concentration is added
to the tablet. The solution is then briefly heated to insure that all of
the antacid reacts. Finally, the remaining unreacted acid is titrated with
base (NaOH) to determine the amount left over and hence, the amount that
reacted with the tablet. The chemical reaction which occurs is:
NaOH + HCl -------> NaCl + H2O
A detailed description of the individual steps in the analysis follows.
Preparation of Sample:
- Weigh an antacid tablet and transfer it to a 250 ml erlenmeyer flask.
Record the weight of the tablets you are going to analyze.
- Add 50.0 ml of hydrochloric acid solution (labeled 0.50 M) with a pipette
to the flask containing the tablet.
- If the tablet does not dissolve readily, cover the flask with a watch
glass and boil gently for 5-10 minutes on a hotplate. Set it aside to cool.
While it's cooling, weigh out another tablet of the same brand and proceed
through steps 1 and 2.
- Check out a buret from the Stockroom. Test it to see if it is clean
by filling with distilled water and letting it drain. If the water does
not drain from the walls, wash carefully ($$$) with warm, soapy water using
a buret brush. Rinse several times with distilled water.
- Using a funnel fill the buret with base (sodium hydroxide) solution
(labeled 0.5 M) to just above the top line.
- Allow it to drain to just below the top line. If air bubbles remain
in the tip, drain more out. If they still are in the tip, your instructor--who
is lurking nearby, will be happy to get them out for you.
- Add several drops of the indicator, thymol blue, to the cooled solution
of antacid tablet containing unreacted acid. The solution will be red.
- Read the initial volume of the buret using the graduations on the buret
and bottom of the meniscus. It is often easiest to put a white piece of
paper behind the buret to sight against. Record this as the initial volume
for the first trial in your notebook in the results section.
- Add a small amount (less than one milliliter) of the sodium hydroxide
to the flask. A spot of colorless solution may appear where the drops hit.
With swirling, this area will disappear. The idea is to add the exact amount
of the base solution needed to cause the solution in the beaker to change
from red to very pale yellow and stay yellow for 15 seconds or more. This
is harder than it may sound. The color change should be observed with the
addition of only one or two drops of the base solution and you can't go
back if you add too much. Watch the solution in the flask. As it gets more
difficult to get rid of the pinkish color when you swirl the flask, add
smaller amounts. Ideally, you should be adding it dropwise when you reach
the point where color finally changes from red to yellow (endpoint). Patience!
The first time is the hardest. Once you know how much it takes for a tablet,
you can add slightly less in the next trial and add the last milliliter
or so dropwise and you won't go past the endpoint. It may be necessary
to refill the buret once.
- Read and record the final volume. Calculate the total volume you used
by subtracting the initial from the final volume. Record this, too, in
- As time permits, do one or two more trials. They should, as you practice,
agree to within about 0.20 ml.
Wastes: The HCl and NaOH used in this experiment are weak. They
can be put into the sink with the water running. The finished titrations
are not hazardous and can also be rinsed down the drain.
The Calculations: The number of grams of stomach acid that
is neutralized can be calculated using the expression:
grams stomach acid per tablet = 10[ 25.0 - (ml base)(.50) ]
If, for example, it took 8.0 ml of base to titrate the sample, then
grams stomach acid = 10[ (25.0) - 8.0 (.50) ] = 210 g
Record the value for each of your samples and the average value with
Conclusion: Does your tablet neutralize 47 times its weight in
stomach acid? You can answer this question for your conclusion by
dividing the mass of stomach acid by the mass of the individual tablet.
This activity has been copied, with permission, from the California State University at Stanislaus
server to ours, to allow faster access from our Web
site. We encourage you to explore the original site
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