Computer
Investigating Indicators
11
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 acid-base titrations using 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.
Prelab
Write a purpose for this lab
Create a table of reagents
MATERIALS
Vernier Drop Counter
Vernier pH Sensor
0.10 M sodium hydroxide, NaOH, solution
0.10 M hydrochloric acid, HCl, solution
0.10 M acetic acid, HC2H3O2, solution
magnetic stirrer
stirring bar or Microstirrer
wash bottle
Universal indicator solution
Thymol blue indicator solution
phenolphthalein indicator solution
bromthymol blue indicator solution
methyl orange indicator solutions
Procedure
1. Obtain and wear goggles.
Part I Strong Acid – Strong Base Titrations
2. Add 50 mL of distilled water to a 100 mL beaker. Make sure there is enough to cover 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 a few drops of thymol blue indicator solution to the
beaker of HCl solution.
Advanced Chemistry with Vernier
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Computer 11
Figure 2
3. Lower the Drop Counter plate onto a ring stand and clamp it into position (see Figure 2).
4. Connect the Drop Counter to DIG/SONIC 1 of the Vernier computer interface. Connect the
pH Sensor to CH 1 of the interface.
5. Start the Logger Pro program on your computer. Open the file “11b Indicators (Drop)” from
the Advanced Chemistry with Vernier folder.
6. 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. 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.
b. Discard the drained NaOH solution in the 250 mL beaker as directed.
7. Calibrate the drops that will be delivered from the reagent reservoir.
8. Assemble the apparatus as shown in Figure 2.
9. Turn on the magnetic stirrer so that the Microstirrer is stirring at a fast rate.
10. You are now ready to begin collecting data. Click
. No data will be collected until the
first drop goes through the Drop Counter slot. 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.
11. Watch carefully to see if the indicator changes color before, at the same time, or after the
rapid change in pH at the equivalence point. Note: Volume is displayed in the live table. If
this is a suitable indicator for the reaction, it should change color at about the same time as
the jump in pH occurs. Note the volume when the indicator changes color.
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Vernier
Advanced Chemistry with
Investigating Indicators
12. After the jump in pH occurs, continue to deliver about 3 more milliliters of NaOH solution,
then click
. Turn the bottom valve of the reagent reservoir to a closed (horizontal)
position. Dispose of the beaker contents as directed.
13. Label the point on your graph where the indicator solution begins to change color and where
the color point is complete to describe the color change. To do this, choose Text Annotation
from the Insert menu.
Part II Weak Acid strong base
14. Repeat the necessary steps to conduct a second and third trial, using the acetic acid instead of
hydrochloric acid. Test the bromthymol blue, phenolphthalein indicator, universal indicator
solutions. Repeat Step 13 to analyze the titration curve for your second and third trials.
15. Print a copy of the titration curves.
DATA ANALYSIS
1. Describe how the 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. During this lab, it was best if the color change did not happen during the equivalence
point. If you wanted to use the indicator for a titration, you would want the color change
on the equivalence point. Explain why.
3. Why is it important to rinse the end of the burette with NaOH before filling. What type
of errors would you expect if you did not?
Advanced Chemistry with Vernier
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