How Much Citric Acid is in Your Soda?
To determine the molarity (concentration) of citric acid (H3C6H5O7) in various different citrus flavored sodas.
Popular sodas all strive for a somewhat sour ('tart') flavor and manufacturers utilize acids to impart this taste. While there are a vast variety of different brands of sodas on the market, they can be broken into two main categories: dark or clear. This differentiation is not only a simple color one but is also one of taste and content. Clear sodas, such as 7-Up, tend to taste like citric fruit and citric acid is used to impart the tart flavor. Dark sodas, such as Coca Cola, tend to use caramel for flavoring and phosphoric acid is used to complement this.
All acids can be classified as monoprotic, diprotic, or triprotic. Shown below are examples of each type of acid undergoing a reaction with sodium hydroxide.
Monoprotic (x=1): HCl (aq) + NaOH (aq) → H2O (l) + NaCl (aq) Diprotic (x=2): H2SO4 (aq) + 2 NaOH (aq) → 2 H2O (l) + Na2SO4 (aq) Triprotic (x=3): H3PO4 (aq) + 3 NaOH (aq) → 3 H2O (l) + Na3PO4 (aq)
Any acid with more than one proton (hydrogen) is called a polyprotic acid. Citric acid is a weak, polyprotic acid that undergoes the following reaction with sodium hydroxide:
H3C6H5O7 (aq) + 3 NaOH (aq) → 3 H2O (l) + Na3C6H5O7 (aq)
You will perform a titration to determine the concentration of citric acid in a clear soft drink. Prior to the titration, the majority of the carbonic acid (carbonation) was removed by allowing the soft drink to go flat so we do not have to consider this when doing our calculations.
A buret is used in a titration experiment to dispense measured increments of one solution into a known volume of another solution. Careful technique will allow you to detect the point where the reaction is complete; in this case, when all of the citric acid has reacted with the base.