PHOTOCHEMICAL REACTION: AMMONIUM OXALATE AND IODINE

PURPOSE

The promotion of a chemical reaction by light is investigated.

This experiment is suitable for first or second-year courses. The effect of light on the reaction between ammonium oxalate and iodine is studied by subjecting samples of the reaction mixture to light from various sources. These are then compared to a sample kept in the dark.

TIME REQUIRED

Fifteen to twenty minutes to set up; two hours for reaction to occur.

MATERIALS

Chemicals: household ammonia; wood bleach, oxalic acid (3/4 teaspoon in 25 mL H₂O)*; tincture of iodine*
Equipment: small beaker; medicine dropper; aluminum foil; test tubes

*See Modifications/Substitutions

HAZARDS

Tincture of iodine should be used with care; iodine stains and irritates skin. Oxalic acid is poisonous. Ammonia vapors are irritating. Goggles must be worn throughout the experiment.

MODIFICATIONS/SUBSTITUTIONS

Pure oxalic acid can also be obtained from hardware stores and radiator or motor shops. Zud cleanser is another source of oxalic acid.
A solution of 0.15 g iodine in 30 mL ethanol can be used in place of tincture of iodine.

PROCEDURE

Add 25 mL of household ammonia to 25 mL of oxalic acid solution to produce an ammonium oxalate solution.
In each of two test tubes place 2 mL of the prepared ammonium oxalate solution.
Wrap one test tube with a piece of aluminum foil large enough to allow part to cover the top of the test tube.
Using a medicine dropper, add 10 drops of the tincture of iodine to each of the test tubes.
As quickly as possible, cover the top of the wrapped test tube and put in a drawer or some dark place.
Expose one unwrapped test tube to strong sunlight, another to fluorescent light, and the third to incandescent light.
After a couple of hours, compare the colors of the four solutions.

DISPOSAL

Solutions should be flushed down the drain with plenty of water.

DISCUSSION

A photochemical reaction is one in which light energy is required for the reaction to occur. In the reaction under investigation, the ammonium oxalate reacts with iodine forming ammonium iodide and releasing carbon dioxide.

(NH₄)₂C₂O₄(aq) + I₂(aq) 2 NH₄I(aq) + 2 CO₂(g)

The oxalate ion is oxidized to CO₂ and the I₂ is reduced to iodide.

Trials in which the type of light, iodine source, and oxalic acid source varied were conducted and observations of the color changes were made. After the addition of iodine (step 4) the color was orange. Results were as follows:

Sunlight lightened the color to a faint yellow (reagent grade oxalic acid) or colorless (Zud cleanser) after two hours.
Fluorescent light caused no change (reagent grade oxalic acid) or light yellow color (Zud cleanser) after six hours.
Incandescent light did not cause any change after six hours.
Samples kept in the dark remained orange.
Tincture of iodine and an iodine-alcohol solution gave identical results.

TIPS

Reagent grade oxalic acid, C₂H₂O₄·2H₂O, does not dissolve easily in water.
Some difficulties obtaining consistent color changes occurred in tests on a cloudy day.
When two consecutive lab periods are not available, one class can read the results from the solutions prepared by the preceding class.
For additional study students might investigate the effect of the concentration of iodine upon the time required for the reaction to occur or the effect of the concentration of iodine upon the appearance of the products. It was found that when excess amounts of the iodine were added to the saturated solution of oxalic acid, colorless crystals eventually precipitated from the solution.

REFERENCES

Alyea, H.N. and Dutton, F.B., Tested Demonstrations in Chemistry, 1962, p. 82.

Pickles, A., J. Chem. Ed. 23, 347 (1935).

Submitted by Robert Kemnitz, S. Margaret Suerth, Irene Walsh, and Doug Wilbur

Woodrow Wilson Leadership Program in Chemistry

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The Woodrow Wilson National Fellowship Foundation

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