CMW Two Stage Clock Reaction
Desciption: Equal volumes of three solutions, one clear and pale blue, two clear and colorless, are added and quickly mixed. After a brief induction period, the solution turns a bright orange. In several more seconds, the solution changes color again to a deep blue/black.
KIO3(aq)+ Na2S2O5(aq) → I–(aq)+ 2SO4-2(aq) + 2Na+(aq) + K+(aq)
IO3–(aq) + 8I–(aq) + 6H+(aq) → 3I3–(aq) + 3H2O(l)
HgCl2(aq) + 2I–(aq) ↔ HgI2(s)
2I3–(aq) + Starch ↔ blue starch-I5–(aq) + I–(aq)
Source: Dr. Chad M. Wilkinson, UW Madison Personal Consult (2012)
Shakhashiri, B. Z., Chemical Demonstrations: A Handbook for Teachers of Chemistry vol 4 (1993) pp26-30
Year: 2012, 1993 Vol: 4 Page: 26 -30
Keywords: Clock Reaction, Precipitate, Complexation, Starch, Kinetics
Rating: 
Hazard: Medium
- Acute toxicity hazard
- Aquatic hazard
- Flammability hazard
- Reproductive hazard
- Germ cell mutagenicity
- Respiratory hazard
- Serious eye damage
- Skin corrosion hazard
Effectiveness: Good
- Good connection to course material
- Results are clearly observable without guidance
- Time to results is low
- High reliability
Difficulty: Medium
- Prior training recommended
- Procedures with some intermediate steps to results
- Adding reagents in concerts or timed
Safety Precautions:
- Eye protection required
- Gloves required
- Prepare reagents in fume hood
- Avoid exposure to mists, dusts, vapor, and droplets
- Perform in a well-ventilated area
- Minimum observation distance is one meter
Class: Kinetics
Division: General
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