LabQuest
Investigating Indicators
11
An indicator solution does what its name suggests – it indicates. In chemistry, a common use of
an indicator is to signal the equivalence point of a reaction between an acid and a base.
Generally, an indicator is itself a weak organic acid or a blend of weak organic acids. The
indicator establishes equilibrium in aqueous solution, which is shifted according to LeChatelier’s
principle as the solution changes in pH. The indicator is one color in the presence of a range of
concentrations of H+ or OH– ions, and another color when the acidity changes. Knowing when an
indicator will change color helps you determine the precise equivalence point of an acid-base
reaction. This knowledge is also important as you select the proper indicator for a given reaction.
OBJECTIVES
In this experiment, you will
 Conduct strong acid-strong base titrations using solutions of hydrochloric acid and sodium
hydroxide, and three different indicator solutions.
 Select the proper indicator to use with a titration involving a weak acid or a weak base,
based on your observations and measurements.
Figure 1
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.
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MATERIALS
Materials for both Method 1 (buret) and Method 2 (Drop Counter)
LabQuest
LabQuest App
Vernier pH Sensor
0.10 M sodium hydroxide, NaOH, solution
0.10 M hydrochloric acid, HCl, solution
0.10 M acetic acid, HC2H3O2, solution
0.10 M ammonia, NH3, solution
250 mL beaker
50 mL graduated cylinder
magnetic stirrer
stirring bar or Microstirrer
wash bottle
ring stand
utility clamp
phenolphthalein indicator solution
bromthymol blue indicator solution
methyl orange indicator solutions
distilled water
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
reagent reservoir
5 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.
Part I Strong Acid – Strong Base Titrations
2. Use a graduated cylinder to measure out 20 mL of 0.10 M HCl solution into a 250 mL
beaker. Add exactly two drops of methyl orange indicator solution to the beaker of HCl
solution. 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).
CAUTION: Sodium hydroxide solution is caustic. Avoid spilling it on your skin or clothing.
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 Units (mL).Select OK.
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Investigating Indicators
8. You are now ready to conduct the titration. During the titration, make careful note of the
color changes in the reaction mixture. Record the volume of NaOH solution added and the
pH at which the indicator changed color.
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 1 mL of the NaOH titrant. When the pH stabilizes, tap Keep and enter the current
buret reading. Select OK to save the second data pair.
d. Continue adding NaOH solution 1 mL at a time, and enter the buret reading after each
increment, until the pH of the mixture is about 5.
e. Add NaOH in 0.5 mL increments until the pH is about 10.
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. Print the graph directly from LabQuest, if possible. Alternately, transfer the data to a
computer, using Logger Pro software. Use the graph to show the point or points at which the
indicator solution changed color. Describe the color, or colors, as precisely as possible. Make
special note of the area around the equivalence point, the region of the graph where the pH
increases drastically. It is crucial for the indicator solution to change color at the equivalence
point of the titration.
12. Repeat the necessary steps to conduct a second and third titration, using the bromthymol blue
and phenolphthalein indicator solutions. Repeat Step 11 to analyze the titration data and
indicators for the second and third trials.
Part II Selecting an Appropriate Indicator for a Titration
13. Obtain a weak acid or weak base solution from your instructor. If you are testing a weak
acid, use the 0.10 M NaOH solution as the titrant. If you are testing a weak base, rinse and
fill your buret with the weak base solution and titrate 20 mL of 0.10 M HCl solution. Use the
results of your previous tests to select the appropriate indicator solution for this titration.
Conduct the titration as you have in the previous three tests. Print a copy of the graph of pH
vs. volume for this final titration. Use the graph to show the point or points at which the
indicator solution changed color. Describe the color as precisely as possible.
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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
5.0 mL of the HCl solution into the 100 mL beaker with distilled water. Add exactly one
drop of methyl orange indicator solution to the beaker of HCl solution. CAUTION: Handle
the hydrochloric acid with care. It can cause painful burns if it comes in contact with the
skin.
Figure 2
Part I Strong Acid – Strong Base Titrations
3. Set up the pH Sensor and Drop Counter.
a. Connect the pH Sensor to LabQuest. Lower the Drop Counter onto a ring stand and
connect it to DIG 1.
b. 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 0.10 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 0.10 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 that 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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Investigating Indicators
5. Calibrate the drops that will be delivered from the reagent reservoir.
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 (vertical). Keep the top valve closed
(horizontal).
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 precise volume of NaOH and select Stop. Record the number of drops/mL for
possible future use 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
with 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 perform the titration. During the titration, make careful note of the
color changes in the reaction mixture. Record the volume of NaOH solution added and the
pH at which the indicator changed color.
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 to view a graph of pH vs. volume.
e. Turn the bottom valve of the reagent reservoir to a closed (horizontal) position.
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8. Dispose of the reaction mixture as directed. Rinse the pH Sensor with distilled water in
preparation for the second titration.
9. Print the graph directly from LabQuest, if possible. Alternately, transfer the data to a
computer, using Logger Pro software. Use the graph to show the point or points at which the
indicator solution changed color. Describe the color, or colors, as precisely as possible. Make
special note of the area around the equivalence point, the region of the graph where the pH
increases drastically. It is crucial for the indicator solution to change color at the equivalence
point of the titration.
10. Repeat the necessary steps to conduct a second and third titration, using the bromthymol blue
and phenolphthalein indicator solutions.
Part II Selecting an Appropriate Indicator for a Titration
11. Obtain a weak acid or weak base solution from your instructor. If you are testing a weak
acid, use the 0.10 M NaOH solution as the titrant. If you are testing a weak base, rinse and
fill your buret with the weak base solution, and titrate 5 mL of 0.10 M HCl solution. Use the
results of your previous tests to select the appropriate indicator solution for this titration.
Conduct the titration as you have in the previous three tests. Print a copy of the graph of pH
vs. volume for this final titration. Use the graph to show the point or points at which the
indicator solution changed color. Describe the color as precisely as possible.
DATA ANALYSIS
1. Describe how the three indicator solutions worked in the titrations. Which indicator solution
is the best for a strong acid-strong base titration? Specify the color changes and the pH
readings where the color changes occurred.
2. For which type of acid-base titrations would the other two indicator solutions be best suited?
Explain.
3. Use the graphs to compare how accurately the indicator solutions “indicated” the equivalence
point of the titrations that you conducted. In your judgment, can an indicator alone accurately
determine the equivalence point of an acid-base reaction? Explain.
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