Acid in Your Soda

      The acid content of many foods and beverages contributes significantly to their taste. Soft drinks often contain varying quantities of several acids, which give sodas their tart flavor. In cola products, these acids are predominantly carbonic acid (from the carbonated water) and phosphoric acid. In sodas such as Squirt and 7-Up, the acids are carbonic acid and citric acid.
        Acids can be classified as monoprotic, diprotic, triprotic, etc.: a monoprotic acid has one proton that can undergo a reaction with a base, a diprotic acid has two such protons, and a triprotic acid has three. Shown below are examples of each type of acid undergoing a reaction with sodium hydroxide.
Monoprotic: HCl(aq) + NaOH(aq)  H2O(l) + NaOH(aq)
Diprotic: H2SO4(aq) + 2NaOH(aq)  H2O(l) + Na2SO4(aq)
Triprotic: H3PO4(aq) + 3NaOH(aq)  H2O(l) + Na3PO4(aq)
Any acid that has more than one proton that undergoes a reaction with a base is called a polyprotic acid. Citric acid is a weak, polyprotic acid that undergoes the following reaction.
H3C6H5O7(aq) + 3 NaOH(aq)  3 H2O(l) + Na3C6H5O7(aq)
In this experiment you will be performing a titration to determine the concentration of citric acid in 7-Up. Prior to the titration the majority of the carbonic acid was removed by allowing the 7-Up to go flat so we do not have to take it into consideration. A burette is used in a titration 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 been reacted with the base. The technique of titration can be applied to other types of reactions such as oxidation-reduction, precipitation, complexation and other acid-base neutralization reactions.
Indicators are used to provide visual information about the amount of acid or base in a solution. You will be using an indicator called phenolphthalein for this experiment. In a basic solution (a solution that contains more base than acid), phenolphthalein is pink. In solutions that are acidic (a solution that contains more acid than base), phenolphthalein is colorless. During the titration performed in this experiment, the point where the indicator first changes from colorless to pink corresponds to the point where the reaction is complete. This point is called the endpoint of the titration.
Preparing a standard base
Prior to performing the titration to determine the amount of acid in a 7-Up sample, you need to know the exact concentration of the base that you will be using. Standardizing the base involves performing a titration with a known amount of a stable acid, which in this case is the monoprotic acid potassium hydrogen phthalate (abbreviated KHP). The reaction of KHP and NaOH is
NaOH(aq) + KHC8H4O4(aq)  H2O(l) + KNaC8H4O4(aq)
Note that one mole of NaOH reacts with one mole of KHP. Note that the chemical formula of KHP is not KHP!

Cardinal Rules Regarding Burets:

- Liquid only comes out bottom, through stopcock.
- Never 'completely' drain a buret.

- When changing liquid, rinse x2 with ~5ml of 'new' liquid first.
- Always fill buret using a funnel, but do not leave funnel in buret during titration.
- Volumes are read to the hundreths place. Always.

Standardization of NaOH
    You will first need to make ~250 mL of a ~0.020 M NaOH solution. You will then standardize it by titrating a known mass of KHP (~0.05 g) in ~35 mL of water into a 125 mL flask and don't forget some indicator. Your end point is a faint persistent pink color. Placing a white piece of paper under your Erlenmeyer flask helps you better see this color change. Calculate the molarity of the NaOH solution. Repeat this process two more times and calculate the average molarity of the NaOH solution.
Titration of 7-Up
     Now that you have your [NaOH], it is time to titrate the soda. The citric acid content in Squirt and 7-Up is quite low, so good technique is critical. You will be titrating 10 mL samples of the supplied soda samples. The sodas in your lab have been opened days in advance to assure they are decarbonated. Slowly add the NaOH solution to the 7-Up until a faint pink color is obtained. Calculate the molarity of the citric acid in 7-Up. Repeat this process two more times and calculate the average molarity.

8 min screencast of experiment

Lab Report
: To be explained in lecture, but it will include electronic data table with words explaining each step, include sample calculations of each calculation, and that whole short form memo style commentary section.

Pre-lab questions (you know where to go)
1. You be making your NaOH solution that you will be using as a titrant. You will make 250 mL of a ~0.02 M NaOH. How many g of NaOH will you need to dissolve in the solution?
2. What is the molar mass of KHP?
3. A 10.0 mL aliquot of clear soda takes 13.5 mL of 0.00234 M NaOH to reach the endpoint. What is the molar concentration of citric acid in the soda?
4. Make (on a seperate piece of paper) a blank data sheet showing all the numbers you will be measureing for this experiment. Put your answer to #1 and #2 above because you will need those during lab.