Lab Report 3: Buffer Systems 1
Lab Report 3: Evaluating the Effectiveness of Buffer Systems
Manager ________________________
Recorder ______________________________
Lab Tech ________________________
Computer Tech _______________________
On campus students: Attach your LoggerPro graph to this Lab Report. Online students: Use graph and data attached to this assignment folder in D2L.
1. State your hypothesis. ____________________________________________________________________ __________________________________________________________________________________________
2a. In this experiment, what was the independent variable? ____________________________________
2b. What was the dependent variable? _______________________________________________________
2c. Circle the independent variable and underline the dependent variable in the following prediction: IF simple chemicals are the best buffer category, THEN their total buffer range will be the smallest.
3a. Enter your team data in the table. Does your data support your hypothesis? ________
What about class data? _________
3b. Which data set (team or class) is more useful when testing your hypothesis ? ___________
Why? _______________________ __________________________________________________________
4. How should the pH of the solution being tested in the Acid beaker compare to that in the Base beaker before any acid or base is added? Why? ___________________________________________ __________________________________________________________________________________________
5. Why was it of value to include the plot of water in acid and water in base with every experiment? _____________________________________________________________________________ __________________________________________________________________________________________
6a. What effect did adding HCl have on the pH of each solution? _______________________________
6b. What effect did adding NaOH have on the pH of each solution? ____________________________
7. Study the figure below in order to answer the following questions. a. What is the initial pH of the solution being tested? ___________
b. Use red to color the line indicating the addition of an acid.
c. Use blue to color the line indicating the addition of a base.
d. What is ΔpH for the base? _____________
e. What is ΔpH for the acid? ____________
f. What is the total buffer range? ________
g. Draw and label a bracket on the figure to indicate the total buffer range. Cat. Substance Buffer Range A B C 14 12 10 8 6 4 2 5 10 15 20 25 30 # drops added pH
2 Lab Report 3: Buffer Systems
8. In category C, the biological organisms, only the yeast was an entire living organism; the rest were just parts. Which had the smallest total buffer range? _________________________ Offer an explanation why. ________________________________________________________________________ 9. If a person is suffering both a headache and acid indigestion, which substance would you recommend? Justify your answer. _________________________________________________________ __________________________________________________________________________________________ 10. Indicate the range of initial pH values of the biological organisms by listing the lowest and highest values for those solutions. ____________________________ Offer an explanation as to why the initial pH range is not from 0 to 14. ______________________________________________________ __________________________________________________________________________________________
11. Why was the acid and base added in 5 drop increments? Why not in 15 drop increments? __________________________________________________________________________________________ __________________________________________________________________________________________ 12. Study the following two graphs. Explain the results of each in the context of this experiment.
Drops of what was added? _________
Initial pH: __________ Final pH: __________ ΔpH: ______________ Buffering ability? ___________________ Explanation of graph: ______________ ___________________________________ ___________________________________ ___________________________________
Initial pH: __________
TOP LINE IN GRAPH: Drops of what was added? ________ Final pH: __________
BOTTOM LINE IN GRAPH: Drops of what was added? ________ Final pH: __________ Total Buffer Range: _______________ Buffering ability? __________________
14 12
10
8
6
4
2
5 10 15 20 25 30 drops added
pH
14 12
10
8
6
4 2
5 10 15 20 25 30 drops added
pH
Lab 3: Buffer Systems 3
Begin analyzing your data by looking for “outliers”. Outliers are values which are far from the rest of the data.
When outliers exist, you must decide your course of action: retest or accept the discrepancy based on a valid scientific explanation.
Look at the buffer ranges by category for outliers. List any you find and decide on your course of action.
Table 2: Initial pH and Buffer Range Cat. Substance Initial pH Buffer Range
A
B
C
Table 3: Initial pH and Buffer Range Cat. Outlier Course of Action Justify your course of action. A
B
C
Table 4: Rankings based on pH Substance Initial pH Rank most acidic 2 3 4 5 6 7 8 9 10 11 least acidic
Table 5: Rankings based on total buffer range Substance Total Buffer Range Rank greatest change 2 3 4 5 6 7 8 9 10 11 least change
Lab 3: Buffer Systems 9
Lab 3: Evaluating the Effectiveness Of Buffer Systems
Organisms are often very sensitive to the effect of acids and bases in their environment. They must maintain a stable internal pH in order to survive, such as in the event of environmental changes. Many naturally occurring biological, geological, and man-made chemicals are capable of stabilizing the environment’s pH. Substances that resist changes in pH are buffers. In an aqueous solution, they form a conjugate of a weak acid and weak base that constitutes a buffer system. In this experiment, your team will be using two pH probes that interface with your computer. You will measure the effect of both an acid and a base on various solutions. Each team will test water as well as 3 other substances, one from each of three categories: formulated medicines, simple chemicals, and solutions from biological organisms. All teams will share their data. _____________________________________________________________________________________________ In this experiment, you will
• Add an acid to a solution and note the extent to which it resists changes in pH.
• Add a base to a solution and note the extent to which it resists changes in pH.
