Quantitative Determination of Sodium Bicarbonate in Alka Seltzer®
Chen, Y-H, and Yaung, J-F. “Alka Seltzer Fizzing- Determination of Percent by Mass
of NaHCO3 in Alka Seltzer Tablets.” J.Chem.Ed. vol 79, no.7, July 2002. p. 848
Introduction:
Alka Seltzer® is an over the counter antacid and pain reliever that is taken after
dissolving in water. It and other generic effervescent tablets contain aspirin, citric acid
and sodium bicarbonate. As the tablet dissolves in water, an acid base reaction between
the citric acid and sodium bicarbonate takes place with the evolution of carbon dioxide
gas. The production of the gas produces bubbles into the atmosphere resulting in a loss
of mass after the reaction is complete. By determining the loss in mass, one should be
able to calculate the amount of sodium bicarbonate reacted and therefore the percent
by mass of NaHCO3 in the tablet. According to the balanced equation, one mole of
carbon dioxide gas is produced from one more of NaHCO3.
Therefore,
Mass NaHCO3 reacted = (mass of CO2 generated/MM of CO2) x MM of NaHCO3
and
Percent by mass of NaHCO3 in a tablet =
(mass of NaHCO3 in tablet/mass of tablet) x 100%
Materials and Equipment per student or pair of students
8 or more Alka Seltzer® or other available effervescent tablets
vinegar (acetic acid about 4.5%) 150 mL
Clear plastic cups ( 9 cm depth, about 230 mL) (Do NOT USE Paper cups)
Electronic balance
Graduated cylinder or burettes to accurately dispense the vinegar samples
Procedure:
Measure 35 mL of water into a plastic cup. Record the mass of the cup and the water.
Determine the mass of one tablet and record it.
Carefully place the tablet in the water in the cup.
When the bubbling has ceased, stir the solution to ensure complete reaction.
Reweigh the cup and contents and record the mass.
Repeat the procedure using 30 mL of water and 5 mL of acetic acid in the cup. Dispense
the acetic acid accurately using a burette. It is best to run several tests concurrently
to allow complete reaction.
The following eight tests should be completed.
Experiment Number Volume of Acetic Acid, Volume of Water,
(mL) (mL)
1 0 35
2 5 30
3 10 25
4 15 20
5 20 15
6 25 10
7 30 5
8 35 0
Calculations:
Write the balanced equation for the reaction between the NaHCO3 and acetic acid.
Calculate the mass of carbon dioxide gas generated in each of the trials.
Calculate the mass of NaHCO3 reacted for each trial.
Calculate the percent by mass of NaHCO3 in each of the tablets.
Calculate the average percent of NaHCO3 in the tablets.
Prepare a graph of Percent by Mass of NaHCO3 in each tablet as a function of the
volume of vinegar used.
Questions:
1. This reaction only requires 35 mL of liquid. Why is the reaction run in a large cup?
2. The effervescent tablet contains citric acid which causes it to react in pure
water. Why is acetic acid added to the reaction?
3. Why must the mixture be stirred after the reaction is completed?
4. Using the graph determine the limiting reactant. Explain your answer.
5. Does the percent of NaHCO3 vary in the different tablets?
6. Carbon dioxide dissolves in water and water evaporates into the atmosphere
during the experiment. How do these two factors affect the results of your
analysis?
Teacher Notes:
1. The quantity of vinegar is not required in the calculation; therefore, the acetic acid
content is not crucial. It should remain constant for all trials. Vinegar with a lower
concentration of acid will result in a later switch of limiting reactant.
2. To improve the results, it is important to carefully measure the amount of vinegar
added. The use of burettes is recommended. Set up several dispensing burettes in
the lab. If they are not available, then provide 10 mL graduated cylinders.
3. The spattering of the mixture may cause an extra loss in mass. Therefore the use of
tall reaction vessels is encouraged. Also do not use paper cups (such as bathroom
cups) as they will absorb water as the reaction is allowed to stand.
4. Swirl or stir the mixture to ensure release of all bubbles.
5. For a 10 minute reaction time, there is a 35-70 mg decrease in mass in the blank
(Trial #1) depending on the temperature and humidity in the room. Part of the
carbon dioxide may dissolve, up to 46 mg of carbon dioxide may dissolve in 35 mL.
While the evaporation of water leads to too large a value of percent mass, the
dissolving of carbon dioxide will result in an opposite effect. The combination of the
two errors should only give a minimal effect on the shape of the graph and the
percent by mass.
6. The reaction mixtures may be diluted with water and poured down the drain.
LABORATORY REPORT FORM
Name:____________________________ Roster #:___ Period: ____ Date:___________________________
Partner’s Name or Lab Group (If applicable)____________________
Volume Mass of Total mass of Total mass of Mass of carbon Mass of
Volume Mass of % NaHCO3
of container container and container and dioxide NaHCO3 in
Exp. # of water Tablet in tablet
vinegar and liquid contents,before contents,after (lost mass) tablet
(mL) (g) %
(mL) (g) (g) (g) (g) (g)
