Introduction

Salute the Stars and Stripes! Mix three sets of two colorless solutions and in half a minute a red, white and blue tribute to Old Glory appears.

Concepts

• Clock reaction
• pH indicators
• Buffers

Materials

Safety Precautions

Formaldehyde is an alleged carcinogen, however, recent studies indicate no significant risk of cancer from low level exposure to formaldehyde. Formaldehyde is a strong irritant; avoid breathing vapor. 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. The magnesium chloride solution is slightly toxic by ingestion. Thymolphthalein solution is a flammable liquid and a dangerous fire risk. Phenolphthalein solution is a flammable liquid and a dangerous fire risk; it 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. 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 formaldehyde solutions may be disposed of according to Flinn Suggested Disposal Method #2. The magnesium chloride solution, the sodium bisulfite solution, and the reaction products may all be disposed of according to Flinn Suggested Disposal Method #26b. Sodium sulfite and its solution may be disposed of according to Flinn Suggested Disposal Method #12b. Magnesium chloride and sodium bisulfite may be disposed of according to Flinn Suggested Disposal Method #26a. The thymolphthalein solution and the phenolphthalein solution may both be disposed of according to Flinn Suggested Disposal Method #18b.

Prelab Preparation

0.3 M Formaldehyde solution: This solution must be prepared at least 2 hours before use. In a 1-L Erlenmeyer flask, dilute 10 mL of the 37% formaldehyde solution to the 600 mL mark with deionized or distilled water. Mix thoroughly. Keep the flask covered.

Sulfite/bisulfite solution: Prepare this solution within 24 hours of use. Dissolve 3.8 g of sodium sulfite and 12.6 g of sodium bisulfite in another 1-L Erlenmeyer flask containing about 400 mL of distilled or deionized water. Mix thoroughly to dissolve and dilute to the 600-mL mark with deionized or distilled water.

Procedure

1. Set up the demonstration by placing the three 600-mL beakers, each with a stirring rod, on display with a 250-mL beaker behind each.
2. Using a 250-mL graduated cylinder, transfer 200 mL of the 0.3 M formaldehyde solution into each 600-mL beaker.
3. Rinse the cylinder with deionized water and use it to transfer 200 mL of the sulfite–bisulfite solution to each of the 250-mL beakers.
4. With the 10-mL graduated cylinder add 3 mL of the phenolphthalein solution to the first 250-mL beaker, 10 mL of the magnesium chloride solution to the second 250-mL beaker and 3 mL of the thymolphthalein solution to the third 250-mL beaker. Stir each solution briefly.
5. Quickly pour the contents of each 250-mL beaker to the 600-mL beaker in front of it and stir each solution.
6. In 20 to 30 seconds, the first beaker will turn bright red, the second will form a white precipitate, and the third beaker will turn brilliant blue.

Student Worksheet PDF

12025_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration as written seven times.

• Only a small fraction of formaldehyde exists in solution as the formaldehyde molecule, CH₂O. Most of it exists as methylene glycol, CH₂(OH)₂.
  {12025_Tips_Reaction_1}

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

  {12025_Tips_Reaction_2}

  When this solution is diluted, depolymerization occurs slowly. Waiting two hours before use allows the solution to build up a sufficient concentration of formaldehyde molecules. It is this molecule that reacts with the sulfite ion in the clock reaction.

• The sulfite–bisulfite solution must be fresh to avoid having both ions oxidized by oxygen in the air to sulfate ions. This reduces the amount of each ion in solution and also produces sulfuric acid, which changes the pH of the initial solution and can interfere with the clock time and color changes.

Answers to Questions

1. Describe the contents of the three 600-mL beakers. Also note the solutions added to each beaker and the color changes each underwent.

Each of the three 600-mL beakers contained 200 mL of formaldehyde solution and 200 mL of the sulfite-bisulfite solution. 3 mL of phenolphthalein solution was added to the first beaker, 10 mL of magnesium chloride solution was added to the second, and 3 mL of thymolphthalein was added to the third. After about 30 seconds, the first beaker turned bright red, the second became a cloudy white, and the third turned bright blue.

2. Write a balanced chemical equation for each of the following reactions.

a. Bisulfite ions reacting with water. Hint: This reaction is reversible.

b. Sulfite ions reacting with water. Hint: This reaction is reversible.

c. Formaldehyde reacting with sulfite to form hydroxymethyl sulfonate ions and hydroxide ions.

3. The third reaction consumes sulfite ions and produces hydroxide ions. What does this do to the first two equations?

The consumption of sulfite ions and the production of hydroxide ions causes the first equation to shift to the right and the second equation to shift to the left.

4. The sulfite/bisulfite solution acts as a buffer. What happens when the bisulfite ions are used up? How are the color changes produced?

When the bisulfite ions have all been used up, the hydroxide ions can no longer be consumed as part of the second reaction. This excess of hydroxide ions causes the pH of the solutions to rise. Phenolphthalein and thymolphthalein are acid-base indicators and are red and blue, respectively, in basic conditions. Magnesium hydroxide forms a white precipitate in solutions with a pH exceeding 9.2.

Discussion

The same clock reaction is occurring in all three beakers. The only difference is the color of the pH indicators used. The pH of the solutions, and the indicator colors, remain constant through the clock period, about 30 seconds. At this time, the pH of the solutions quickly increases, causing a sudden change in the indicator colors.

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

The initial solution of sulfite/bisulfite buffer is slightly acidic with a pH of approximately 6.4.

Formaldehyde reacts with sulfite 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. This causes a shift to the right of Equation 1 and a shift to the left of Equation 2. This buffering keeps the pH of the solution essentially constant until all the bisulfate ions (HSO₃^–) are consumed. Without the bisulfite ion, no buffering occurs. Hydroxide ions produced in reaction 3 are therefore not consumed in Reaction 2 and the pH rapidly rises to approximately 10.5.

The color changes in all the solutions occur as the solution pH changes from 7 to 10. In beaker 1, phenolphthalein changes from colorless to red-purple in the pH range of 8.5–9.6.

In beaker 2, the white solid is magnesium hydroxide [Mg(OH)₂], which precipitates under reaction conditions when the solution pH exceeds 9.2.

In beaker 3, thymolphthalein changes from colorless to blue in the pH range of 9.3–10.6.

References

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