Lab: Selecting Indicators for Acid-Base Titrations
You have a separate lab handout from the book Laboratory Experiments for AP Chemistry from the Flinn company. This sheet gives additional instructions or clarifications in order to make the lab work go more smoothly and to modify the original lab so that each group performs just two titrations: one titration of a weak acid with a strong base and one titration of a weak base with a strong acid.
Before you begin you must complete the calculations in the pre-lab on the lab handout. They will prepare you by letting you know what to expect from the data you collect and what indicator to use in each titration.
Calibrate your pH meter using the Vernier software, Logger Lite. Solutions buffered at a pH of 7.00 and 4.00 (or 10.00) have been provided to you by your instructor. Carefully rinse the pH meter with deionized water and allow excess liquid to drip off before switching from one solution to another.
Acetic Acid Titrated with Sodium Hydroxide: At your lab table there should be two lab groups. One group will set itself up to perform the titration of acetic acid, the other to perform the titration of ammonia. If you are in the latter group, read the instructions below marked with bold text. After you have performed your first titration you will switch burets with the other group at your lab table in order to save the time that would otherwise be used washing the buret in order to switch from NaOH to HCl. Be sure to refill the buret you obtain from the other group at your lab table.
Get about 30 mL of acetic acid in a small beaker; use a volumetric pipet to measure exactly 25.00 mL of the acetic acid into a 250-mL beaker.
Fill your buret with the sodium hydroxide solution your teacher has provided. Only fetch enough to fill the buret, about 50 mL. Fill to above the 0.00 mark then drain some out to fill the tip. Mark your initial volume on your lab handout. (Most data will be collected directly in the Logger Lite software but this starting value is important).
Set your beaker on a magnetic stirrer and place a stir bar in the beaker.
Clamp your pH meter so that the probe is just above the bottom of the beaker containing the acetic acid. Be sure it will not be struck by the spinning stir bar. Add two drops of the indicator your calculations have shown you should use for this titration. Set the whole thing under the tip of the buret so you can easily add liquid from the buret without splashing. Start the stirrer to gently mix the solution.
Use the Logger Lite software to record your data using the Events with Entry mode, starting with the pH at zero volume added. You will need to enter the total volume delivered up to and including each point at which you record the pH. This will require some calculations on your part. You may use your lab handout to help you but remember to subtract correctly to be able to enter the data correctly. For example, your first few data points might be: 0 mL, 1.00 mL, 2.04 mL, 2.92 mL, … None of these numbers are read directly from the buret: all volumes are calculated by difference.
You will have calculated the volume needed to reach equivalence and you will know the pH to expect at equivalence from your calculations. When you get close, measure pH with smaller volume increments between successive measurements. Roughly speaking, switch from approx. 1 mL increments to approx. 0.20 mL increments when the change in pH for 1 mL added is 0.3 or more.
Increase the increments of the volume added once you are two milliliters past equivalence. Keep adding liquid from the buret in 1 mL increments. The total volume you add should end up being about twice the volume needed to reach equivalence.
Finish the titration, store the data in Logger Lite. Save the Logger Lite file to your personal drive in case the data you export get lost.
Copy and paste your data into Excel and then follow the instructions in the post-lab to format the X-Y scatterplot you will make using the data.
Ammonia Titrated with Hydrochloric Acid: for this titration you will be following the same instructions as above but instead of filling the buret with sodium hydroxide solution, you fill it with hydrochloric acid solution. You will measure out 25.00 mL of ammonia solution, rather than acetic acid solution. You will use a different indicator than with the acetic acid titration. If you are doing this titration after completing the one with acetic acid remember to refill the buret you have obtained from the other group at your lab table with hydrochloric acid solution.
Disposal and clean-up: all solutions used in this lab may be washed down the drain with plenty of running water. Please clean all of your glassware, being especially careful to rinse many times.
The post-lab information on your handout is not very extensive and leaves a lot to the imagination. Follow the instructions below.
Type up your pre-lab calculations for the pH at the equivalence point for the titration of acetic acid with sodium hydroxide and the titration of ammonia with hydrochloric acid. Keep your calculations brief: only show the initial set-up and result for each calculation.
Explain how to use the calculations in the first question to select the correct indicator for each titration.
Provide a graph of the titration of acetic acid with sodium hydroxide with volume of NaOH added on the x-axis and pH on the y-axis. The graph should be an X-Y scatterplot created with a smooth curve, rather than individual data point. Add labels to the graph showing:
Range of pH Corresponding to Indicator Color Change
Create and provide a similar graph with all of the same features labeled for the titration of ammonia with hydrochloric acid.
Consider the possibility that the pH meter was inaccurate. How would the indicator have helped you to discover this fact?