Element of the Month - Iodine

"In honor of the International Year of the Periodic Table this series of articles details the Element of the Month project developed by Stephen W. Wright (SWW), Associate Research Fellow at Pfizer Inc., and Marsha R. Folger (MRF), chemistry teacher (now retired) at Lyme – Old Lyme High School in Connecticut. Read The Element of the Month - An Introduction for an overview of the project and links to the other articles in the series." - Editor 

Iodine is the Element of the Month for January in our program. January is usually a no-fun month for high school students. The fall and winter holidays are over, and the school year seems to stretch on interminably. In many areas, the winter weather brings rain, snow, overcast skies, or unwelcome cold. The inclusion of iodine as the Element of the Month allows for some color and fun to be injected into the classroom at this time. Beyond its occasional use as a disinfectant, the students will have little familiarity with iodine. Despite this, by now the students are generally quite enthusiastic about the Element of the Month program and the posters produced for iodine are some of the most colorful and well executed that we have seen.

Occurrence in Nature 

Iodine is a scarce element and does not generally occur in mineral deposits. It is found in seawater at low concentrations, in seaweeds and kelp, in marine organisms, and in some salt deposits. Nevertheless, it is found in the human body in the hormone thyroxin, which is synthesized by the thyroid gland and is critical for metabolic control.

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Figure 1: Ampoule of solid Iodine

Uses

Iodine is critical for life because of its use by the thyroid gland, which scavenges iodide ion very efficiently from the exceedingly low concentrations in the environment. We note that iodine deficiency can cause goiter, thyroid disease and growth and metabolism disorders. We briefly discuss the importance of thyroxin and the frequency of thyroid – related diseases, hyperthyroidism and hypothyroidism. We explain that iodine is supplied in the diet by seafood and by the use of table salt supplemented with iodide ion (“iodized”), a public health measure that has greatly reduced the frequency of hypothyroid disease in inland areas. The human body contains about 50 mg of iodine. Iodine is also used directly in disinfectant solutions and in the synthesis of organic chemicals and intermediates. On the desk we display a bottle of an iodine disinfectant solution, an empty medicine bottle, and a box of iodized table salt.

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Figure 2: Iodine vapor in a round bottom flask

Physical Properties

We note that iodine is a non – metallic, very dark violet crystalline solid. In fact, its name comes from the Greek word for “violet”. We show the class a sample of elemental iodine sealed in a glass tube (figure 1). When heated, iodine typically sublimes from the solid phase to the gas phase rather than melting and passing through a liquid phase. We show this by sprinkling a few crystals of iodine into a 1 L or larger flask and briefly heating the flask over a flame or heat gun. The flask will quickly fill with a beautiful violet vapor (see figure 2).¹ Iodine is insoluble in water, but dissolves in other liquids like alcohols and hydrocarbon solvents. This behavior is generally unfamiliar to students, whose experience with dissolution has been largely limited to water soluble ionic solids and polar substances such as sugar. We add a few iodine crystals to a beaker of water and to a beaker of ethyl alcohol and note the colors produced.² If desired, a third beaker containing some mineral spirits may also be used. Next, we add some potassium iodide or sodium iodide to the beaker containing the water and observe that suddenly the iodine is now dissolving. We question the class as to why this may be and guide them to the idea that a chemical reaction must have occurred. On the board, we briefly explain the following reaction:

NaI _(aq) + I_{2 (s)} → NaI_{3 (aq)}

This reaction, along with the use of ethyl alcohol as a co-solvent, is used to dissolve iodine in the preparation of the disinfectant tincture of iodine.³

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Figure 3: Iodine reacts with starch in a slice of bread

Chemical Properties

First, we remind the students that the customary test for iodine is the starch test, which they will often remember from an earlier biology class. We demonstrate this by adding a drop of the iodine solution we just prepared to some starch solution in a large test tube, and note the deep blue color. We remind them that any starch source will do, and conversely the test for starch is to add iodine. We show this by marking a slice of white bread with the iodine solution, using a glass stirring rod to write the formula “I₂” on the bread (see figure 3). Next we explain that iodide salts are very easily oxidized by oxygen in air to afford elemental iodine, writing the equation for the reaction on the board and noting that the reaction not only requires oxygen but also hydrogen ions:

