4.2: Exercise
Exercise A: Using pH Indicators to Measure pH
Part 1: Making a pH Indicator
Solutions of anthocyanins (the plant pigments responsible for red, blue, and purple colors in flowers, fruits, autumn leaves, etc.) can be used as pH indicators. In low pH, these solutions turn red, whereas in high pH, they turn blue. In some flowers, however, soil pH impacts the update of certain metals that can complex with anthocyanin and alter its color expression. For example, in hydrangeas, low soil pH stimulates the uptake of aluminum, which then complexes with the anthocyanins and causes the flowers to now appear blue. Raising the pH of soil prevents aluminum uptake and a pink color will now be expressed. When the soil pH values are neutral, the flowers are intermediate in color.
A prepared solution of anthocyanins extracted from red cabbage will be used as a pH indicator in this lab. To use an indicator to determine the pH of unknown solutions, you must first determine the color changes that occur when the cabbage indicator is used with substances whose pH is known (your standards). You can then determine the pH of unknown substances by comparing color changes with your standards.
- As a group, label eight clean test tubes with the following pH values: 2, 4, 6, 7, 8, 10, 12, and 14. Use the labeled pipettes to measure 5 mL of the appropriate buffer into each tube.
- Use a pipette to measure 3 mL of cabbage extract and add it to each test tube. Cover each tube with Parafilm and mix well.
- Record the color of each tube below. Note: record both initial and final colors at pH 12 and 14, since the pigments are not stable at these pH values. Save these tubes for the entire lab period .
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pH: |
2 |
4 |
6 |
7 |
8 |
10 |
12 |
14 |
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Color: |
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Part 2: Measuring pH - pH (alkacid) Test Paper versus Cabbage Extract
- Label 2 clean test tubes: “A” and “B”
- Transfer 2 mL of unknown solution A into tube A. Hold a test paper with forceps. Use a clean stirring rod to apply a drop of solution A to the test paper. While the paper is still wet, compare its color with the standard pH color scale on the label of the pH paper’s container. Record your pH in the table below.
- Add 1 mL of cabbage extract into tube A and mix well.
- Compare the color in tube A with the colors of the standards from part 1. The approximate pH of the unknown solution is the pH of the standard whose color most closely matches the color in tube A. Record your pH below.
- Repeat the steps above for solution B.
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Solution |
pH (pH paper) |
pH (cabbage extract) |
[H + ] (use pH from pH paper to calculate) |
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Unknown solution A |
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Unknown solution B |
- Do the results obtained for solutions A and B using the pH paper match those of the cabbage extract method? Yes / No Explain: __________________________________________________________________________________________________________________________
- Use the pH from the pH paper to calculate [H + ]. Record your value in the table.
- Which solution is more acidic? Unknown solution A / Unknown solution B
Exercise B: Determining the pH of Some Common Solutions
Part 1: pH of Beverages – test one per group (assigned beverage: _____________________)
Many beverages differ in their pH levels due to CO 2 content (if carbonated) or differences in their levels of organic acids.
- Use the cabbage indicator method (2 mL of assigned beverage + 1 mL cabbage extract) to determine pH levels of Sprite and wine. Use pH paper to determine the pH of apple juice and coffee. Record your values below:
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Beverage |
pH |
List beverages in order of increasing [H + ] |
|---|---|---|
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Apple juice |
Lowest: |
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Coffee (black) |
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Sprite |
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White wine |
Highest: |
Part 2: pH and Activity of Common Medicines – (assigned medication: _____________________)
Some medications, such as those used to guard against stomach acidity, are fairly alkaline, whereas others tend to be acidic. Use the cabbage indicator method (2 mL of assigned medication + 1 mL cabbage extract) to determine pH values for the following medications:
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Medication |
pH |
|---|---|
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Aspirin |
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Alka-Seltzer / Sodium bicarbonate (NaHCO 3 ) |
- Health care providers often recommend that aspirin be taken with a large glass of milk or water. Based on your results above, why might this be important? _________________________________________
- Would apple juice be a good replacement for water or milk while taking aspirin? Yes / No Explain: __________ ______________________________________________________________________________
- The normal pH of the stomach is around 2-3. How might the Alka-Seltzer affect the stomach and/or digestion? _______________________________________________ What might happen if someone uses too much Alka-Seltzer? _____________________ ______________________________________________________________________________
Part 3: pH and the Action of Cleaning Solutions – (assigned solution: _____________________)
Based on your experiences with the cleaning solutions below, predict their pH value. Then, use the cabbage indicator method (2 mL of assigned cleaning solution + 1 mL cabbage extract) to determine the actual pH values of the cleaning solutions:
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Cleaning solution |
Predicted pH |
Actual pH |
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Drano |
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Ivory Liquid |
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Cascade |
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Tide |
Exercise C: The pH Meter - test one per group (assigned solution: _______________________)
In the previous exercises, you learned how the pH of a solution can be estimated by comparing indicator colors with known standards. Sometimes, however, greater accuracy is needed. Electronic methods of pH determination using pH meters provide more precise results. The pH meters provided in our lab will already have been standardized for you using standardization buffers close to the pH of the samples that will be tested.
