Vitamin C Iodine Clock Reaction Ryan Piotrowski and Robin E. Renzi
University of New Hampshire, Manchester


Plan A:
Our plan A was to meet with a High School Science Teacher who we would have worked with to "green" the iodine clocks reaction and share our results with her class. Unfortunately we were unable to make contact with our assigned teacher. Research was done on the traditional clock reaction and green alternatives were found to exist. We presented our findings to our professor and choose to focus future research on the already "greened" Vitamin C clock and make the information available to our high school teacher via a video demonstration of the lab. Since we were provided the lab protocol for the Vitamin C Clock reaction there was no need to determine a Plan B.

The iodine clock reaction has been used by high school chemistry teachers for years to help students visualize concepts pertaining to reaction rates. This task is accomplished by mixing two colorless solutions together, and timing the sudden color change. The iodine clock reaction can be manipulated by changing variables such as concentration, temperature, and the presence of a catalyst. The simplicity of the experiment works well in a high school lab setting as it allows the student to observe, compare, and analyze data collected from multiple experiments in a single lab session.

The Traditional Clock Reaction:
The iodine clock reaction is one of a series of three reactions which takes place after mixing the two solutions in order to turn it blue. Equation 1 indicates that in an acidic solution, iodide ions are oxidized by Bromate which converts iodine ions to Iodine. In Equation 2 the iodine is reduced back to iodide ions by thiosulfate ions. Thiosulfate is reducing the iodine as fast as it is being produced. In Equation 3 all the thiosulfate ions have been consumed and iodine has time to react with starch to dye the solution purple.

(1) 6I- (aq) + BrO3- + 6H(aq) → 3I2- (aq) + Br- (aq) + 3H2O

(2) I2- (aq) + 2S2O32- (aq) → 2I- (aq) + S4O62- (aq)

(3) 3I2- (aq) + starch → starch-I5- complex + I- (aq)

Reaction Time/ Kinetics:
After completing the clock reaction, each and every duplicate experiment should produce a similar outcome. By manipulating the concentrations of the reactants students have the chance to alter the reaction rate. Increased concentrations of iodide, hydrogen peroxide, or acid will accelerate the reaction, while increasing thiosulfate concentrations will increase the time it takes to see a color change.

The Green Chemistry:
Traditionally to create the acidic conditions necessary to start the clock reactions hydrochloric acid has been utilized. Hydrochloric acid is considered a toxic chemical and can be harmful to ones health as well as the environment. When used in this experiment the greatest risk of exposure is contact with skin or the eyes which may result in burns and blindness. The traditional clocks reaction also requires students to neutralize the end product before it can be discarded. Also the previous method could be demonstrated with a catalyst, ammonium molybdate, which created toxic byproducts that could not disposed of immediately through natural processes.The vitamin C iodine clock has been greened in the following ways:

Hydrochloric acidAscorbic acid
Bromate ionHydrogen peroxide
Ammonium molybdatePlatinum wire

(1) 2H​+ (aq) + 2I- (aq) + H2O2 (aq) → I2 (aq) + 2H2O(l)

(2) I2 (aq) + C6H8O6 (aq) → 2H+ (aq) + 2I- (aq) + C6H6O (aq)

Safety Precautions and Hazards:
  • Wearing a lab coat and proper eye protection
  • Wearing gloves to prevent the staining of the skin is recommended due to Iodine's staining property
  • Be aware of hot plate surfaces and allow for glass to adequately cool before handling beakers
  • When hot plates are not in use be sure to turn off and unplug from outlet
  • As always be sure to follow any additional instructions given by your instructor

Equipment List:
Several 250-ml beakers
1000mg vitamin C tablets (or equivalent to)
Mercury thermometer
Ice cubes
Shallow bucket or tub for both ice/warm bath
Several hot plates with stiring capabilities orange-slices-row-lg.jpg

Vitamin C stock:
1000mg crushed vitamin C tablet Ascorbic Acid MSDS Sheet
60mls Distilled water Distilled Water MSDS Sheet

Starch stock:
Saturated homologous solution of
distilled water and potato starch Potato Starch MSDS Sheet

