12. Analysis of Vinegar by Titration
What you will accomplish in this experiment
You’ll use a chemical analysis method known as “titration” to determine the concentration of acetic acid in the
aqueous solution that is common household vinegar.
Concepts you need to know to be prepared
Vinegar contains acetic acid, HC2H3O2. This molecular compound is classified as a weak acid because when acetic
acid molecules are dissolved in water, only a small fraction of those acid molecules “dissociate.”
You should recall from Lab 11 that “dissociation” is the term used to describe the acid molecules donating their
protons (H+ ions) to the water molecules to create hydronium ions (H3O+) and the conjugate base of the acid.
In this case, the conjugate base is the acetate polyatomic ion (C2H3O2 -).
HC2H3O2 (aq) + H2O (l) H3O+ (aq) + C2H3O2 - (aq)
Titration
Titration is a method that’s used to determine the concentration of an acid or a base in an aqueous solution.
For an acidic solution (such as vinegar) a titration experiment would involve adding a base (such as sodium
hydroxide, NaOH) to the vinegar until all of the acid in the vinegar solution has reacted with that added base.
The acid-base reaction between the acetic acid molecules in the vinegar and the hydroxide ions in the NaOH
solutions is a called a “neutralization” reaction.
HC2H3O2 (aq) + NaOH (aq)
H2O (l) + NaC2H3O2 (aq)
In neutralization reactions, an acid molecule will donate a proton (H+) to the hydroxide ion (OH-) of the base to
make a water molecule (H2O). The other product in a neutralization reaction is a “salt” – an ionic compound.
In this case, the cation is the sodium ion (Na+) and the anion is the acetate ion (C2H3O2 -), the conjugate base of
acetic acid.
The strategy of our titration experiment is as follows:
•
If we know both the volume and the molarity (# moles/liter) of the added base, then we know the number of
moles of hydroxide ion needed to neutralize the acetic acid in the vinegar solution.
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C. Graham Brittain
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•
In a successful neutralization reaction, the number of moles of acid will be exactly equal to the number of
moles of added base.
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•
If we know the volume of the sample of acid, and we know the number of moles of the acid, we can calculate
the molarity of the acid
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Thus the experimental method used for a titration will require a method to accurately measure the volume of the
sample of acid, as well as the volume of base added to that acid.
As you consider the various volume measurements you’ve made in lab, you should recall that the volumes of
liquid you’ve delivered from a buret were recorded to two places past the decimal. So you’ll use a buret to
carefully measure out a sample of vinegar. You’ll then add the sodium hydroxide solution to that sample from a
second buret.
But you’ll also need a way to detect the instant when the acetic acid in the vinegar sample has been completely
“neutralized” – meaning that all of the acetic acid molecules have reacted with the added base (and the number of
moles of acid is exactly EQUAL to the number of moles of added base).
You should recall from Lab 11 that acid-base indicators are molecular compounds that change color when the pH
of the solution containing the indicator reaches a certain value.
The pHydrion paper you used in that experiment was filter paper that had been impregnated with a mixture of
indicator compounds. These compounds changed color across a full range of pH values. By matching the color of
the paper to the pHydrion indicator color chart, you were able to determine the pH of various solutions.
In this experiment, we’ll use just one indicator compound that changes color at precisely the desired pH.
The figure below illustrates how pH changes when a strong base (such as NaOH) is added to a weak acid (such as
acetic acid). You can see that when the titration experiment approaches the “equivalence point” – when the
number of moles of acid is equal to (equivalent to) the number of moles of base – there is a sudden change in pH
from approximately pH 6 to pH 9.
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Thus the appropriate indicator to detect this sudden pH change is phenolphthalein, which changes from colorless to
a pinkish-red at pH 8 – 9.
Procedure that you will follow
A. Preparation of Vinegar Sample
1. Three different vinegars will be available in the reagent fume hood: white vinegar, red wine vinegar, and apple
cider vinegar. Thus the fume hood will contain three burets – one for each of these vinegars. Your lab
instructor will refill these burets as needed. You’re to select one of three vinegars for your titration
experiment, and use this vinegar for both of the titrations you’ll perform.
2. Each time you take a sample of vinegar, you’ll need to record the initial volume from the buret. Then you’ll
open the stopcock and deliver a 2-mL sample of vinegar into your 125-mL Erlenmeyer flask. You’ll then
record the final volume from the buret. Realize that you may deliver slightly less than 2 mL of vinegar, or
slightly more than 2 mL. Some variation in this volume is fine – as long as you correctly record both the initial
and final volume readings (to the appropriate number of significant figures), and so have accurately measured
the volume of vinegar delivered from the buret. Realize this volume is essential to your post-lab calculations,
so the measurement must be made carefully and recorded properly.
3. After obtaining your vinegar sample, add several drops of phenolphthalein indicator to the Erlenmeyer flask.
Then use your graduated cylinder to add 10 mL of distilled water to the flask. This water is added to dilute the
natural color that some commercial vinegars have, so that you’ll not confuse the color of the vinegar with the
color change of the phenolphthalein indicator.