• Collaborate with other teams to compare the ability of different materials to resist pH changes. PROBLEM OF THE DAY
Which of the 3 categories listed below will most effectively minimize pH change?
Develop a hypothesis concerning which of the 3 categories of solutions has the best buffering ability. Hypothesis: ______________________________ _________________________________________ Prediction (If, then): ______________________ _________________________________________ Null Hypothesis: __________________________ _________________________________________ Prediction (If, then): ______________________ _________________________________________
Next, determine the independent and dependent variables in this experiment based on the following explanations. Record these on the Lab Report. Dependent Variable This is what you measure or record. This value depends upon your manipulation of the independent variable. Independent Variable This variable affects the value of the dependent variable. It serves as the “predictor” variable and is the one that is manipulated by the experimenter.
THREE CATEGORIES OF TEST SOLUTIONS Item # A = Formulated Medicines B = Simple Chemicals C = Biological Organisms
1 Alka Seltzer vitamin C (ascorbic acid) spinach leaves
2 aspirin gelatin (protein) potato
3 Bufferin (buffered aspirin) artificial salt water chicken egg yeast (unicellular fungus)
4 Tums carbonated drink
10 Lab 3: Buffer Systems
PROCEDURE Divide your team into two groups. One group will always use the A probe to test the effects of adding an acid to a liquid. The other group will use the B probe to test the effect of adding a base. One person will also have to enter data into the computer. NOTE for computer operator (ON CAMPUS) Your jobs are indicated with this icon . Setting Up the LoggerPro Program
1. Plug the 2 pH probes into the computer interface, if not already connected. Open LoggerPro.
2. Find the word “Latest” located at the top of the gray data table. Double click on it. When the window appears, replace “Latest” with “Control”. Click “OK.”
3. Change label of first column heading by double clicking on “pH 1.” When the window opens, change “pH 1” to “Acid”, and click “Done”. Repeat this step to change “pH 2” to “Base”.
4. Adjust the scale for the horizontal X-axis by double clicking anywhere on the graph. When the window appears, click on “Axes Options” tab. In the Y-axis label window, type in “pH”. Look for the X-axis information located at the bottom of the window. Change “Right” value to 30 but do NOT click “Done”.
5. Select the “Graph Options” tab in the same window. Under “Examine”, check “Legend.” Under “Appearance”, click in the open square next to “Connect Points”, if it is not already checked. Then click “Done.”
6. On the top toolbar, under “Experiment”, select “Data Collection”. On the “Collection” tab, select “Events with Entry”. When the window appears, type in “Drops” for the “Column Name” and “#” for “Units. Click “Done”.
7. Open a Word doc and type the proper heading: • first and last name of each team member, stacked in the upper left hand corner • title of this lab (centered 2 spaces below names) Establishing Controls You will test the effects of adding an acid or base to distilled water (dH2O).
8. Before and after each use of a pH probe, hold its tip over the large rinse jar. Thoroughly rinse with distilled water. Important: Do not let the pH electrode dry out. Keep it in a 500 mL beaker containing pH 7 buffer any time that it is not in use.
9. In your kit are two 50 mL beakers, one labeled acid and the other labeled base. Group A, add 20 mL of dH2O to the A beaker. Group B, add 20 mL of dH2O to the B beaker
10. Rinse the pH probe thoroughly with distilled water, then hold it in the beaker to be tested: • Group A: Hold your probe in the beaker labeled acid. • Group B: Hold your probe in the beaker labeled base. Since the probe is top heavy, the beaker will fall over if you don’t hold it.
11. Click to begin making pH measurements. Wait for the pH readings to stabilize (they may bounce up and down a small amount). Then click . DO NOT CLICK “STOP” until all the data has been collected. Should you accidentally click “Stop”, you must click again and “Append to Latest”.
Lab 3: Buffer Systems 11
12. A data window will appear. Type “0” in it. Press ENTER. The “0” means that you have not added any drops of acid or base. Record your data in Table 1. NOTE: Don’t worry if the probes don’t give exactly the same pH, but it should be pretty close. If the two readings are more than 1 pH value apart, try thoroughly rinsing out the beakers, drying them, and getting new water.
13. Group A, you will need the dropper bottle of the acid HCl (hydrochloric acid). Group B, you will need the base NaOH (sodium hydroxide). CAUTION: Handle the acid and base with caution. If you spill either solution on your skin or clothing, wash it immediately and thoroughly. Notify the instructor.
14. Gently swirl your beaker as you add acid or base. Do not add acid or base while a probe is in one of the beakers. Always remove the probe first. If you don’t, the acid or base will damage the probe tip. • Group A: Add 5 drops of acid to the beaker labeled acid and swirl. • Group B: Add 5 drops of base to the beaker labeled base and swirl.
15. When the pH readings at the lower left of the display are fairly stable (usually 5-10 seconds), click . Enter the total number of drops of acid or base you added to the beaker. Type “5” in the text box and press ENTER. Record your data in Table 1.
