Acid-Base Titration
The goal of today’s lab is to precisely determine the concentration of acid in an aqueous solution
by titration of the acid (acetic acid in vinegar) with a solution of base (sodium hydroxide).
Once enough base has been added to neutralize the acid, the addition of even one more drop of
base will make the indicator phenolphthalein turn pink. The first pink color indicates the equivalence
point of the titration, which means that there has been exactly enough base added to react with all of the
acid. Although phenolphthalein is a useful indicator for many titrations, it is not appropriate for all
titrations.
A second, more reliable, method to determine the equivalence point of a titration is through the
use of a pH electrode. The pH electrode monitors the pH of the solution being titrated. When excess acid
is present in the solution (before the equivalence point) the solution will be buffering and the pH will
rise slowly. Near the equivalence point, the acid will have been consumed, the solution will lose its
buffering ability, and the pH will experience large pH changes. The point of the titration curve with the
most rapid rise in pH as base is added is the location of the titration equivalence point.
You will perform at least three vinegar titrations. The first run will be quick and rough, intended
to give you an approximate volume of base needed to neutralize the acid. In the subsequent runs, you
will use the volume from the first titration to save time, to avoid performing an entire titration drop-bydrop.
Before titrating vinegar, you will need to determine the exact molarity of the sodium hydroxide
solution in use. Therefore, a solid, stable acid, benzoic acid, will be used as an acid standard.
Calibrate the pH probe
Connect the pH probe to the computer's LabPro box. Prepare the computer to collect pH data in
the “Events with Entry” mode, with “volume of NaOH” entered from the keyboard for the x-axis.
Calibrate the pH probe with the two available buffer solutions.
Procedure for standardization of the NaOH solution

Obtain approximately 100 mL of ≈ 0.1 M sodium hydroxide solution. Rinse the barrel of a buret
with ten mL of the NaOH solution to clean it out. Dispense the NaOH solution from the bottom of
the buret, and then fill the buret with the NaOH solution and lower the level of the liquid to within 1
mL below the zero mark. Record the initial reading on the buret with 0.01 mL precision.

Weigh between 0.3 and 0.4 g of benzoic acid into a clean 150 mL beaker. Record the mass of the
benzoic acid with 0.001 g precision. Add 20 mL of ethanol to dissolve the benzoic acid, then add 10
mL of water, three drops of phenolphthalein solution and a magnetic stir bar to the beaker. Position
the bulb of the pH probe in the solution.

While the beaker is being stirred, slowly dispense NaOH solution into the acid solution in the
beaker. After each addition, click KEEP, and enter the volume (cumulative total) of added base.
Begin adding the base 0.5 mL at a time.

When the solution begins turning pink immediately after the base addition, decrease the volume of
base in each addition.

Once the pH of the solution in the beaker begins to “flatten out,” stop adding NaOH solution. Note
the amount of added NaOH solution when the pink color persists for at least 30 seconds.

Print the plot of the titration data and locate the equivalence point. Record in your notebook the
initial buret reading and the buret reading at the equivalence point.

The volume of NaOH solution added to reach the equivalence point in this titration will be used in
the following calculation.
Using the equation for the neutralization reaction,
NaOH(aq) + HC7 H5 O2(aq)  H2 O(l) + NaC7 H5 O2(aq)
the volume of added base at the equivalence point, and the known mass of the benzoic acid, calculate the
concentration of NaOH in the base solution, in units of molar (moles NaOH per liter.)
Procedure for analysis of the HC2H3O2 solution

Refill the buret with sodium hydroxide solution. After the previous procedure, you now know the
exact concentration of NaOH in the solution.

Use a volumetric pipette to measure 3.00 mL of vinegar into a clean 150 mL beaker. Add 30.0 mL of
water and three drops of phenolphthalein. Position the bulb of the pH probe in the solution.

While the beaker is being stirred, slowly dispense NaOH solution into the acid solution in the
beaker. After each addition, click KEEP, and enter the volume (cumulative total) of added base.
Begin by adding the base 0.5 mL at a time.

Once the pH of the solution in the beaker begins to “flatten out,” stop adding NaOH solution.

Print the plot of the titration data and locate the equivalence point. Record in your notebook the
initial buret reading and the buret reading at the equivalence point.
Knowing the concentration of the NaOH solution (which you calculated above) and the reaction
equation,
NaOH(aq) + HC2 H3 O2 (aq)  H2 O(l) + NaC2 H3 O2 (aq)
calculate the concentration of acetic acid in the 3.00 mL of vinegar that you started with. Be sure to use
the correct number of significant figures.
Before continuing, confer with your instructor to ensure that your titration was sufficiently
accurate. If you are too far “off” from the actual concentration of the vinegar, you may need to retitrate.
In this case, it is definitely worth your while to do it right the first time.
Repeat the vinegar titration until you have collected three good equivalence points, calculate the
concentration of vinegar from each of the equivalence points.