B. Titration of the Vinegar Sample with Sodium Hydroxide
1. Use an appropriate-sized beaker to take approximately 75 mL of the sodium hydroxide stock solution to your
lab bench. Record the molarity of the sodium hydroxide solution on your Report Sheet; you’ll need this
concentration for your post-lab calculations.
2. As described in the Remedies document, first rinse and then fill your own buret with the sodium hydroxide
solution. Then use the stopcock to lower the meniscus to the zero calibration mark (or below). Collect the runoff in a waste beaker, to be disposed of in the appropriate waste container.
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3. When you’re ready to begin titrating a vinegar sample, carefully read and record the initial volume reading
from your sodium hydroxide buret.
4. With one hand, you’ll hold the neck of the Erlenmeyer flask and swirl the flask. With the other hand, you’ll
need to open the stopcock of the buret slightly, to permit dropwise addition of the base to the acid solution in
the flask. At the spot where the base first hits the vinegar solution, the color may temporarily turn pink, but
this color will disappear as you mix the solution by swirling.
5. You’ll continue the titration, adding base as needed until a faint, PERMANENT pink color appears. (Placing a
white sheet of paper beneath the Erlenmeyer flask will help you to see this color change; the faint pink color is
difficult to discern when the flask is on the black surface of the lab bench.) You want to be careful not to add
too much sodium hydroxide. This error is called “over-titration.” If the indicator color becomes a deep pink
or purple, you have over-titrated, and passed the desired end-point. If this happens, you’ll need to repeat the
entire titration with a new sample of vinegar.
6. When you’ve reached this titration “end-point,” carefully read and record the final volume reading from your
sodium hydroxide buret.
7. Now repeat all of these procedures: Obtain/prepare a second sample of vinegar and titrate it with sodium
hydroxide. It’s helpful to keep the flask from the first titration close by, so you can compare the colors of the
titration end-points, and try to achieve the same pink color in your second titration.
8. When you’ve successfully titrated two samples of vinegar to a consistent end-point color, your experimental
work is complete. Be sure to empty your buret into your bench-top waste beaker, and then rinse the buret
thoroughly with distilled water. Dispose of all liquid waste in the appropriate waste container.
9. Following the “strategy-of-titration” discussion in the lab handout, calculate the molarity of the vinegar for
each titration. Determine the average molarity, and the percent difference of the two titration results.
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Report Sheet 12: Analysis of Vinegar by Titration
Student ______________________________ Lab Partner__________________________ Date Lab Performed__________
Section #_________ Lab Instructor__________________________________________ Date Report Received ___________
Lab Notebook: Data and Observations
A. Preparation of Vinegar Sample
Volume of Vinegar Sample 1:
Initial volume reading for vinegar in buret
_______________________
Final volume reading for vinegar in buret
_______________________
Volume of vinegar sample (from subtraction)
_______________________
Volume of Vinegar Sample 2:
Initial volume reading for vinegar in buret
_______________________
Final volume reading for vinegar in buret
_______________________
Volume of vinegar sample (from subtraction)
_______________________
B. Titration of the Vinegar Sample with Sodium Hydroxide
Stock concentration of sodium hydroxide solution (moles/liter, M)
________________________________
Volume of Sodium Hydroxide used to Titrate Vinegar Sample 1:
Initial volume reading for sodium hydroxide in buret
_______________________
Final volume reading for sodium hydroxide in buret
_______________________
Volume of sodium hydroxide delivered from buret (from subtraction)
_______________________
Volume of Sodium Hydroxide used to Titrate Vinegar Sample 2:
Initial volume reading for sodium hydroxide in buret
_______________________
Final volume reading for sodium hydroxide in buret
_______________________
Volume of sodium hydroxide delivered from buret (from subtraction)
_______________________
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Formal Report: Results and Conclusions
Calculation of # MOLES of NaOH used to neutralize the vinegar sample
Vinegar Sample 1:
Vinegar Sample 2:
Determination of # MOLES of Acetic Acid in the vinegar sample
Vinegar Sample 1:
Vinegar Sample 2:
Calculation of Molarity of Acetic Acid in the vinegar sample
Vinegar Sample 1:
Vinegar Sample 2:
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Average of the two Acetic Acid molarities:
Show calculation:
_______________________
Percent Difference of the two Acetic Acid molarities:
Show calculation:
_______________________
Additional Questions:
1. In the Pre-Lab Assignment, you converted the concentration of acetic acid in vinegar from 5.0% w/v to
molarity units. Using that molarity as the “accepted value,” calculate the Percent Error of your experimentallydetermined concentration.
Accepted Value (from Pre-Lab) = _____________ Experimental Value (from Average above) = ____________
Show calculation of Percent Error:
2. You’ll recall that distilled water was added to each vinegar sample to dilute the natural color of the vinegar, so
it would not be confused with the color change of the phenolphthalein indicator. This added volume was not
considered in the calculations. Explain why this volume could be ignored.
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