- Transfer the estimated pH values from Exercise C to the table below. Then, transfer 20 mL of solution into a 50 mL beaker, and add a small magnetic stirring bar. Turn the stir function on.
- Remove the pH electrode from the resting solution, gently rinse with distilled water, and then measure the pH of the assigned solution. Make sure that the stir bar does not hit the electrode.
- Measure the pH by changing the setting from “standby” to “pH” and record your pH.
- Repeat these steps with the remaining solutions (if applicable). Make sure to switch back to “standby” and rinse the electrode with distilled water between solutions.
- When you are finished, please rinse the electrode and replace it into the resting solution.
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Solution (20 mL) |
Estimated pH |
Measured pH |
[H + ] |
[OH - ] |
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Apple juice |
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Sprite |
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Maalox |
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Tide |
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Ivory Liquid |
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6. How that we have a more accurate measure of pH, we can calculate [H + ] or [OH - ] with our calculators. Since pH is expressed as a negative logarithm of [H + ], you need to determine the antilogarithm of the pH expressed as a negative number. Fill in your calculations above.
7. More practice:
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Solution |
[H + ] |
pH |
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Solution |
pH |
[H + ] |
|---|---|---|---|---|---|---|
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Urine |
5.73x10 -5 M |
Lemon juice |
2.7 |
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Pancreatic juice |
7.98x10 -8 M |
Milk |
6.5 |
8. If the pH of a solution is 6, what is its OH - concentration? First, find the antilog of -6. This will give you the H+ concentration of the solution. Then, divide 1x10 -14 by [H + ] to get [OH - ]. Calculate this value: _______________
- If a solution has an OH - concentration of 1x10 -4 M, what is its pH? ___________________
- If a solution has an [H + ] = 1 x 10 -6 M, what is its [OH - ]? _____________________________
Exercise D: Buffers
pH levels must remain relatively constant for most physiological processes. For example, the pH of our blood is usually maintained between 7.35-7.45. However, blood returning to the heart contains CO 2 picked up from our tissues, our diets, as well as the normal metabolic reactions in our cells, and may contribute to an excess of hydrogen ions. Buffer systems help maintain constant pH in the body.
A buffer is a solution that resists changes in pH when small amounts of acid or base are added. Bicarbonate, phosphate, and protein buffer systems maintain our blood pH. A buffer is made by mixing a weak acid with its salt in order to have in solution something that can act both as an acid (donate hydrogen ions) and something that can act as a base (accept hydrogen ions). For example, KH 2 PO 4 can be ionized to K + + H + + HPO 4 2 - . If hydrogen ions are added to the solution, they can be picked up by HPO 4 2 - , which acts as a base. If hydroxyl ions are added to the solution, they can be picked up by H + .
- Find the unknown solutions marked C and D. Pour 40 mL of unknown solution C into a 100 mL beaker. Add a stir bar and place the beaker on the stirring plate.
- Add 1 drop of Congo red. Record the initial color of the solution in the table below.
- Measure the initial pH using the pH meter and record this value.
- Using a pipette, add 2 mL of 0.1 N HCl. Record the final color of the solution and its final pH.
- Dispose of this solution, and wash and dry the beaker and stir bar thoroughly.
- Pour another 40 mL of the same unknown solution into the beaker, add the stir bar, and return it to the stirring plate. Add 1 drop of phenolphthalein and again measure the initial color and pH and record in the table below.
- Now use a pipette to add 2 mL of 0.1 N NaOH. Record the final color and pH.
- Dispose of this solution, and wash and dry the beaker and stir bar thoroughly.
- Repeat these steps for unknown solution D.
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Unknown solution |
Initial color |
Initial pH |
Solution added |
Final color |
Final pH |
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C + Congo red |
0.1 N HCl |
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C + Phenolphthalein |
0.1 N NaOH |
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D + Congo red |
0.1 N HCl |
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D + Phenolphthalein |
0.1 N NaOH |
- Which unknown solution contains buffers? C or D How could you tell? _____________________ ______________________________________________________________________________
- Which indicator would be best to use to determine the presence of an acid? _______________ What color would it appear at pH 2? ___________ pH 7? _____________ pH 9? _____________
- Which indicator would be best to use to determine the presence of base? _______________ What color would it appear at pH 2? ___________ pH 7? _____________ pH 9? _____________