Name: Kevin Scully
Title: Part A: The Disappearing Rainbow
Part B: Yamada’s Universal Indicator
Curriculum Outcomes:
Chemistry 621A
320-5 Describe the interactions between H+ ions and OH- ions using Le
Chatelier’s principle
320-7 Explain how acid-base indicators function.
Equipment and Materials:
Part A
6 x 400-mL beakers
2 x 1000-mL beakers
6 dropping bottles
6 x 30-mL indicator solutions
Supply of 95% ethanol
Phenolphthalein indicator
Thymolphthalein indicator
p-nitrophenol indicator
1L of 0.01M NaOH
1L of 0.01M HCl
50-mL of 3M NaOH
Part B
1 x 400-mL beaker
50-mL of 0.05M NaOH
Thymol Blue indicator
Methyl Red indicator
Bromothymol Blue indicator
Phenolphthalein indicator
Part A: Disappearing Rainbow
Procedure:
Preparing indicator solutions (prior to class)
Violet – 0.45 g of phenolphthalein and 0.2 g thymolphthalein in 30 mL of 95% ethanol.
Blue – 0.2 g of thymolphthalein in 30 mL of 95% ethanol.
Green – 0.2 g of thymolphthalein and 2 g of p-nitrophenol in 30 mL of 95% ethanol.
Add 5 drops of 1M HCl to acidify.
Yellow – 1 g of p-nitrophenol in 30 mL of 95% ethanol. Add 5 drops of 1M HCl to
acidify.
Orange – 2 g of p-nitrophenol and 0.15 g phenolphthalein in 30 mL of 95% ethanol. Add
5 drops of 1M HCl to acidify.
Red – 1.5 g of p-nitrophenol and 0.75 g phenolphthalein in 30 mL of 95% ethanol. Add
5 drops of 1M HCl to acidify.
Place each solution in a dropping bottle to be given out in the lab.
1. Set out six 400-mL beakers and place 50 mL of 0.01M HCl in each of them.
2. Pick a specific color of indicator for each of the six beakers and add three drops to
the 50 mL of HCl. Note: at this time all beakers should contain 50 mL of
clear HCl solution
3. Add 75 mL of 0.01M NaOH solution to each of the six beakers. Each solution
should now change from clear to the bright color of the indicator.
4. By adding a few drops of 3M HCl the process reverses and the solutions will turn
clear once again.
5. Addition of a few drops of 3M NaOH to each beaker will result in expression of
the colors once again.
6. When finished waste can be disposed of down the drain with water.
Safety issues:
It should be noted p-nitrophenol is toxic by ingestion. Dilute hydrochloric acid
(HCl) solution is corrosive to eyes, skin and other tissues. Dilute sodium hydroxide
(NaOH) is corrosive; skin burns are possible. Indicator solution contains ethanol, a
flammable liquid and a fire risk; keep away from flames and other sources of ignition.
Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron.
Questions to promote student thinking and interaction:
1. Ask students to comment on the amount of acid and base required to cause color
transition of the indicator.
2. Why did the indicators change color?
3. Where did all the different indicator colors come from?
4. How could this concept be applied in the field of chemistry or more generally,
science?
Scientific Explanation:
There are three different indicators used in this experiment. These three
indicators all indicate for a basic solution medium with a different color transition. The
pH ranges of transition for the three indicators are very close with some overlapping
considerably. The consequence of this event is the generation of different colors
depending on the ratios of the various indicators in the indicator solution. The
p-nitrophenol indicates yellow under basic conditions. The phenolphthalein indicates red
under basic conditions and the thymolphthalein indicates blue under basic conditions.
The all other observed colors are some combination of the three aforementioned
indicators.
Part B: Yamada’s Universal Indicator
Procedure:
1. Place the following amounts of each indicator in a flask for mixing.
Thymol Blue 5.0 mg
Methyl Red 12.5 mg
Bromothymol Blue 60 mg
Phenolphthalein 100 mg
2. Add 100 mL of 95% ethanol to the flask containing the four indicators.
3. Neutralize the red solution with 0.05M NaOH solution.
4. The solution will turn green in color when the pH reaches the desired level.
5. Use deionized water to make up a final volume of 200 mL of solution.
6. Add several drops of Yamada’s universal indicator to an acidic solution and add
0.05M NaOH to transition through the various colors.
Safety issues:
Yamada’s Universal Indicator is a flammable liquid. The indicator dyes should
be considered toxic by ingestion and skin contact should also be avoided. Protective
gloves and glasses should be worn to prevent contact with sodium hydroxide (NaOH)
solutions.
Questions to promote student thinking and interaction:
1. How does the universal indicator compare to those used in Part A?
2. Why is the universal indicator so useful for determining pH?
3. Why would you want a universal indicator instead of a single indicating solution?
4. Who uses universal indicators?
Scientific explanation:
The theory for a Universal indicator is the same as mentioned in part A. The
biggest difference here is the indicators have been selected to have sequentially
increasing pH values of indication. There is slight overlap at the edges of each indicators
range of detection. This allows a sequence of colors to be observed instead of a single
event. The colors should follow the trend
pH10: violet
pH9: indigo
pH8: blue
pH7: green
pH6: yellow
pH5: orange
pH4: red
References: The idea for this demonstration came from
http://en.wikipedia.org/wiki/Universal_indicator
http://www.flinnsci.com/Documents/demoPDFs/Chemistry/CF0591.00.pdf