Do this before you come to lab (have at the top of your notebook page to begin the lab):

1. Safety: Concentrated hydrochloric acid (HCl, 12.1 M) fumes continuously; quickly corroding metal and posing a serious hazard. Dilute HCl solutions are considered safer and easier to handle, use, and store. Answer the following questions about making dilute HCl solutions from concentrated HCl.
   1. Where in the lab should dilute HCl solutions be prepared? Why?
   2. When preforming the dilution, should concentrated HCl be added to water, or water be added to concentrated HCl? Why?
2. Purpose: Write a few sentences about the purpose of this lab: What is the “big picture” viewpoint of the lab experiment? What question am I trying to answer? What data will be collected to answer the questions? What technique will I use to obtain these data?
3. Preparation: If you are asked to prepare a buffer with a pH = 3.3 — a similar pH to apple sauce. Using the table of pKa values below, select an appropriate acid/base pair and clearly state the pair that you selected. Calculate the volume of the acid solution and the volume of the conjugate base solution that would be needed to prepare a buffer with a total volume of 50 mL. Assume all stock solutions are 0.10 M. Show all work

A. Your Laboratory Challenge:

Part 1: Preparing a Buffer

Your TA will demonstrate the calibration and proper use of a pH meter. Please read Topic 10 Appendix A: Use and Care of a pH Meter before using a pH meter. The electrodes are fragile (glass) so handle them with care. After calibration, you can proceed with measuring your prepared buffer solution.

Your TA will ask you to prepare a buffer of a specific pH within the range 2-12. Select appropriate acid/conjugate base pairs from the following table to form your buffer system:
Prepare 50.0 mL of the desired buffer by mixing appropriate volumes of the 0.100 M solutions provided. Carry out the following measurements:

Table 10.1  Reagents available in lab for buffer preparation

1. Measure the pH of your buffer using a pH meter. (Your TA may want to witness this measurement.)
2. Measure the change in pH when 5.0 mL of 0.100 M HCl is added via graduated cylinder to the buffer solution.

Part 2: Analyzing lake water samples

Lake water that resides over limestone bedrock has higher levels of CO₃^2- and HCO₃^–. This makes lake water from these sources have a natural buffer capacity, especially compared to lakes with lower levels of dissolved solids, such as ones found over granite bedrock. This leaves some lakes more sensitive to changes in pH from acid rain or human sources. 

C. Communicating Your Results

For a permanent record, both your data and report should be written in your notebook.

1. Safety
2. Purpose
3. Preparation
4. Data and Observations for Part 1:
   1. Record the pH of the buffer that was assigned to you either from the posted spreadsheet or from your TA.
   2. Record the chemical reagents used for the buffer preparation.
   3. In a summary table, record the pH values that you obtained for the following solutions: i) the buffer you made, ii) your buffer after HCl was added
5. Data and Observations for Part 2:
   1. Record the pH of each lake water sample and the volume of HCl added. Plot the pH vs mL of HCl added for both lake water samples.
6. Calculations and Results for Part 1:
   1. Show your calculations for the preparation of your buffer.
   2. Calculate what the pH of your buffer solution should be after the addition of 5.0 mL of the 0.100 M HCl. Be sure to show all calculations.
7. Calculations and Results for Part 2:
   1. Show your calculations for the buffer capacity of the buffered lake water sample starting at a pH of around 7.(look in the procedure for the details of calculation) (hint: you will have to scale your volume to make it approximate for 1L)
8. For part 1, address the following:
   1. How did you choose the chemical system with which you prepared your buffer?
   2. Discuss whether or not the prepared buffer achieved its objectives (e.g. correct pH, pH change when acid was added to each).
   3. Compare the measured pH change when acid was added to the predicted pH change you calculated.
   4. Write a balanced, net ionic chemical equation for the addition of acid to the buffer system you used.
9. Identify which lake water sample is buffered and which is not. Use data (plots) you collected, as well as your observations for the behavior of the BCG indictor, to justify your claim.
10. For the buffered lake water sample, for what pH range was the sample retaining some buffering capacity. Use data you collected to justify your claim.
11. Write a brief conclusion paragraph: comment on whether or not you achieved the goal of the experiment, respond to the questions posed in the introduction.  Support your conclusion using your data.

References

1. Chemistry 7 Laboratory Manual, Harvard, Spring, 1991.
2. Silva, C. R., Pereira, R. B., and Sabadini, E. Journal of Chemical Education, 2001, 78, pp. 939 – 940.
3. Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 3, Madison, WI, the University of Wisconsin Press, 1989.
4. Brown, T. L., LeMay, H. E., Bursten, B. E., and Burdge, J. R. Chemistry the Central Science, 2003, upper Saddle River, NJ: Prentice Hall, pp. 574 – 659.