16. Repeat steps 14 and 15, adding 5 drops at a time until each team has added a total of 30 drops. When you “keep” each set of readings, type in the total number of drops you’ve added (0, 5, 10, 15,…) Record all data in Table 1.
17. Click when you have recorded data for a total of 30 drops.
18. Rinse the pH probes thoroughly and place them in the beaker of water. Swirl the probe in the water to wash off any residue of acid or base.
19. Dump the liquids in beakers A and B into the labeled waste disposal containers. Take both beakers to the sink and rinse thoroughly. Return to your work area and rinse with dH2O, then dry the inside of each with a paper towel. If you are not thorough at this step, residual acid or base will affect your next data set.
20. The data you collected for water will be used as a control for comparing experimental data. Make sure the data for water are displayed on the next 3 graphs that you will generate.
21. To keep the water data, click on the Experiment menu. Choose “Store Latest Run”. NOTE: You will still see the lines on the graph, but the data in the data table seems to disappear. It is still there but has been moved to the right to make room for the next data set. At the bottom of the data table is a horizontal scroll bar. If you scroll to the right, you will see your data for water.
Collecting Experimental Data You will test the effect of an acid and a base on one substance from each of the three categories of materials listed at the beginning of the lab: formulated medicines, simple chemicals, and biological organisms. Your instructor will assign your team an “Item #” so that you will know which test solution to use from each category. Labeled stock solutions are on the side counter. Use only the matching graduated cylinder to measure each solution. Be sure you swirl the stock solution before obtaining your sample.
1. Start with your Category A test solution located on the side counter.
2. Team A, measure 20 mL of the solution and pour it into Beaker A. Team B, do the same for Beaker B. 3. Give the graph a title to indicate the substance you are testing. Simply double click on the graph. A window will appear with the cursor blinking in the graph title
12 Lab 3: Buffer Systems
window. Type in the name of the solution being tested and click “Done”.
4. On the gray data table, click on the heading “Latest” and rename according to the test solution. Click “Done.”
5. Perform steps 10 – 19 of the previous section for the category A solution.
6. As you collect data, your graph will have 4 lines (water with acid, water with base, test solution with acid, and test solution with base).
7. After adding all 30 drops of acid or base to the solution, click on . Record all data in Table 1.
8. Right mouse click on the graph and choose “Copy”. Paste the graph into your Word doc
. 9. Go to “Experiment” on the upper toolbar and select “Store Latest Run”. This will clear the data table for the next solution.
10. Before testing the next solution, clear the graph for the first solution by choosing “Hide Data Set” from the Data menu located on the upper toolbar. Click on the name of the solution you tested.
11. Repeat steps 2 – 10 of “Collecting Experimental Data” for your category B and C solutions. If the line colors in your graph are not distinct, you can select alternate colors by double-clicking on the column heading in the data table. Click on the “Options” tab to choose a color on the drop-down menu to the right. Click “Done”. IMPORTANT: Upon completing this experiment, you will have 3 different graphs, one for each category of test solution. Each graph will have 4 lines: 2 for the control (water) and 2 for the test solution. 12. Print your Word doc.
13. Close both LoggerPro and Microscoft Word.
CLEAN UP
1. Empty rinse container in sink. 2. Organize white bin: Both 50 mL beakers HCl dropper bottle NaOH dropper bottle 3. Refill squirt bottle with dH2O.
∆pH when acid is added negative value Table 1: Data Table
Solution Tested Add
pH, after adding this many drops 0 5 10 15 20 25 30 ΔpH Total Buffer Range
1. water (control)
base acid
2.
base acid
3.
base acid
4.
base acid ∆pH = final pH – initial pH Total Buffer Range = ∆pH (base) – ∆pH (acid) ∆pH when base is added positive value
Lab 3: Buffer Systems 13
total buffer range
Analyzing Data
1. Determine the ∆pH (change in pH) for the first solution (water) when the base was added by subtracting the 0-drop value from the 30-drop value. Record the difference in Table 1, in the ∆pH column for the base. ∆pH = final pH – initial pH
2. Repeat step 1 for the addition of acid to the water.
3.To determine the “Total Buffer Range” for water, subtract the ∆pH of the acid from the ∆pH of the base. Record the range as an absolute value. = ∆pH (base) – ∆pH (acid)
4. Do the same calculations for the 3 solutions you tested.
5. Recorder, enter the initial pH for each of your 3 solutions as well as the “Total Buffer
Range” on the class data chart on the front computer. (NOTE: If you have two different values for the initial pH, take the average.)
6. Once all the teams have reported their data, record the data for all the solutions tested in Table 2 (located on the last page of the Lab Report).
7. Complete Table 4 (located on last page of lab report) by listing each solution that was tested in your class. Place the most acidic solution at the top of the list and the most basic solution at the bottom. Use the pH value prior to adding any drops of acid or base, as this value represents the initial pH of the solution.
8. Complete Table 5 (located on last page of lab report) by listing the solutions with the largest “Total Buffer Range” at the top of the list and the smallest range at the bottom of the list.
14 Lab 3: Buffer Systems
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