Iodine/Ascorbic Acid Reaction

Author(s): John Olson
Discrepant Event - Teacher's Guide
SED 695B

Ascorbic Acid

Detailed Explanation of Discrepant Event

Principles ilustrated

  • Vitamin C (ascorbic acid) is necessary to your body for proper teeth, bone, and red blood cell formation. It is also a part of enzyme systems that regulate chemical reactions in your cells.
    When ascorbic acid reacts with iodine, the ascorbic acid is oxidized (looses electrons) and the iodine is reduced (gains electrons).
  • As long as the solution contains ascorbic acid, the iodine is used up in a rapid. reaction with ascorbic acid, during which dehydroascorbic acid and iodide ion are formed.
  • Iodine is relatively insoluble, but this can be improved by complexing the iodine with iodide to form triiodide:

    I2 + I- <--> I3-

    Triiodide oxidizes vitamin C to form dehydroascorbic acid:

    C6H8O6 + I3- + H2O --> C6H6O6 + 3I- + 2H+

  • As a result of this reaction, the Iodine in the solution seems to vanish.
  • What actually happens to the iodine?

Discrepant Event: Diluting solutions demonstration (wmv)

What appears to happen. When the iodine water is poured into the first cylinder, it turns, as expected, a lighter shade. When that diluted mixture is poured into the second cylinder, it again, as expected, turns a lighter shade. The expectation would be that when it is poured into the third cylinder, the fluid would again appear as a more diluted shade. Instead, the fluide in #3 completely absorbs all of the coloration from both of the first two cylinders. If poured into the remaining orginal mixture, that would also turn clear.

Set up for this D-event:
*The dark fluid in the graduated cylinder is povidone iodine (apx 2 ml) diluted in 30 ml of water, enough to give the water a dark brown coloring.
*The first and second clear cylinders have apx 30 ml of tap water. *The third has 30 ml of a solution of vitamin C (4000 mg/liter). The high concentration of ascorbic acid allows all of the iodine to react with the vitamin C. (see left panel)

Quick version: This demo is a variation on the one above,
using the same concentrations in the two solutions..

Different Approach version: In this clip, the set up for the discrepant event is shown at the start. In a class, you would prepare the solutions ahead of time.

What appears to happen. When you pour the dark fluid into the pink water, there is no apparent change in the coloration of the water in the GC.

What actually occurs: The coloration comes from using water with food coloring (in the pitcher at start of video). The first cylinder is topped off with water and the second is topped off with the vitamin C solution. Half of the vitamin C solution is poured into a second cylinder to serve as the color comparison. Most of the water only solution is set aside and the remainder is darkened with povidone iodine. When this darkened red water is poured into the vitamin C solution, the iodine reacts with the ascorbic acid and the illusion is that it vanishes, leaving behind the water which is the same shade as the water in the control cylinder.


8th Grade Physical Science Standards

  • 3b: Students know compounds are formed by combining two or more different elements and that the compounds have properties that are different from their constituent elements

    5a: students know reactant atoms and molecules interact to form products with different chemical properties

    5b: atoms explain the conservation of matter: In chemical reactions the number of atoms stays the same no matter how they are arranged, so their total mass stays the same.

Questioning Script

Prior knowledge & experience:

  • What happens when you pour a dark or colored liquid into clear liquid?

Root question:

  • What do you predict will happen when I pour this dark brown liquid into the clear liquid?
  • What will happen when I pour that first liquid into the second? The second into the third?

Target response:

  • 1: The liquid should get lighter, turn light brown.
    2: The second liquid should be lighter than the first.

Common Misconceptions:

The liquid will continue to change to lighter and lighter shades of brown.

When the liquid remains clear, students will need to rethink their assumptions about what type of liquid is in each container. Seeing liquids become diluted is a common experience. When a dark liquid vanishes in a clear liquid, it does not fit with our preconception about diluted liquids.


References & Links:

MSDS for Povidone Iodide
Povidone Iodine
Ascorbic Acid
Iodine titration experiment