Rainbow Connection

Age

Elementary School, Middle School, High School

Format

Stage Show

Materials

   2.85 g phenolphthalein.
   21.0 g m-nitrophenol
   2.5 g thymolphthalein
   590 mL 95% ethanol
   250 mL of 0.05 M sulfuric acid
   1200 mL of 0.015 M NaOH
   10 mL of 18 M H2SO4
   20 mL of glycerol
   100 mL of 0.2 M NaOH
   7 30 mL dropping bottles
   20 oz clear soda bottle
   A clear Pitcher
   6 glass tumblers (12 oz)
   A glass stir rod
   White backdrop
   Towel
   Copy of the Rainbow Connection song by Kermit.
   Something to play the song with (e.g. speaker setup) 

Safety Precautions

Keep the audience at safe distance there is a splash hazard with these acids and bases. You MUST were splash proof goggles and a lab coat. Avoid contact of acids with the skin; it can cause burns. The solutions are at low concentrations but still take care when handling them. Care must be taken when mixing the HSO-glycerol solution since the solution will become hot. Allow it to cool overnight. Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.

   Phenolphthalein MSDS
   m-Nitrophenol MSDS
   Thymolphthalein MSDS
   95% Ethyl Alcohol (190 proof) MSDS
   .05M Sulfuric Acid MSDS (<.1M)
   .0015M Sodium Hydroxide MSDS (<.02M)
   18M Sulfuric Acid MSDS
   Glycerin MSDS
   .2M Sodium Hydroxide 

Preparation

The indicators must first be dissolved in the following proportions with 30 mL of ethanol.

RED

   1.5 g phenolphthalein with 3.0 g m-nitrophenol 

ORANGE

   0.45g phenolphthalein with 6.0 g m-nitrophenol 

YELLOW

   6.0 g m-nitrophenol 

GREEN

   0.6 g thymolphthalein with 6.0 m-nitrophenol 

BLUE

   1.5 g thymolphthalein 

VIOLET

   0.9 g phenolphthalein with 0.4 g thymolphthalein 

   Store the solution in a dropping bottle label with the correct color.
   Mix 250 mL of 0.05 M sulfuric acid with 250 mL of Ethanol and store in the soda bottle.
   Transfer to 1200 mL of 0.012 NaOH into the pitcher.
   Put 100 mL of 0.2 M NaOH solution into a dropper bottle.
   Dissolve the HSO in the glycerol and store in a dropper bottle. WILL BECOME HOT
   Clean the glass tumblers and arrange them in a row. Place 2 drops of the red indicator into the first tumbler and proceed with the orange indicator in the next tumbler. Repeat for each indicator in an individual tumbler. Place them in the correct order red – violet.
   Allow the indicator solutions to dry in the tumblers (2 hours)
   Place the tumblers in succession on the white backdrop, red – violet. 

Demonstration

Begin to play the song. Pick up every tumbler in succession pretending to clean in with the towel. Place them back into the correct place. Do not break the film on the bottom.

Add equal volumes (1.5 oz) of the acid- alcohol solution into each tumbler. Try not to pour directly onto the film so it does not break up and react to quick in the next step.

Add the 0.012 M NaOH solution from the pitcher to each tumbler. Fill them halfway and make sure to add the solution on the side of the tumbler so the colors do not flash the color.

The demonstrator should look at the tumblers (should to clear still) and the pitcher to observe that more solution can be added to the tumblers. Then fill the tumblers to ¾th’s full with the remaining NaOH solution in the pitcher pouring rapidly so the colors instantly become colored.

Take the dropping bottle of HSO-glycerol and add 3-4 drops to each tumbler (titrate to determine actual amount). This solution should drop to the bottom since it is denser than the basic solution.

Stir the tumblers rapidly to break up to the HSO-glycerol droplets which with make the solution acidic once more and return the tumblers to the original clear state. (DO NOT want to long to move to the next step or the solution will be permanently clear.)

Using the dropper bottle of 0.2 M NaOH and a stirring rod, rapidly titrate the solution back to the original color.

The tumblers are then poured into the pitch. The resulting solution is colorless

What to Say

Nothing is said during the demonstration. Afterward, the presence of acid/ base indicators can be discussed. Each indicator used turns a different color in the same pH range and the mixture of indicators allows different wavelengths of light to be absorbed. With these different wavelengths absorbed by the solution we see a new color.

Why It Is

“An acid-base indicator is a weak acid or a weak base. The undissociated form of the indicator is a different color than the iogenic form of the indicator. An Indicator does not change color from pure acid to pure alkaline at specific hydrogen ion concentration, but rather, color change occurs over a range of hydrogen ion concentrations. This range is termed the color change interval. It is expressed as a pH range.”[2] As before, each indicator has a different color for the same pH range. Then the mixture of the indicators allow for new colors to be observed.

Real Life Examples

Mixing water color paints gives the same effect as mixing the indicators.