Acid in Your Soda
The acid content of many foods and
contributes significantly to their taste. Soft drinks often
contain varying quantities of several acids, which give sodas their
In cola products, these acids are predominantly carbonic acid (from the
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
with sodium hydroxide.
Monoprotic: HCl(aq) + NaOH(aq) →
H2O(l) + NaOH(aq)
+ 2NaOH(aq) →
H2O(l) + Na2SO4(aq)
H2O(l) + Na3PO4(aq)
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
3 H2O(l) + Na3C6H5O7(aq)
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.
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.
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
a standard base
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
+ KHC8H4O4(aq) →
H2O(l) + KNaC8H4O4(aq)
that one mole of NaOH reacts with
one mole of KHP. Note that the chemical formula of KHP is not
Cardinal Rules Regarding
- 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
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
piece of paper under your Erlenmeyer flask
helps you better see this color change. Calculate the molarity
the NaOH solution. Repeat this
process two more times and calculate the average molarity of the NaOH
Now that you have your [NaOH], it is time to
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
pink color is obtained. Calculate
the molarity of the citric acid in 7-Up. Repeat this process two more
and calculate the average molarity.
8 min screencast of experiment
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.
questions (you know where to go)
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
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
4. Make (on a seperate piece of paper) a blank data
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.