A Qualitative Study of Indicators

Description

A series of solutions of pH 1 through 12 is prepared, and pH indicators are added systematically. From the resulting information, the pH range of each indicator is determined.

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Some of the indicators are poisonous. Bases and acids can cause blindness.

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Handle acids and bases with caution. Handle indicators carefully to avoid stains.

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Three alternate procedures are possible for this experiment. The instructor will assign the procedure appropriate for your equipment.

Test Tube Scale

1. Prepare 5 test tube racks. Place one 13- x 100-mm test tube for each indicator to be studied into each rack. Place 2 mL of distilled water into each test tube. Add 6 drops of 1 M NaOH to each test tube in the first rack. Add 1 drop of 1 M NaOH to each tube in the second rack. Add 1 drop of 1 M HCl to each tube in the fourth rack. Add 6 drops of 1 M HCl to each tube in the fifth test tube rack.
2. Place 4 drops of one indicator in the first tube of each of the five racks. Add the 4 drops of the next indicator in the second tube of each rack. Continue until all indicators have been added. Shake each of the tubes to mix its contents. Note the resulting colors.
3. You may wish to view the tubes by indicator. Place the tubes of the same indicator in one test tube rack.

Microscale Serial Dilutions

1. Make serial dilutions of 0.1 M HCl and 0.1 M NaOH in 96-well plates.
2. For the HCl serial dilutions, find out how many indicators you must test. If 4 indicators are to be tested then fill 4 wells in the "1" column (i.e., A1, B1, C1, etc.) with 0.1 M HCl.
3. One drop is removed from each well and placed in the neighboring "2" well. (For example one drop is removed from C1 and placed in C2.) 9 drops of distilled water are added to the "2" well. (For example, 9 drops of water are placed in C2.) The "2" wells are mixed. This places 0.01 M HCl in the column of "2" wells.
4. By repeating the procedure, one gets 0.001 M HCl in the "3" wells, and 0.0001 M HCl in the "4" wells. Beyond this point, the concentrations of the acid are too low to permit a valid assignment of pH to the well.
5. Use NaOH to prepare the corresponding strongly basic solutions.
6. Add 2 drops of indicator to each well. Mix.
7. Create a custom-blended "universal" indicator mixture.

Microscale Buffer Capsules

1. Since the columns in a 96-well plate are numbered 1 to 12, assign a column of wells to a particular pH buffer. In this case, pH = 11 is assigned to wells labeled "11."
2. Add 6 drops of pH 11 buffer to each well labeled with an 11 (i.e., A11, 035, etc.)
3. Add 2 drops of indicator to each well. Mix.
4. Create a custom-blended "universal" indicator mixture.
5. Record the colors obtained by other groups using different indicators.

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• Chemical reagents used as acid/base indicators are themselves weak acids or bases. These substances therefore interact with water according to the following simplified equilibrium reactions:
  • H₂O + HIn In^- + H₃O⁺
• Each species, HIn and In^-, has a distinctive color so that the predominant species in a mixture can be detected by eye.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 100 A Qualititative Study of Indicators

Closure Questions:

1. Identify the indicators that showed: a single color change; two color changes; three or more color changes.
2. List indicators best suited to test bases.
3. List indicators best suited to test acids.
4. State the properties characterizing a useful indicator.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 100 A Qualititative Study of Indicators

Watch the movie and record the colors observed.

Closure Questions:

1. Identify the indicators that showed: a single color change; two color changes; three or more color changes.
2. List indicators best suited to test bases.
3. List indicators best suited to test acids.
4. State the properties characterizing a useful indicator.

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Purpose

To examine pH indicators under acidic, neutral, and basic conditions.

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• 200 mL 1 M HCl (16 mL concentrated HCl in enough distilled water to make 200 mL of solution)
• 200 mL 1 M NaOH (8.0 g NaOH in enough water to make 200 mL of solution)
• distilled water in a squeeze bottle
• 100 13- x 100-mm test tube; test tube racks
• a variety of indicator solutions (dispensed in small squeeze bottles or Beral pipet dispensers) Indicator solutions are usually made by dissolving a small amount of powdered indicator in one part ethanol and diluting with one part distilled water. The following are suggested: phenolphthalein; bromthymol blue; alizarin yellow; bromcresol green; methyl red; methyl orange; litmus; orange IV; indigo carmine; and methyl violet. Several household indicators should be included: freshly-brewed strong tea; grape juice; and juice from freshly-boiled red cabbage leaves.

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• Check the indicators. Sometimes the solutions spoil with time. The solutions should be clear, not cloudy.
• This experiment may be run as a cooperative learning activity with different groups of students using different assigned indicators followed by sharing and comparison. Each student should record observations from all indicators. Three alternate procedures are included for you to select. If equipment is available you may wish to assign different groups different procedures for sharing and discussion.
• Encourage students to create custom-blended "universal" indicator mixtures.

Microscale

• The microscale version of this experiment offers a great many advantages. One approach is to make serial dilutions of HCl and NaOH. A better approach is to use buffer solutions prepared by dissolving the contents of premeasured capsules of solid chemicals into distilled water and using these to fill the wells.
• Not only are the resulting displays in the 96-well plates very dramatic, but they may be viewed using an overhead projector. Place an opaque mask on the stage of the projector to reduce the background light.

Microscale Serial Dilutions

• Beyond 0.0001 M, the concentrations of the acid are too low to permit a valid assignment of pH to the well. Also, students complain about the tedium of making the dilutions.

Microscale Buffer Capsules

• These results will be more reliable in the pH 4-10 range than will those obtained by dilution of strong acids or bases.

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Teacher preparation: 30 minutes

Class time: 40-45 minutes

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The materials suggested for use in this experiment may be disposed of safely at the sink after neutralization.

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Closure Questions:

1. Identify the indicators that showed: a single color change; two color changes; three or more color changes.
2. List indicators best suited to test bases.
3. List indicators best suited to test acids.
4. State the properties characterizing a useful indicator.

Answers to Closure Questions:

1. Phenolpthalein and litmus show a single color change. Bromthymol blue gives two color changes. Cabbage juice gives three, four or more color changes.
2. Phenolphthalein, cabbage juice.
3. Bromthymol blue, litmus.
4. Useful indicators exhibit intense colors that depend upon the acidity of their environment.

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Acid base indicators are used to check the acidity of swimming pools and tropical fish aquaria so that appropriate management of the water acidity can be accomplished.

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^a Note: methyl red should be yellow at pH 7 water must be slightly acidic in the experiment shown.

^b Note: red cabbage solution is lavender-blue in this range but the solution in the movie is too dilute to pick out the color.

See closure questions for answers to the questions.

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• pH
• pH indicators
• universal indicator
• color change
• litmus
• acidity
• dilutions
• buffer solutions

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