LabQuest
Acid-Base Titration
7
A titration is a process used to determine the volume of a solution that is needed to react with a
given amount of another substance. In this experiment, your goal is to determine the molar
concentration of two acid solutions by conducting titrations with a base of known concentration.
You will be testing a strong acid, HCl, solution and a weak acid, HC2H3O2, solution. You will
use the sodium hydroxide, NaOH, solution that you standardized in Lab 6 as your base of known
concentration. The reaction equations are shown below in net ionic form.
H+ (aq) + OH– (aq) → H2O(l)
HC2H3O2(aq) + OH– (aq) → H2O(l) + C2H3O2– (aq)
The stoichiometry of the two reactions is identical; thus, your calculations will be
straightforward. However, you will observe a significant difference in how the two acid solutions
react with NaOH.
In this experiment, you will monitor pH as you titrate. The region of most rapid pH change will
then be used to determine the equivalence point. The volume of NaOH titrant used at the
equivalence point will be used to determine the molarity of the acid solutions.
OBJECTIVES
In this experiment, you will
 Accurately conduct acid-base titrations.
 Determine the equivalence point of a strong acid-strong base titration.
 Determine the equivalence point of a weak acid-strong base titration.
 Calculate the molar concentrations of two acid solutions.
Figure 1
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CHOOSING A METHOD
If you choose Method 1, you will conduct the titration in a conventional manner. You will
deliver volumes of NaOH titrant from a buret. You will enter the buret readings manually to
store and graph each pH-volume data pair.
If you choose Method 2, you will use a Vernier Drop Counter to conduct the titration. NaOH
titrant is delivered drop by drop from the reagent reservoir through the Drop Counter slot. After
the drop reacts with the reagent in the beaker, the volume of the drop is calculated and a
pH-volume data pair is stored.
MATERIALS
Materials for both Method 1 (buret) and Method 2 (Drop Counter)
LabQuest
LabQuest App
Vernier pH Sensor
0.100 M NaOH solution
hydrochloric acid, HCl, solution, unknown molarity
acetic acid, HC2H3O2, solution, unknown molarity
magnetic stirrer
stirring bar or Microstirrer
wash bottle
distilled water
ring stand
utility clamp
250 mL beaker
10 mL pipet with pump
50 mL graduated cylinder
Materials required only for Method 1 (buret)
50 mL buret
10 mL pipet
buret clamp
Materials required only for Method 2 (Drop Counter)
Vernier Drop Counter
60 mL reagent reservoir
5 mL pipet or graduated 10 mL pipet
100 mL beaker
10 mL graduated cylinder
a second 250 mL beaker
METHOD 1: MEASURING VOLUME USING A BURET
1. Obtain and wear goggles.
2. Obtain about 25 mL of a hydrochloric acid solution of unknown concentration. Add 50 mL
of distilled water to a 250 mL beaker. Use a pipet bulb (or pipet pump) with a pipet to
transfer 10 mL of the HCl solution into the beaker. CAUTION: Handle the hydrochloric
acid with care. It can cause painful burns if it comes in contact with the skin.
3. Place the beaker on a magnetic stirrer and add a stirring bar. If no magnetic stirrer is
available, stir the reaction mixture with a stirring rod during the titration.
4. Connect the pH Sensor to LabQuest and choose New from the File menu. If you have an
older sensor that does not auto-ID, manually set up the sensor.
5. Set up a ring stand, buret clamp, and 50.0 mL buret to conduct the titration (see Figure 1).
Rinse and fill the buret with 0.100 M NaOH solution. CAUTION: Sodium hydroxide
solution is caustic. Avoid spilling it on your skin or clothing.
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Acid-Base Titration
6. Use a utility clamp to suspend the pH Sensor on the ring stand, as shown in Figure 1.
Position the pH Sensor so that its tip is immersed in the HCl solution but is not struck by the
stirring bar. Gently stir the beaker of acid solution.
7. On the Meter screen, tap Mode. Change the data-collection mode to Events with Entry. Enter
the Name (Volume) and Unit (mL) and select OK.
8. Conduct the titration carefully, as described below.
a. Start data collection.
b. Before you have added any NaOH solution, tap Keep and enter 0 as the buret volume in
mL. Select OK to store the first data pair.
c. Add the next increment of NaOH titrant (enough to raise the pH about 0.15 units). When
the pH stabilizes, tap Keep, and enter the current buret reading as precisely as possible.
