Lab Background

The Alka-Seltzer® Reaction

Everyone is probably familiar with the old Alka-Seltzer® commercial starting with “Plop-plop, fizz-fizz, oh what a relief it is…” However, what you’ve most likely never thought about is the actual mechanism by which Alka-Seltzer® provides its relief. According to its packaging, an Alka-Seltzer® tablet contains 1916-mg of sodium bicarbonate (NaHCO₃), 325-mg of acetylsalicylic acid (HC₉H₇O₄), and 1000-mg of citric acid (H₃C₆H₅O₇). In particular, when the tablet is dissolved in water, the sodium bicarbonate reacts with acetylsalicylic and citric acid to generate the following chemical reaction:

Though the above chemical equation appears absolutely terrifying at first glance, it is rather simplistic once we look at what is actually reacting in greater detail. Each reactant in the equation above is a weak electrolyte, meaning that when dissolved in water each species breaks down into its corresponding ions. For example, the sodium bicarbonate (NaHCO₃) gets broken down into a sodium ion (Na⁺) and bicarbonate (HCO₃^-), while the acetylsalicylic acid breaks down to H⁺ and C₉H₇O₄^- and the citric acid decomposed to H⁺ and H₂C₆H₅O₇^-. In fact, the molecule holding the key to the relief of an Alka-Seltzer® tablet is the bicarbonate ion. When dissolved in water, the sodium bicarbonate undergoes an acid-base reaction with the two acids, which for the sake of simplicity will now just be referred to as H⁺. Thus, our frightening equation above can be dramatically simplified to:

Continuing, the product in this equation is known as the extremely unstable carbonic acid (H₂CO₃) which quickly breaks down into water (H₂O) and carbon dioxide (CO₂) gas. Thus, our chemical equation now takes the form:

This release of CO₂ gas is what produces the bubbles seen when we add the tablet to water, and since the CO₂ molecule was originally part of the mass of the Alka-Seltzer® tablet, its release into the atmosphere should result in a net loss of mass after the reaction is complete. Thus, if we know the mass of the water and tablet before adding them together we can subtract the mass of the water and the tablet after the reaction is complete to get the mass of CO₂ lost. Since there is a 1:1 ratio of CO₂ to HCO₃^- in the reaction, we can then calculate the amount of NaHCO₃ reacted and therefore finally determine the mass percent of this species in our Alka-Seltzer® tablet.

Idealistically, we would hope that all the sodium bicarbonate would be reacted, however this is not the case as some is always left over. This is because the mole amounts of the acids in each tablet is deliberately less than the mole amount required to react with all of the NaHCO₃. The purpose of this excess is to make sure that there is always enough sodium bicarbonate available to neutralize the stomach acids responsible for heartburn and stomachaches. The bile in the stomach responsible for such illness is around a pH of 1.0, and not readily available. Therefore, in this experiment vinegar, also known as acetic acid (HC₂H₃O₂), will be used to replace our stomach juices. Overall, the reaction we will therefore be dealing directly with in this experiment is now shown:

Overall, in this experiment, Alka-Seltzer® tablets will be added to 8 different solutions with increasing amounts of vinegar. The amount of CO₂ produced by the reaction of NaHCO₃ with HC₉H₇O₄ and H₃C₆H₅O₇ can then be determined as previously described. With the mass of carbon dioxide known, stoichiometry can then be utilized to work backwards in order to determine the mass of NaHCO₃ reacted in each trial. The mass percent, which is defined as shown below, will then be calculated for each trial with a plot of mass percent versus volume of vinegar being generated to experimentally determine the total amount of sodium bicarbonate in an Alka-Seltzer® tablet.

Mass Percent NaHCO3