Indicator Change Points

Description

A solution containing two indicators is titrated. By choosing concentrations and indicators judiciously, it is possible to show that one indicator changes color before another.

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• Phenolphthalein and bromcresol green are two common acid/base indicators. Phenolphthalein's color change from pink (base) to colorless (acid) occurs near pH 8-10; bromcresol green's transition occurs from about pH 4 to 5.5. As a result, a solution with pH of 6 or 7 produces the acidic color of phenolphthalein but the alkaline color of bromcresol green.
• Chemists have developed universal indicators by combining indicators. These are used to estimate pH over a wide range of values.

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1. Place 100 mL of distilled water in a 250-mL Erlenmeyer flask. Set the flask on a circle of white filter paper to aid in determining the color of the solution.
2. Add 20 drops of 1% ethanolic phenolphthalein. Swirl and record the color.
3. Use a clean Pasteur pipet or other dropping pipet to add 1 drop of 1.0 M NaOH. Swirl vigorously. Note and record any color.
4. Use a clean dropping pipet to add 0.005 M HCl dropwise, while counting drops.
5. Swirl vigorously and continue adding acid dropwise until a color change is noted. Record the change and the number of drops.
6. Assemble aqueous 0.2% bromcresol green and 0.005 M HCl with the flask with the phenolphthalein solution titrated above.
7. Add 20 drops of aqueous 0.2% bromcresol green to the flask, swirl, and note the color. Record the change.
8. Add 0.005 M HCl dropwise to the flask, counting drops and swirling constantly until the color stops changing.
9. Record the color change and the number of drops.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 101 Indicator Change Points

Watch the movie.

Record the color at each step in the demonstration.

Solutions color:

Phenolphthalein+ water:

Phenolphthalein+base:

Phenolphthalein+acid(Soln. A):

bromcresol green+soln. A:

bromcresol green+soln. A+acid:

The phenolphthalein solution tests acidic when the bromcresol green is added. Explain in your own words how the bromcresol can change color as more acid is added.

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Purpose

To illustrate that a given solution may produce the acidic form of one pH indicator but the basic form of another.

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• phenolphthalein solution (add 1 g phenolphthalein to 50 mL alcohol, then add 50 mL distilled H₂O)
• bromcresol green solution (dissolve 0.2 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.)
• 1 M NaOH (4.0 g NaOH in 100mL)
• 0.005 M HCl (place 0.4 mL of concentrated HCl in 1 L of distilled water)
• 250-mL Erlenmeyer flask
• 2 Pasteur pipet with bulbs (Beral pipets, other dropping pipets)
• distilled water
• several circles white filter paper

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• This demonstration works well with a Pasteur pipet. A "pulled" thin stem polyethylene transfer pipet (Beral pipet) also works very well. Leave a long stem on the beral pipet.

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Teacher preparation: 15 min

Class time: 40-45 minutes

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Sodium hydroxide may cause blindness. The indicators are toxic. The bromcresol green may cause stains.

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Wear eye protection at all times. Have eye wash facilities available. Do not ingest chemicals. Wear old clothing and laboratory aprons.

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

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Mixtures of different pH indicators are used as "universal" indicators. The color of a universal indicator gives an indication of the pH of a solution. pH test papers are prepared by using universal indicators.

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Not all indicators change at the same pH. Phenolphthalein changes color at a more basic pH then bromcresol green. (Students may also suggest a polyprotic indicator. This possibility is consistent with the demonstration even though it is not what is happening.)

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• pH
• pH indicator
• titration
• color change
• acid
• acidic
• base
• basic

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