Blinking Holiday Colors

Introduction

A green and a red solution are prepared and after 30 seconds they suddenly switch colors.

Concepts

• Clock reaction

• pH indicators
• Buffers

Materials

Safety Precautions

Formaldehyde is a probable carcinogen; however, recent studies indicate no significant risk of cancer from low level exposure to formaldehyde. Formaldehyde is a strong irritant; avoid breathing vapor and avoid skin contact. Formaldehyde is highly toxic by ingestion, inhalation and skin absorption. The use of formaldehyde in this demonstration does not present an unnecessary risk. Use a fume hood to prepare the solution. Sodium sulfite is moderately toxic and a possible skin irritant. Sodium bisulfite is slightly toxic; a severe irritant to skin and tissue as an aqueous solution. The indicator solutions contain ethyl alcohol which is moderately toxic by ingestion. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulation that may apply, before proceeding. Formaldehyde may be disposed of according to Flinn Suggested Disposal Method #2. Sodium sulfite may be disposed of according to Flinn Suggested Disposal Method #12b. Sodium bisulfite may be disposed of according to Flinn Suggested Disposal Method #26a. The reaction products may be disposed of according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

Formaldehyde solution: Measure out 10 mL of 37% formaldehyde in a 10-mL graduated cylinder and transfer to a 500-mL Erlenmeyer flask. Fill to the 500-mL mark with deionized or distilled water. Stir to mix. This solution must be prepared at least two hours before use.

Sulfite/bisulfite solution: Prepare this solution within 24 hours of use. Dissolve 3.2 grams of sodium sulfite and 10.5 grams of sodium bisulfite in another 500-mL Erlenmeyer flask containing about 300 mL of distilled or deionized water. Fill to the 500-mL mark with distilled or deionized water. Stir to mix.

Worksheet: (Optional) Make copies of the demonstration worksheet and pass out to students before beginning the demonstration.

Procedure

1. Place the two hydrometer cylinders or tall-form beakers on display. Put a 400-mL beaker, with a stirring rod, behind each.
2. Use a 250-mL graduated cylinder to transfer 250 mL of the dilute formaldehyde solution into each hydrometer cylinder or tall form beaker.
3. Rinse the 250-mL graduated cylinder with distilled or deionized water and then use it to transfer 250 mL of the sulfite/bisulfite solution into each 400-mL beaker.
4. Use a 10-mL graduated cylinder to transfer 5 mL of the “green-to-red” indicator solution into the first 400-mL beaker. Stir to mix. The solution will be green.
5. Rinse the 10-mL graduated cylinder with distilled or deionized water and then use it to transfer 5 mL of the “red-to-green” indicator solution into the second 400-mL beaker. Stir to mix. This solution will be bright red.
6. Quickly pour the contents of each 400-mL beaker into the hydrometer cylinder or tall form beaker in front of it. Stir each solution.
7. In 20 to 30 seconds, the solution in the first cylinder or beaker will suddenly turn bright red, and the solution in the second beaker will switch to green.

Student Worksheet PDF

12753_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration as written at least seven times: 75 mL of 37% formaldehyde, 25 g of sodium sulfite, 80 g of sodium bisulfite, 50 mL of red-to-green indicator solution and 50 mL of green-to-red indicator solution.

• The sulfite/bisulfite solution must be prepared fresh before use to avoid having both ions oxidized by oxygen in the air. Oxidations will decrease the amount of each ion in solution and will also produce sulfuric acid, which changes the pH of the initial solution and interferes with the clock time and color changes.
• Only a small fraction of formaldehyde exists in solution as the formaldehyde molecule, CH₂O. Most of it exists as methylene glycol, CH₂(OH)₂.
  {12753_Tips_Equation_1}

  In a 37% solution of formaldehyde, the methylene glycol polymerizes to form polyoxymethylene glycols.

  {12753_Tips_Equation_2}

  When this solution is diluted, depolymerization occurs slowly. Waiting two hours before use allows the solution to build up a sufficient concentration of free formaldehyde molecules, which react with sulfite ions in the “clock reaction.”

• The green colors may not be a bright green and the reds may have an orange tint.
• The reaction starts as soon as the sulfite/bisulfite solution is added. Fill the two cylinders as quickly as possible. This will keep the timing of the color changes in the two cylinders close.

Answers to Questions

1. Describe what happens in the demonstration.
   

   Two equal volume solutions, one colored green and the other red, are each combined with an equal volume of a clear solution in two separate tall cylinders. The resultant solutions remain green and red for approximately 30 seconds. Both then change colors—the red solution becoming green and the green solution becoming red.

2. Both solutions contain indicators that change colors near a solution pH value of 10. What causes the solution color and pH to rapidly “blink” after 30 seconds?
   

   The solution pH stays below 10 until a sudden increase in hydroxide ion concentration occurs to increase the solution pH to above 10.

3. The initial solution contains a buffer. Does this fact help explain your answer to Question 2?
   

   If the reaction in each cylinder produces hydroxide ions, the buffer would consume the hydroxide ions and prevent the pH from increasing. Once the buffer is consumed, the hydroxide ions remain in solution and rapidly increase the pH to 10 and above.

Discussion

The same “clock reaction” occurs in both beakers. The only difference is the color of the pH indicators used. The pH of the solutions, and the indicator colors, remain constant throughout the initial period, about 30 seconds. At this time the pH of the solutions quickly increases, causing a sudden change in the indicator colors.

The clock reaction is a formaldehyde–sulfite/bisulfite reaction. The sulfite (SO₃^2–) and bisulfite (HSO₃^–) create a buffer system, where the bisulfite ion is the weak acid and the sulfite ion its conjugate base.

The general expression for the pH of a buffer system is:

Insert the initial concentration of the sulfite ions (SO₃^2–) and the bisulfite ions (HSO₃^–) as well as the initial pH of the solution into Equation 2:

pH = 7.2 – 0.60 = 6.6

The initial sulfite/bisulfite buffer solution is slightly acidic, with a pH of approximately 6.6.

When the formaldehyde solution is added, formaldehyde reacts with the sulfite ions to form hydroxymethyl sulfonate ions and hydroxide ions, according to Equation 3.

As the reaction proceeds, the sulfite ions are consumed and hydroxide ions are produced. As hydroxide ions are formed, the weakly acidic bisulfite ions react with them to form water, thus buffering the solution.

This buffering keeps the pH of the solution essentially constant until all the bisulfite ions (HSO₃^–) have been consumed. Without the bisulfite ion, no buffering occurs and the hydroxide ions are not produced in Equation 3. Without hydroxide ions, the pH rapidly rises to approximately 10.5.

Both indicator solutions are mixtures of two or more indicators. For the green-to-red indicator solution, the acid color is green, the basic color is red, and the transition occurs at approximately pH 10. For the red-to-green indicator solution, the acid color is red, the basic color is green, and again, the transition occurs at approximately pH 10.

References

Shakhashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison; 1985; Vol. 4, pp. 70–74.