pH Indicators
Data Sheet
Write a summary of your observations from this lab which includes the colors of the various indicators tested and the pH at which those colors were observed.
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Introduction
An indicator solution changes color at a specific pH depending upon the indicator used. Ideally, one would like to have an indicator change color at a pH of 7 to see where the solution changes from acidic to basic. However, this is not usually the case. One also seeks an indicator in which the color change is quite abrupt -- over a short pH change the color changes sharply. In this experiment, you will be observing the color changes of several indicators over a given pH range.
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Procedure
- Place the 96-well microplate on a white paper background with the numbered columns at the top and the lettered rows on the left side.
- In rows A, B, C, E, F, and G, place 9 drops of distilled water in wells #2 - #11. In the first well of each row place 10 drops of 0.1 M HCl and in well #12 place 10 drops of 0.01 M NaOH.
- Using a clean pipet, transfer one drop of acid from the first well to the second well in row A. Mix thoroughly by drawing up the entire contents of the second well into the pipet and then returning it to the well. Transfer one drop of the liquid from the second well to well 3, mixing as before and continue in this way until you reach well #6 which will be the last one of the acidic dilutions. Repeat this procedure for the other rows.
- Repeat the dilution procedure using the 0.01 M NaOH in the same row as the acid dilution, but working backwards from #12 to #8, making #8 the last of the basic dilutions. Note that you have not added anything to well # 7. What does this tell you about the solution in that well?
- You have now diluted each well by a factor of 10. In well #1, the [ H+ ] = 1 x 10-1 moles/liter, in well #2, the [ H+ ] = 1 x 10-2 moles/liter. In well #12 the [ OH- ] = 1 x 10-2 moles/liter, in well # 11 the [ OH- ] = 1 x 10-3 moles/liter.
- Add one or two drops of indicator to each well, using the chart above as a guide.
- Many indicator compounds are found as pigments in the flowers or fruits of plants. In row G, test the extract of the natural material you brought or one of the ones available in the lab. If time permits, you may make up dilutions to test other natural indicators.
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Safety
Wear safety goggles and aprons in the lab at all times. Use caution in dealing with the acids, bases, and indicators. Wash your hands immediately upon contact with the chemicals.
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TG Data Table
|
Acid |
Base |
Range |
| bromthymol blue |
yellow |
blue |
6.0-7.6 |
| methyl orange |
red |
orange/yellow |
3.0-4.4 |
| bromcresol green |
yellow |
blue |
4.0-5.6 |
| phenolphthalein |
colorless |
red |
8.2-10.0 |
| alizarin yellow |
yellow |
violet |
10.0-12.0 |
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TG Lab Hints
Although the procedure described in the student handout usually gives adequate results, it is based upon an assumption that can cause difficulty. In wells 4, 5, and 6, for example, the [H+] is thought to be 10-4, 10-5, and 10-6 respectively. Because these concentrations are derived from the dilution of a strong acid, there is no buffering capacity. If the added indicator contains a substantial amount of acid or base, the [H±] is likely to change significantly.
For these reasons, using buffers in the wells of the plate is a preferred procedure. Buffer powders are commercially available at relatively low cost in gelatin capsules. One capsule provides enough chemical for over 100 students.
Prepare the buffers with distilled water. Distribute them from pipet chemical containers.
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TG Materials
- 96-well plate
- clean, pulled, transfer pipet
- The following prepared solutions in pulled Beral pipets:
- 0.1M HCl (dissolve 4.2 mL concentrated HCl in 400 mL of distilled water, and add enough water to bring the total volume to 500 mL)
- 0.01M NaOH (dissolve 0.4 g NaOH in enough distilled water to make 1000 mL of solution)
- bromthymol blue (dissolve 0.1 g bromthymol blue in a mixture of 20 mL 95% ethanol and 50 mL distilled water. Add enough distilled water to bring the final volume to 100 mL.)
- methyl orange (dissolve 0.1 g methyl orange in 100 mL of distilled water)
- bromcresol green (dissolve 0.1 g bromcresol green in a mixture of 20 mL 95% ethanol and 50 mL distilled water. Add enough distilled water to bring the final volume to 100 mL.)
- phenolphthalein (dissolve 1 g phenolphthalein in 60 mL 95% ethanol, and add enough distilled water to bring the final volume to 10 mL)
- Alizarin yellow (dissolve 0.1 g alizarin yellow R in 100 mL of distilled water)
- natural indicator (red cabbage juice, blueberries, petals from flowers, etc.)
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