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Invisible Painting

Last updated

Aug 15, 2020
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- Instant Fire
- Iodine Clock Reaction

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Required Training	Required PPE
UC Lab Safety Fundamentals	Lab coat, safety glasses/goggles, nitrile gloves

Equipment	Chemicals
White poster board	500 mL 0.01N NaOH in 95% EtOH in spray bottle
3-4 Paint brushes (1”)	Phenolphthalein indicator solution – 0.5 g in 20 mL 95% EtOH – Red
	Thymolphthalein indicator solution – 0.5 g in 20 mL 95% EtOH – Blue
	m-nitrophenol indicator soulution – 2.0 g in 20 mL 95% EtOH – Yellow

Procedure:

Write a message on the board using any or all of the indicators. It should dry clear or very faint. Use a different brush for each indicator to avoid contamination
Once dry, spray the poster with the dilute NaOH to reveal the message.
Alternatively, you can use another brush to apply the NaOH solution.

Discussion:

This demonstration uses acid-base indicators that are colorless when acidic or neutral but colored when sufficiently basic. A painting or message is formed on the board or paper with concentrated solutions of colorless indicators. When the basic solution is applied the painting appears because the basic solution causes the indicator to take on their basic colors.

At pH values below their pKa values the indicators are colorless. At pH values above their pKa values the indicators become colored.

Indicator Basic Color pKa

Phenolphthalein pink 9.2

Thymolphenolphthalein blue 10.1

The image produced is not very long lasting. The fading is caused by the carbon dioxide in the atmosphere. Carbon dioxide, which dissolves in water to form carbonic acid, gradually neutralizes the colored indicators returning them to their colorless state.

Hazards:

Ethanol is a flammable liquid; be sure to spray away from ignition sources. If skin comes into contact with the sodium hydroxide/ethanol solution, wash with copious amounts of water.

If developing solution comes in contact with eyes, immediately flush with water for at least 15 minutes; get medical attention.

SOP

Carcinogen - Phenolphthalein

Corrosive – Sodium Hydroxide

Reproductive Hazard – Phenolphthalein

Disposal (by Storeroom)

The board or paper can be disposed in regular waste containers.

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