Colorful Indicators

Purpose:

To demonstrate several reactions with carbon dioxide and indicators that result in a variety of color changes.

Materials:

• Insulated gloves
• 12 demo glasses or 250mL beakers

Reagents:

• Bromothymol blue indicator
• Dry ice (CO_2(s))
• Methyl red indicator
• Phenol red indicator
• Phenolphthalein indicator
• Thymolphthalein indicator
• Universal indicator
• 0.1M Sodium hydroxide (NaOH)

Hazards and PPE:

• Dry ice is harmful to the touch and produces gas rapidly, producing a risk of suffocation in an enclosed space. Store and handle with care.
• Sodium hydroxide is corrosive and toxic. Handle with care.
• Wear approved safety goggles and gloves.

Protocol:

1. Fill each of 12 demo glasses as follows:
   1. 1st pair: 1mL thymolphthalein
   2. 2nd pair: 1mL phenophthalein
   3. 3rd pair: 1mL phenol red
   4. 4th pair: 1mL bromothymol blue
   5. 5th pair: 1mL methyl red
   6. 6th pair: 1mL universal indicator
2. Add 1mL 0.1M NaOH to each demo glass.
3. Dilute each demo glass with 100mL DI H₂O.
4. Wearing gloves, drop a chunk of dry ice into one demo glass of each pair.

Additional Notes:

• The indicator color changes should occur as follows:

• Test Tube Pair	Indicator	Color Change	pH Range
  1	Thymolphthalein	Colorless → Blue → Colorless	10.6 - 9.4
  2	Phenolphthalein	Colorless → Pink → Colorless	10.0 - 8.2
  3	Phenol red	Red → Red → Yellow	8.0 - 6.6
  4	Bromothymol blue	Blue → Blue → Yellow	7.6 - 6.0
  5	Methyl red	Orange → Yellow → Red	6.0 - 4.8
  6	Universal indicator	Green → Purple → Orange	N/A

• Initial pH in all tubes should be ~11.

Disposal:

• Solutions can be poured down the sink with excess water.

Reactions:

• CO_2(aq) + H₂O_(l) → H₂CO_3(aq)
• H₂CO_3(aq) + H₂O_(l) → H₃O⁺_(aq) + HCO₃^-_(aq)
• HCO₃^-_(aq) + H₂O_(l) → H₃O⁺_(aq) + CO₃^2-_(aq)

Citations:

1. Shakharshiri, Bassam; Chemical Demonstrations, Volume II, 1985.