The Roy G Biv Clock Reaction

Introduction

Take seven different dry chemical mixtures and add each to a separate beaker. Pour a clear liquid into each beaker and, in less than a minute, a rainbow of colors appears.

Concepts

• Clock reaction
• pH indicators
• Buffers

Materials

Safety Precautions

Formaldehyde is a known carcinogen. 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 solution. Sodium sulfite is moderately toxic; possible skin irritant. Sodium bisulfite is slightly toxic; severe irritant to skin and tissue as an aqueous solution. meta-Nitrophenol is moderately toxic by ingestion, inhalation and skin absorption; body tissue irritant. Phenolphthalein acts as a laxative upon ingestion; a body tissue irritant. Avoid body tissue contact. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. 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 regulations that may apply, before proceeding. The reaction products may all be disposed of according to Flinn Suggested Disposal Method #26b. The formaldehyde solutions may be disposed of according to Flinn Suggested Disposal Method #2. Sodium sulfite/sodium bisulfite dry powder mixtures and solutions may be disposed of according to Flinn Suggested Disposal Method #12b.

Prelab Preparation

Formaldehyde solution: Dilute 7 mL of the 37% formaldehyde solution with 1.7 liters of deionized water in a 2000-mL Erlenmeyer flask. Stir the solution. The final formaldehyde solution concentration is approximately 0.05 M. Prepare this solution at least 2 hours before use in the demonstration.

Procedure

1. Set up the demonstration by placing the seven 400-mL beakers, each with a stirring rod, on display. Label the beakers 1–7.
2. Add, in numerical order, 1 gram of each bisulfite/sulfite dry mixture to each 400-mL beaker. For example, add dry mixture 1 to beaker 1.
3. Using a 25-mL graduated cylinder, measure 20 mL of the 28% isopropyl alcohol solution and transfer to each of the 400-mL beakers. Swirl each beaker to dissolve the solids.
4. Once all the solids have dissolved, quickly and carefully fill each beaker to the 250-mL mark with the dilute formaldehyde solution and stir each. The solutions should all be colorless.
5. In about one minute or less, the solutions in the beakers will change colors, as listed in Table 1.

Student Worksheet PDF

12028_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration as written seven times: 70 mL of 37% formaldehyde, 10-g of each dry chemical mixture and 1 liter of 28% isopropyl alcohol.
• The reaction starts as soon as the formaldehyde solution is added. Fill the remaining glasses as quickly as possible. This will keep the timing of the color changes in the seven beakers close. Another option is to have the formaldehyde solution in seven separate beakers to make the addition quicker.
• Place a white background behind the beakers or place the beakers on a light box to show the colors more effectively.
• A two-hour wait is needed to allow for a sufficient buildup of formaldehyde concentration in solution. In the 37% formaldehyde solution, formaldehyde undergoes polymerization to polyoxymethylene glycols, HO(CH₂O)_nOH. When this solution is diluted below a concentration of 4%, the methylene glycols slowly depolymerize to formaldehyde.
• In the Materials section, each dry mixture (noted with †) is a 4 to 1 combination of sodium bisulfite powder, NaHSO₃, and sodium sulfite powder, Na₂SO₃, along with various amounts of three indicators: phenolphthalein, thymolphthalein and meta-nitrophenol.

Answers to Questions

1. List the color changes that occurred in each of the seven beakers. Also, note the chemicals that were used in this demonstration.
   

   Each beaker contained a clear solution of a dry mixture in isopropyl alcohol. Formaldehyde was added to each solution to produce the color changes.
   Beaker 1 – Red
   Beaker 2 – Orange
   Beaker 3 – Yellow
   Beaker 4 – Green
   Beaker 5 – Blue
   Beaker 6 – Indigo
   Beaker 7 – Pink

2. Give the chemical equation for the clock reaction of formaldehyde with sulfite ions and water.
   

   H₂O(l) + HCHO(aq) + SO₃^2–(aq) → HOCH₂SO₃^–(aq) + OH^–(aq)

3. Each dry mixture contained bisulfite ions and their conjugate base, sulfite ions. What is the special name for this kind of chemical? What does it do?
   

   A substance composed of a weak acid (bisulfite ion) and its conjugate base (sulfite ion) is a buffer. Buffers are solutions that resist pH changes when acids or bases are added to them.

4. Eventually, the bisulfite ions are used up in the reaction. Knowing this, how would you explain the color changes that occurred in the beakers?
   

   Each solution contained a buffer, but when either the weak acid or the conjugate base are used up, the buffer can no longer resist changes in pH. In this case, the weak acid was used up, meaning that the solutions then became much more basic. Therefore, the color changes were probably due to acid-base indicators, or combinations of indicators, that were included in each dry mixture and are colorless in an acid but different colors in a base.

Discussion

In this clock reaction, formaldehyde reacts with sulfite ions to form hydroxymethyl sulfonate ions and hydroxide ions.

The bisulfite ion is a weak acid and it is in solution with its conjugate base, the sulfite ion, setting up a buffer condition in the solution.

Initially, the solution is slightly acidic, with a pH of approximately 6.4. As the reaction proceeds (Equation 1), sulfite ions are consumed and hydroxide ions are produced. This reduction in sulfite ion concentration causes a shift to the right in Equation 2, producing sulfite ions and hydronium ions (H₃O⁺). The hydronium ions, in turn, react with the hydroxide ions to produce water.

This buffering action holds the pH of the solution relatively constant until all the bisulfite ions have been consumed. The hydroxide ion concentration then builds up causing a rapid rise in solution pH.

The color changes of the indicators in the solution all occur as the solution pH changes from 7 to 10. Phenolphthalein changes from colorless to red-purple in the pH range of 8.0–9.6, thymolphthalein changes from colorless to blue in the pH range of 9.3–10.6, and meta-nitrophenol changes from colorless to yellow in the pH range of 6.8–8.6. The seven colors of the final solutions are a result of the different combinations of these three indicators.

References

Special thanks to Steve Spangler, Director of the National Hands-On Institute, Regis University, Denver, CO, for providing the idea and procedure for this demonstration.

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