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Investigating Titration Curves

Description of Activity

Title: Investigating Titration Curves
Overview: In this laboratory exercise students will investigate the process of titration. They will be asked to follow the procedure carefully, and then to draw some conclusions about what they did, what they learned, the information they collected, etc. This lab activity is intended to allow students to draw conclusions, and learn by experimentation instead of teacher lecture.
Duration: This activity requires about 50 minutes to complete.
Materials: TI-Calculator (T1-82 or T1-85) and CBL with pH probe. Program pH (found in the CBL System Experiment Workbook).

50-mL Buret
25-mL graduated cylinder
Ring Stand
Buret Clamp
Utility Clamp
Several 250-mL beakers
Magnetic stirrer and stir bar (optional but highly recommended)
0.1 M HCl
0.1 M NaOH

Background

Titration is a process which consists of the gradual addition of a standard solution to a measured quantity of a solution of unknown concentration. In an acid-base titration, the point at which the moles of hydroxide ion and the moles of hydronium ions are equal is the equivalence point.

Teaching and Learning Strategies

This activity is intended to teach the process of titration through very guided inquiry. If students already have ANY understanding of the titration process, the inquiry value of this activity is lost.

Prerequisite Instruction :

Students should have had little (if any) experience with the use of burets. Students should be shown how a buret is read, and how to properly rinse and fill the buret.

Invitation to Learn

In this laboratory exercise you will investigate the process known as titration. Many of the ideas presented here will be new to you. You will be asked to follow the procedure carefully, and then to draw some conclusions about what you did, what you learned, the information you collected, etc. There are no ÒincorrectÓ answers if you follow the procedure carefully. Explore titration (what ever that word means) to the best of your ability. This lab activity is intended to allow you to draw conclusions, and learn by experimentation instead of teacher lecture.

Procedure
Equipment Setup

Follow the instructions provided in the CBL workbook for proper equipment setup. To develop a titration curve it is probably not necessary to calibrate the pH probe.

Procedure
1. Obtain about 100 mL of both the acid and the base solutions.
2. Rinse and properly fill the buret with NaOH. Adjust the volume of NaOH until the meniscus is at 0.0-mL.
3. Using a graduated cylinder pour 25.0-mL of the acid solution into a 250-mL beaker. Add 20.0 mL of distilled water. A few drops of an indicator may be added to the beaker. Place a stir bar in the beaker to mix all the ingredients (if a stirrer is available).
4. Place the pH probe into the acid solution. Place the beaker on the magnetic stir plate under the buret containing the base solution and begin stirring. Position the pH probe so that it does not interfere with the stirring bar. To avoid splashing, be sure the tip of the buret is in the beaker.
5. Make sure the CBL is turned on. Start the program PH on the calculator. When prompted, enter the number of the channel to which the probe is connected. Enter 40 for the number of samples to collect.
6. Start the titration and take an initial pH reading of the solution. At the ML? prompt, enter zero (0 mL volume).

Note: To take pH readings, press TRIGGER on the CBL. At the ML? prompt, enter the volume of NaOH that you have added to the solution. The data point is plotted in the calculator display, and the program pauses until you press TRIGGER again. Wait a few seconds between each TRIGGER key-press to allow the reading to stabilize.

7. Stirring gently begin the titration by adding NaOH to the flask drop by drop. Test for pH every 1.0 mL by pressing TRIGGER on the CBL unit, reading the buret, and entering the volume (read from the buret) when prompted. Continue until a total of 40 readings have been taken. (39 mL of NaOH have been added.)
8. Transfer the data and graph to a computer via graph link. The pH is in L5 and the mL of NaOH is in L4. Print the results for reference as you complete the lab.
9. Discard the solution in the flask as instructed. Wash and rinse the flask.
10. Repeat the titration. You may wish to adjust the amounts of the NaOH solution in regions of your titration where you would like to investigate the pH more closely.
11. Transfer the data and graph to a computer and print your results.

Safe Operating Procedures:

Wear your safety glasses and an apron. Acids and bases can be harmful to the skin. Care is required at all times. If any acid or base is accidentally spilled, inform your instructor immediately and wash with copious amounts of water. Dispose of all solutions properly. See Flinn Chemical Catalog.

Summary of Learning

1. Based on the procedure that you followed (which was a titration), what is meant by the term titration?
2. On the graph, label the areas and/or points which you believe to be of interest. Label at least three such points or areas.
3. Write a question that you would like to have answered about each of the areas you identified on your graph. Avoid the use of the work ÒwhyÓ. Focus on statements like Òwhat would happen ifÓ.
4. To each of the questions that you wrote above, hypothesize a reasonable answer.
5. Keeping in mind that you started with an acidic solution and added a basic solution, what conclusions can you draw about pH?
6. What conclusions can you draw about the concentration of the two solutions that you were given?


1. A 100 mL sample of 1.0 M HCl is being neutralized by the addition of measure amounts of NaOH. The most abrupt change in the titration curve occurs when how many moles of NaOH have been added.

A. 0.05
B. 0.1
C. 0.15
D. 0.2

2. The pH of the equivalence point in the titration curve for the titration of a strong acid with a strong base is nearest to:

A. 6
B. 7
C. 2
D. 9

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Updated September 26 1997 by Michelle Dumas