A Flash of Blue: A Classic Clock Reaction

Introduction

In a flash, a colorless solution turns a dramatic deep-blue color! Amaze your students with this popular starch–iodine clock reaction.

Concepts

  • Clock reactions
  • Indicators

Materials

Potassium iodate solution, 0.01 M, KIO3, 100 mL
Sodium meta-bisulfite, Na2S2O5, 0.02 g
Starch solution, 0.5%, aqueous, 100 mL
Sulfuric acid solution, 1 M, H2SO4, 3 mL
Water, distilled or deionized
Balance
Beaker, 250-mL
Graduated cylinder, 10-mL
Graduated cylinder, 100-mL
Stirring rod

Safety Precautions

Potassium iodate solution is an oxidizer. It is moderately toxic by ingestion and a body tissue irritant. Sodium meta-bisulfite is a skin and tissue irritant. Sulfuric acid solution is corrosive to eyes, skin and other tissues and moderately toxic by ingestion. 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 regulations that may apply, before proceeding. Dispose of the resulting solution according to Flinn Suggested Disposal Method #12a.

Procedure

  1. Pour 100 mL of the starch solution in a 250-mL beaker. Add 0.02 g of sodium meta-bisulfite to the starch solution. Stir to dissolve.
  2. Add 3 mL of 1 M sulfuric acid solution to the beaker and stir.
  3. Quickly, but carefully, add 100 mL of 0.01 M potassium iodate solution to the beaker. Stir to mix. Observe the appearance of the deep-blue color which suddenly appears (after about 10–15 seconds).

Further Extensions

Performing the procedure above as indicated is an attention-getting demonstration that shows the classic characteristics of a clock reaction. However, this reaction is also ideal for studying kinetics—the effects of concentration, temperature, and the presence of a catalyst on the rate of reaction. For more information, please request Flinn publication no. 10245, Iodine Clock Reaction.

Correlation to Next Generation Science Standards (NGSS)†

Science & Engineering Practices

Developing and using models
Planning and carrying out investigations
Constructing explanations and designing solutions
Engaging in argument from evidence
Obtaining, evaluation, and communicating information

Disciplinary Core Ideas

MS-PS1.B: Chemical Reactions
HS-PS1.B: Chemical Reactions

Crosscutting Concepts

Stability and change
Cause and effect

Performance Expectations

MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
MS-PS1-4. Develop a model that predicts and describes changes in particle motion, temperature, and state of a pure substance when thermal energy is added or removed.
HS-PS1-2. Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
HS-PS1-5. Apply scientific principles and evidence to provide an explanation about the effects of changing the temperature or concentration of the reacting particles on the rate at which a reaction occurs.

Discussion

This reaction is a classic example of a clock reaction. A clock reaction is a reaction characterized by an initial period with no noticeable change, followed by a sudden change, commonly in the color of the solution. The time period during which no noticeable change occurs is called the clock period, and the sudden change is called the alarm. What actually triggers the alarm varies from clock reaction to clock reaction.

In this reaction, potassium iodate and sodium meta-bisulfite react to form iodine. The starch solution serves as an indicator of the end of the reaction, forming a deep-blue colored starch–iodine complex in the presence of iodine. The chemical pathway for the formation of iodine is complicated and not completely understood, but the following mechanism serves as an outline.

  1. Sodium meta-bisulfite contributes hydrogen sulfite ions, HSO3–, while potassium iodate contributes iodate ions, IO3–, to the solution.
H2O + Na2S2O2(s) → 2HSO3–(aq) + 2Na+(aq)
KIO3(aq) → IO3–(aq) + K+(aq)
  1. The iodate ions react with the hydrogen sulfite ions to produce iodide ions, I–.
IO3–(aq) + 3HSO3–(aq) → I–(aq) + 3H+(aq) + 3SO42–(aq)
  1. In the presence of hydrogen ions, H+, the iodide ions react with excess iodate ions to produce iodine, I2.
6H+(aq) + 5I–(aq) + IO3–(aq) → 3I2(aq) + 3H2O(l)
  1. Before the iodine can react with the starch to produce a dark-blue colored complex, it immediately reacts with any hydrogen sulfite ions still present to form iodide ions.
I2(aq) + HSO3–(aq) + H2O(l) → 2I–(aq) + SO42–(aq) + 3H+(aq)
  1. Once all of the hydrogen sulfite ions have reacted, the iodine is then free to react with the starch to form the familiar dark-blue colored complex.
I2(aq) + starch → dark-blue colored complex

The deep-blue color of the complex is due to the presence of the pentaiodide anion, I5–. By itself, the pentaiodide anion is unstable; however, it is stabilized by forming a complex with the starch. The appearance of the deep-blue color in solution indicates that all of the reactants have been consumed and the reaction has gone to completion.

Recommended Products

Item No. Description
AP8657 A Flash of Blue—Chemical Demonstration Kit

†Next Generation Science Standards and NGSS are registered trademarks of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.