Solution A:
5mls Lugols Reagent or Tincture of Iodine Lugols Reagent / Iodine MSDS Sheet
5mls Vitamin C Stock
Distilled water quantity;
  • 60mls distilled water ( baseline concentration)
  • 30mls distilled water ( high concentration)
  • 90mls distilled water ( low concentration)

Solution B:
15mls Hydrogen Peroxide Hydrogen Peroxide MSDS Sheet
2mls Starch Stock
Distilled water quantity;
  • 60mls distilled water ( baseline concentration)
  • 30mls distilled water ( high concentration)
  • 90mls distilled water ( low concentration)

Procedure: Baseline
  1. Make the Vitamin C and the starch stock solutions by referring to the instructions listed above.
  2. Prepare solutions A and B seperately as listed above using the baseline concentrations .
  3. Combine solutions A and B in a 250mL beaker placed on a hot plate only utilizing the stirring function.
  4. Begin timing once the solutions are combined. Stop timing once the solution turns deep purple in color.
  5. Repeat steps one through four three times to establish a baseline reaction rate, and record results.

Procedure: Concentration
  1. Repeat the experiment, but this time refer to high concentration for solutions A and B.
  2. Begin timing once the solutions are combined. Stop timing once the solution turns deep purple in color
  3. Record the results.
  4. Repeat the experiment, but this time refer to low concentration for solutions A and B.
  5. Mix, time, and record the results.

Procedure: Temperature
  1. Repeat the original (baseline) experiment, but cool solutions A and B to 15°C in an ice bath.
  2. Begin timing once the solutions are combined. Stop timing once the solution turns deep purple in color.
  3. Record results
  4. Repeat the experiment, but this time heat solutions A and B to 25°C in a hot water bath.
  5. Mix, time, and record the results.

The clock reaction factors that we explored were both concentration and temperature. By using different quantities of distilled water, the length of time for the solution to turn blue changed. A less dilute solution resulted in a faster iodine clock. The respective was also true: a more dilute solution resulted in a slower iodine clock. Temperature affected the iodine clock in the same manner. Higher temperatures yielded a faster clock while lower temperatures yielded a much slower iodine clock.

  • 6 seconds
  • 8 seconds
  • 7 seconds
Average: 7 seconds

  • 43 seconds
  • 40 seconds
  • 41 seconds
Average: 42 seconds

  • 3 seconds
  • 4 seconds
  • 4 seconds
Average 3.75 seconds

  • Instant
  • Instant
  • Instant
Average: Instant

  • 27 seconds
  • 31 seconds
  • 30 seconds
Average: 29 seconds

As a group we have found a viable green alternative to the iodine clocks reaction. The reaction presents itself in the same manner and offers the same dramatic color change once the limited reactant has been utilized. Several draw backs we encountered when using the greened Vitamin C clock were variations in the concentration of ascorbic acid of the vitamin C stock that altered the reactions run time. This is not beneficial to the student as he is trying to determine a constant, before he attempts to experiment with other variables such as temperature and concentration. ANother problem we foresee is the length of time needed to prepare a homogenous solution of potato strach. This can be a difficult task which can turn messy fast with high school students limited by a forty five minute class period.

Future experimental alterations:
To further green the experiment we suggest the use of orange juice as a substitution for the vitamin C tablets would be preferred. Several baches of vitamin C stock had to be made due to inconsistency in the concentration of asorbic acid within the stock. We feel orange juice would provide for a more homogenous solution while still providing the acid conditions necessary to start the reaction, and eliminate any concentration inconsistencies. We would also suggest teachers who choose to use potato starch prepare it ahead of time. A slightly less green alternative would be to use liquid laundry starch which would save time and would not require heating before use.

1. B.Z. Shakhashiri (1992) Chemical Demonstrations: A Handbook for Teachers of Chemistry, vol. 4. pp 37-43.
2. Source of picture 1
3. Wright, Stephen W. The Vitamin C Clock Reaction: J. Chem. Educ. 2002, 79,41-43
4. Wright, Stephen W. Tick Tock, a Vitamin C Clock: J. Chem. Educ. 2002, 79,40A-40B
5.Source of picture 2