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Investigating Indicators
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)
LabPro or CBL 2 interface
TI graphing calculator
Vernier pH Sensor
0.10 M NaOH solution
0.10 M HCl solution
0.10 M HC2H3O2 solution
0.10 M NH3 solution
250 mL beaker
50 mL graduated cylinder
magnetic stirrer (if available)
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.
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.
Part I Strong Acid – Strong Base Titrations
4. Connect a pH Sensor to Channel 1 of the LabPro or CBL 2 interface. Use the link cable to
connect the interface to the TI graphing calculator.
5. Use a utility clamp to suspend the pH Sensor on a ring stand as shown in Figure 1. Position
the pH Sensor in the HCl solution and adjust its position so that it is not struck by the stirring
bar.
6. Rinse and fill the buret with 0.10 M NaOH solution. CAUTION: Sodium hydroxide solution
is caustic. Avoid spilling it on your skin or clothing.
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7. Set up the calculator and interface for the pH Sensor.
a. Turn on the calculator and start the DATAMATE program. Press CLEAR to reset the program.
b. Select SETUP from the Main screen. If CH 1 displays PH, proceed directly to Step 8. If it
does not, continue with this step to set up your sensor manually.
c. Press ENTER to select CH 1.
d. Select PH from the SELECT SENSOR menu.
8. Set up the data-collection mode.
a. To select MODE, press
until the cursor is to the left of MODE and press
b. Select EVENTS WITH ENTRY from the SELECT MODE menu.
c. Select OK to return to the Main screen.
ENTER
.
9. Select START to begin data collection. Conduct the titration carefully, as described below.
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. Before you have added any NaOH solution, press ENTER and type in “0” as the buret
volume in mL. Press ENTER to save the first data pair for this experiment.
b. Add 1 mL of the NaOH titrant. When the pH stabilizes, press ENTER and type in the
current buret reading. You have now saved the second data pair for the experiment.
c. 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.
d. Add NaOH in 0.5 mL increments until the pH is about 10.
e. Continue adding NaOH solution until the pH value remains constant.
10. Press
STO
when you have finished collecting data.
11. When you have completed the titration, dispose of the reaction mixture as directed. Rinse the
pH Sensor with distilled water in preparation for the second titration.
12. Print a copy of the graph of pH vs. volume for the first trial. Use the printed 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.
13. Repeat the necessary steps to conduct a second and third titration, using the bromthymol blue
and phenolphthalein indicator solutions. Repeat Step 12 to analyze the titration data and
indicators for the second and third trials.
Part II Selecting an Appropriate Indicator for a Titration
14. 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 printed 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. (You can add less, about 20 mL, if you
will be using a stirring bar instead of the Microstirrer.) Use a pipet bulb (or pipet pump) to
pipet 5.00 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. Connect a pH Sensor to Channel 1 of the LabPro or CBL 2 interface. Lower the Drop
Counter onto a ring stand and plug it into the DIG/SONIC 1 port (see Figure 2). Use the link
cable to connect the interface to the TI graphing calculator.
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 0.100 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 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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5. Set up the calculator and interface for the pH Sensor.
a. Turn on the calculator and start the DATADROP program. Press CLEAR to reset the program.
b. Select SETUP from the Main screen. If CH 1 displays PH, proceed directly to Step 6. If it
does not, continue with this step to set up the sensor manually.
c. Press ENTER to select CH 1.
d. Select PH from the SELECT SENSOR menu.
6. Calibrate the drops that will be delivered from the reagent reservoir. Note: If you are using
the stored calibration (20 drops per mL), then skip this step.
a. Select CALIBRATE DROPS from the Main screen.
 If you have done a previous calibration with this particular apparatus, you can choose
CALIBRATE DROPS and manually enter the drops/mL ratio. Select OK and proceed to
Step 7.
 If you want to perform a new calibration, continue with this step.
b. Fill the reagent reservoir with titrant.
c. Select CALIBRATE DROPS from the Main screen.
d. Select CALIBRATE NOW.
e. Open the bottom 2-way (on-off) valve to begin releasing drops from the vessel through
the Drop Counter (the top valve finely adjusts the drop rate).
f. Continue to release drops until there are between 9 and 10 mL of liquid in the graduated
cylinder.
g. Close the bottom valve on the vessel to stop the drops.
h. Press ENTER .
i. Type the precise volume of liquid in the graduated cylinder in milliliters and press ENTER .
j. The number of drops/mL will be displayed. It may be helpful to record the value for that
particular reagent reservoir for future titrations.
k. Select OK to return to the Main screen.
l. Discard the NaOH solution in the graduated cylinder as indicated by your instructor and
set the graduated cylinder aside.
7. 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 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.
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8. You are now ready to perform the titration.
a. Select START. 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 data table to see that the first data pair was recorded.
9. 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, then press STO . Turn the bottom valve of the reagent
reservoir to a closed (horizontal) position.
10. When you have completed the titration, dispose of the reaction mixture as directed. Rinse the
pH Sensor with distilled water in preparation for the second titration.
11. Print a copy of the graph of pH vs. volume for the first trial. Use the printed 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 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 printed 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.
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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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