Select OK to save the second data pair.
d. Continue adding NaOH solution in increments that raise the pH by about 0.15 units and
enter the buret reading after each increment. When a pH value of approximately 5.0 is
reached, change to a one-drop increment. Enter a new buret reading after each increment.
e. After a pH value of approximately 10 is reached, again add larger increments that raise the
pH by about 0.15 pH units, and enter the buret level after each increment.
f. Continue adding NaOH solution until the pH value remains constant.
9. Stop data collection to view a graph of pH vs. volume.
10. Dispose of the reaction mixture as directed. Rinse the pH Sensor with distilled water in
preparation for the second titration.
11. Examine your titration data to identify the region where the pH made the greatest increase.
The equivalence point is in this region.
a. To examine the data pairs on the displayed graph, select any data point.
b. As you move the examine line, the pH and volume values of each data point are displayed
to the right of the graph.
c. Identify the equivalence point as precisely as possible and record this information.
d. Store the data from the first run by tapping the File Cabinet icon.
12. An alternate way of determining the equivalence point of the titration is to take the first and
second derivatives of the pH-volume data.
Determine the peak value on the first derivative vs. volume plot.
a. Tap the Table tab and choose New Calculated Column from the Table menu.
b. Enter d1 as the Calculated Column Name. Select the equation 1st Derivative (Y,X). Use
Volume as the Column for X and pH as the Column for Y. Select OK.
c. On the displayed plot of d1 vs. volume, examine the graph to determine the volume at the
peak value of the first derivative.
Determine the zero value on the second derivative vs. volume plot.
d. Tap Table and choose New Calculated Column from the Table menu.
e. Enter d2 as the Calculated Column Name. Select the equation 2nd Derivative (Y,X). Use
Volume as the Column for X and pH as the Column for Y. Select OK.
f. On the displayed plot of d2 vs. volume, examine the graph to determine the volume when
the 2nd derivative equals approximately zero.
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13. Repeat the titration with a second HCl solution. Analyze the titration results in a manner
similar to your first trial and record the equivalence point in your data table.
14. Conduct two titration trials with the HC2H3O2 solution. Note that the equivalence point of
this titration will not be identical to the HCl titration. Analyze the titration results and record
the equivalence point in your data table.
15. Print a copy of the graph of each titration. Print the graph directly from LabQuest, if possible.
Alternately, transfer the data to a computer, using Logger Pro software.
METHOD 2: MEASURING VOLUME USING A DROP COUNTER
1. Obtain and wear goggles.
2. Add 40 mL of distilled water to a 100 mL beaker. Use a pipet bulb (or pipet pump) to pipet
5.00 mL of the HCl solution into the 100 mL beaker with distilled water. CAUTION:
Handle the hydrochloric acid with care. It can cause painful burns if it comes in contact with
the skin.
Figure 2
3. Connect the pH Sensor to LabQuest. Lower the Drop Counter onto a ring stand and connect
it to DIG 1. Choose New from the File menu. If you have older sensors that do not auto-ID,
manually set up your sensors.
4. Obtain the plastic 60 mL reagent reservoir. Close both valves by turning the handles to a
horizontal position. Follow the steps below to set up the reagent reservoir for the titration.
a. Rinse the reagent reservoir with a few mL of the 0.100 M NaOH solution and pour the
NaOH into an empty 250 mL beaker.
b. Use a utility clamp to attach the reservoir to the ring stand.
c. Fill the reagent reservoir with slightly more than 60 mL of the 0.100 M NaOH solution.
d. Place the 250 mL beaker, which contains the rinse NaOH, beneath the tip of the reservoir.
e. Drain a small amount of the NaOH solution into the 250 mL beaker so it fills the
reservoir’s tip. To do this, turn both valve handles to the vertical position for a moment,
then turn them both back to horizontal.
f. Discard the drained NaOH solution in the 250 mL beaker as directed.
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Acid-Base Titration
5. Calibrate the Drop Counter so a precise volume of titrant is recorded in units of milliliters.
a. Choose Calibrate from the Sensors menu and select Drop Counter.
 If you have previously calibrated the drop size of your reagent reservoir and want to
continue with the same drop size, select Equation. Enter the value for the Drops/mL
and select Apply. Select OK and proceed directly to Step 6.
 If you want to perform a new calibration, select Start and continue with this step.
b. Place a 10 mL graduated cylinder directly below the slot on the Drop Counter, lining it up
with the tip of the reagent reservoir.
c. Open the bottom valve on the reagent reservoir. Keep the top valve closed.
d. Slowly open the top valve of the reagent reservoir so that drops are released at a slow rate
(~1 drop every two seconds). You should see the drops being counted on the screen.
e. When the volume of NaOH solution in the graduated cylinder is between 9 and 10 mL,
close the bottom valve of the reagent reservoir.
f. Enter the volume of NaOH and select OK.
g. Discard the NaOH solution in the graduated cylinder as indicated by your instructor and
set the graduated cylinder aside.
