Name ___________________________________________________ Date __________ Period _____
Investigating Indicators and Polyprotic
Acids1,2
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
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
1
2
magnetic stirrer
stirring bar or Microstirrer
wash bottle
ring stand
Jack Randall, et al. Advanced Chemistry with Vernier. Vernier Software and Technology, Beaverton, OR, 2007
Modified by David Scott
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David Scott, Rev. 2011
Name ___________________________________________________ Date __________ Period _____
0.10 M hydrochloric acid, HCl, solution
0.10 M phosphoric acid, H3PO4, solution
0.10 M acetic acid, CH3COOH, solution
0.10 M ammonia, NH3, solution
250 mL beaker
50 mL graduated cylinder
50 mL buret
10 mL pipet
utility clamp
phenolphthalein indicator solution
bromthymol blue indicator solution
methyl orange indicator solutions
distilled water
buret clamp
Procedure
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.
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.
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 (cumulative volume)
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.
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David Scott, Rev. 2011
Name ___________________________________________________ Date __________ Period _____
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. 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. If you are titrating phosphoric
acid, continue your titration to pH 13. 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.
Part III Titration of Phosphoric Acid
14. Indicators are not necessary for this titration. The objective here is to observe the first and
second hydrogen ionizations of the acid. When adding NaOH, you will approach the first
equivalence point when you have added 16-18 mL. Add smaller amounts at this point untile
the first equivalence point is noted. Again, add smaller amounts after 36-38 mL have been
added in order to observe the second equivalence point.
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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David Scott, Rev. 2011
Name ___________________________________________________ Date __________ Period _____
3. Find the equivalence point for each titration using the first derivative method learned in
earlier titrations. Write them below.
4. Use the 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.
5. Comment on the titration curve for phosphoric acid. How is it different from the titration
of HCl? How can you explain this difference?
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David Scott, Rev. 2011