Acetic Acid Concentration of “JUICY ORCHARD”
Apple Cider
Collin College
Bassam Attili, PhD, Dawn Richardson, PhD and Fred Jury, PhD
JUICY ORCHARD, home of the world’s sweetest cider, has contracted us to help
with a problem from their past shipment of apple cider. It seems that 30,000 gallons of
premium grade A cider has been returned due to taste (acidity) concerns. It will be our
task to determine the level of acidity in these samples.
Cider becomes more acidic through the formation of acetic acid. Hence, you will
be titrating the acetic acid to determine its concentration in the cider samples. To carry
out this task, you will need to use the titration process to calculate the concentration of
acetic acid. You need to titrate six samples; three for standardizing the NaOH solution to
be used to determine the acid in three cider samples.
Objectives:
The main objective is to determine the concentration of acetic acid in apple cider
using a standardized NaOH solution. In addition, you will learn
how to prepare solutions.
the fundamental concepts associated with titration.
the useful technique of titration.
how to obtain titration data and perform titration calculations at the end point to
determine the molarity of NaOH
Materials:
NaOH pellets, apple cider, weighing boats, 250 ml volumetric flasks, 50 mL burettes,
burette clamps, 250 ml plastic bottles to hold the prepared NaOH solution, ring stands, 3
Erlenmeyer flasks (125 or 250 mL) for each group, 50 mL graduated cylinders,
phenolphthalein solution, potassium hydrogen phthalate (KHP).
Introduction:
Sodium Hydroxide:
Many analytical procedures involve comparison of an unknown with a standard.
NaOH is not a suitable primary standard. Solid NaOH is highly hygroscopic (it absorbs
water from the air). It also absorbs carbon dioxide from the air, forming sodium
carbonate and thereby reducing the amount of sodium hydroxide present. When weighing
a sample of NaOH, you do not know the exact amount of NaOH in the sample due to the
previously stated properties of solid NaOH. This means that an exact known molarity of
NaOH cannot be directly prepared. We can only prepare a solution of NaOH that has an
approximate concentration.
In this case, an acid that can be weighed directly is used as the primary standard
to determine the exact concentration of the NaOH solution. The primary standard used
here is potassium hydrogen phthalate (KC8H5O4, often abbreviated KHP). Potassium
hydrogen phthalate is a weak monoprotic acid. It is available as a pure, stable, fine
crystalline solid that can be accurately weighed. A KHP sample of known mass (and,
therefore, known moles) can be titrated with the NaOH solution to determine very
precisely the concentration of the NaOH. This procedure is called standardizing the
NaOH solution.
KHP reacts with NaOH as shown below.
The stoichiometry of the reaction indicates that one mole of potassium hydrogen
phthalate reacts with one mole of NaOH. The acidic solution of KHP is colorless when
phenolphthalein indicator is added. When enough NaOH has been added to react
stoichiometrically with the acid, the indicator in the solution will be very slightly pink for
30 seconds or longer. This is called the endpoint of the titration. If just one more drop of
NaOH (or even a portion of a drop) is added, the solution will possess excess NaOH
reactant, which will make the solution appear more intensely pink. At the endpoint, the
moles of acid will equal the moles of base added (practically if not exactly). The molarity
(M) of the NaOH can then be determined using the moles and the volume of NaOH in the
sample used. This is the standardization of a base.
M NaOH = [(mol of NaOH) / (L of NaOH solution)]
Once this concentration is determined, the average of three trials, the NaOH
solution can serve as a standard to determine the concentration of vinegar (acetic acid) in
apple cider.
Apple cider:
The exact volume of apple cider must be accurately measured. NaOH standard
solution should be added until the end point; a light pink color appears and stays for 30
seconds or more. Three samples should be titrated and the average should be reported as
the concentration of the acetic acid in the cider.
Caution: Put on your CHEMICAL SPLASH-PROOF SAFETY
GOGGLES!
NaOH reacts with the proteins in the top layer of your skin making it feel slippery. You
will not feel any pain or itching. When this occurs, simply wash your skin with plenty of
soap and water. Keep washing the affected area for no less than 10 minutes.
Procedures:
Begin by preparing 250. mL of approximately 0.1 M NaOH using a volumetric
flask. This solution will be used to titrate a measured mass of KHP (about 0.3 grams).
Wash your burette with soap and water till there are no beads of water on the
inside of the burette. Rinse the burette with two one-mL portions of your NaOH solution
then fill the burette with your NaOH solution up to the zero mark (it need not be exactly
at 0.00 mL). Add about twenty milliliters of distilled water to the weighed KHP sample
in an Erlenmeyer flask. Stir until all the KHP has dissolved. You may have to slightly
heat for all the KHP to dissolve. Add 2-3 drops of the phenolphthalein indicator to the
solution. Start titrating the KHP with your prepared NaOH solution. Be sure to record the
exact initial burette volume before you begin. Rinse the sides of the flask as you go to
make sure all the added NaOH is in solution. The phenolphthalein will change color from
colorless to pink when enough base is added. Record the volume of NaOH that you added
to obtain a color change of the solution that persists for 30 seconds or longer. Repeat the
titration two times, each time with a weighed sample of KHP and refill your NaOH
burette.
Use the same procedure to titrate thee samples of apple cider (acetic acid) with the
standardized NaOH.
Pre-lab Questions:
1. Why should the buret be rinsed with NaOH before it is filled?
2. Why does a pink color appear, and then disappear quickly, at the point where the
NaOH solution comes in contact with the solution in the flask?
3. What is the concentration of a NaOH solution if 32.47 mL of it are required to
neutralize 1.27 g of KHP?
4. Write the balanced chemical equation of the reaction between acetic acid and
NaOH.
5. Why must air bubbles be expelled from the burette tip?
6. If the NaOH was diluted to half of its original concentration, what affect would
this have on the titration?
Post-lab Questions:
1. Determine the number of moles of acid used in each titration.
2. Calculate the molarity (mol/L) of the NaOH solution for each titration as well as
the mean molarity for the trials.
3. An impure sample of potassium hydrogen phthalate was titrated to determine the
percent composition KHP. What is the percentage of potassium hydrogen
phthalate if 2.81 g of the mixture required 35.61 mL of 0.152 mol/L NaOH to
reach the end point?
4. Calculations of acetic acid concentration (M) for each trial and calculate the
average.