6. Assemble the apparatus.
a. Place the magnetic stirrer on the base of the ring stand.
b. Insert the pH Sensor through the large hole in the Drop Counter.
c. Attach the Microstirrer to the bottom of the pH Sensor. Rotate the paddle wheel of the
Microstirrer, and make sure that it does not touch the bulb of the pH Sensor.
d. Adjust the positions of the Drop Counter and reagent reservoir so they are both lined up
near the center of the magnetic stirrer.
e. Lift up the pH Sensor, and slide the 100 mL beaker containing the HCl solution onto the
magnetic stirrer. Lower the pH Sensor into the beaker.
f. Adjust the position of the Drop Counter so that the Microstirrer on the pH Sensor is just
touching the bottom of the beaker.
g. Adjust the reagent reservoir so its tip is just above the Drop Counter slot.
h. Turn on the magnetic stirrer so that the Microstirrer is stirring at a fast rate.
7. You are now ready to conduct the titration.
a. Start data collection. No data will be collected until the first drop goes through the Drop
Counter slot.
b. Fully open the bottom valve. The top valve should still be adjusted so drops are released at
a rate of about 1 drop every 2 seconds. When the first drop passes through the Drop
Counter slot, check the graph to see that the first data pair was recorded.
c. Continue watching your graph to see when a large increase in pH takes place—this will be
the equivalence point of the reaction. When this jump in pH occurs, let the titration
proceed for several more milliliters of titrant.
d. Stop data collection, then turn the bottom valve of the reagent reservoir to a closed
(horizontal) position.
8. Dispose of the reaction mixture as directed. Rinse the pH Sensor with distilled water in
preparation for the second titration.
9. Examine your titration data to identify the region where the pH made the greatest increase.
The equivalence point is in this region.
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a. To examine the data pairs on the displayed graph, select any data point.
b. As you move the examine line, the pH and volume values of each data point are displayed
to the right of the graph.
c. Identify the equivalence point as precisely as possible and record this information.
d. Store the data from the first run by tapping the File Cabinet icon.
10. An alternate way of determining the equivalence point of the titration is to take the first and
second derivatives of the pH-volume data.
Determine the peak value on the first derivative vs. volume plot.
a. Tap the Table tab and choose New Calculated Column from the Table menu.
b. Enter d1 as the Calculated Column Name. Select the equation 1st Derivative (Y,X). Use
Volume as the Column for X and pH as the Column for Y. Select OK.
c. On the displayed plot of d1 vs. volume, examine the graph to determine the volume at the
peak value of the first derivative.
Determine the zero value on the second derivative vs. volume plot.
d. Tap Table and choose New Calculated Column from the Table menu.
e. Enter d2 as the Calculated Column Name. Select the equation 2nd Derivative (Y,X). Use
Volume as the Column for X and pH as the Column for Y. Select OK.
f. On the displayed plot of d2 vs. volume, examine the graph to determine the volume when
the 2nd derivative equals approximately zero.
11. Repeat the titration with a second HCl solution. Analyze the titration results in a manner
similar to your first trial and record the equivalence point in your data table.
12. Conduct two titration trials with the HC2H3O2 solution. Note that the equivalence point of
this titration will not be identical to the HCl titration. Analyze the titration results and record
the equivalence point in your data table.
13. Print a copy of the graph of each titration. Print the graph directly from LabQuest, if possible.
Alternately, transfer the data to a computer, using Logger Pro software.
DATA TABLE
HCl
Trial
7-6
Volume
HCl
(mL)
Equivalence
point
(mL)
HC2H3O2
Trial
1
1
2
2
Volume
HC2H3O2
(mL)
Equivalence
point
(mL)
Advanced Chemistry with Vernier
Acid-Base Titration
DATA ANALYSIS
1. Calculate the molar amounts of NaOH used in the reaction with the HCl solution and with
the HC2H3O2 solution.
2. Calculate the molar concentration (molarity) of the HCl solution and the HC2H3O2 solution.
3. Compare the actual molar concentrations of your two acid solutions with your calculated
molarities. Were the calculated molarities of your acid solutions within a reasonable range
(about 5%) of the actual values? If not, suggest reasons for the inaccuracy.
4. The equivalence points of the two titration curves were not in the same pH range. Explain.
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