2 I^- _(aq) + 2 H⁺ _(aq) + O_{2 (g)} → 2 I_{2 (aq)} + 2 H₂O _(l)

The reduction of iodine to iodide is also possible, by the addition of an appropriate reducing agent. To show this, we add some Vitamin C to the iodine solution we prepared previously. The iodine color disappears and a starch test is negative. Next, we pick up the box of iodized salt and discuss the ingredients listed on the box. Salt is the first ingredient as expected. There is usually an anti-caking agent, often sodium silicoaluminate. Next there is a substance, generally dextrose, included, followed lastly by potassium iodide. We ask why the dextrose is included in the salt, and guide the students to the concept that it is included to reduce any iodine that may be formed and prevent the discoloration of the salt. Also, the sodium silicoaluminate is weakly basic and this helps suppress the oxidation of the potassium iodide by raising the pH slightly and disfavoring the availability of hydrogen ions. Having discussed the ingredients in the table salt, we can test the table salt to see if it really is “iodized” or not. To emphasize the “kitchen chemistry” aspect of this experiment, we use kitchen utensils and household “reagents”.⁴ We stir 4 tablespoons of iodized table salt in one cup of water, acidify with one tablespoon of vinegar (making note that this weak acid provides the needed hydrogen ions), and then add one-half teaspoon of laundry starch solution. Lastly we add two teaspoons of supermarket 3% hydrogen peroxide, noting that it is easier and faster than trying to bubble oxygen through the mixture. The development of a blue color shows that indeed the salt was iodized (see figure 4). 

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Figure 4: Testing for Iodide in Table Salt, A) testing set up, B) salt, vinegar & starch have been stirred into water, C) after hydrogen peroxide has been added to the solution

Next, we remind the class that iodine, being a non-metal and a halogen, will combine with most metals to form iodides. We remind the class of the reaction they saw in December between the metal zinc and the non-metal sulfur (See Element of the Month - Sulfur). We demonstrate the reaction of iodine with zinc by combining two grams of 20 – 30 mesh granulated zinc with seven grams of iodine in a large test tube clamped to a ring stand.⁵ A 1 L flask is then clamped upside down over the mouth of the test tube to contain most of the iodine vapor evolved, and the reaction is initiated by the addition of a little water into the test tube. Occasionally a student will realize that this reaction was shown in the 1939 production of “The Wizard of Oz” as a special effect. We note that almost all the metallic elements form colorless, water soluble iodide salts, with the exceptions being lead, silver, copper, and mercury. This is shown to the class by adding some potassium iodide solution to large test tubes containing some dilute lead nitrate or lead acetate solution, some dilute silver nitrate solution, and some cupric sulfate solution. At the time we started these experiments, we were also able to include dilute mercuric chloride solution to show the precipitation of bright orange mercuric iodide but mercury compounds are now prohibited from the school. The precipitation of mercuric iodide was all the more interesting because further addition of potassium iodide causes the HgI₂ to dissolve, much to the surprise of the class. This is due to the formation of the highly water soluble complex ion HgI₄^2-, and this offered an opportunity for further discussion.⁶

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Figure 5: A) Test tubes containing (left to right) dilute solutions of lead I nitrate, silver nitrate, copper II sulfate and mercuric chloride. B) Potassium iodide solution has been added to all of the test tubes seen in A.

Lastly, we admit that a significant reason why iodine is included as an Element of the Month is because it participates in many fun and colorful reactions. We end the class with a demonstration of the “Vitamin C clock reaction”,⁷ followed by the Briggs – Rauscher “Oscillating Clock Reaction”.⁸

Vitamin C Clock Reaction:

Solution A: Dissolve 2.55 g of KI and 0.18 g of ascorbic acid in 400 mL of water and add 2 mL of glacial acetic acid and 2 mL of 1% w/v starch solution.

Solution B: Combine 150 mL of 3% hydrogen peroxide with 250 mL of water.

Combine both solutions in a 1 L beaker and stir by hand for a few seconds.

Oscillating Clock Reaction:

Solution A: Dilute 410 mL of 30% hydrogen peroxide to 1000 mL with water. Prepare immediately before use (can also dilute 344 mL of 30% hydrogen peroxide to 1000 mL with 3% hydrogen peroxide).

Solution B: Dissolve 43 g of potassium iodate, KIO₃, in 1000 mL of water and add 5 mL concentrated H₂SO₄.

Solution C: Dissolve 16 g of malonic acid, 3.4 g of MnSO₄ monohydrate, in 700 mL of water, add 30 mL of 1% w/v starch solution, and dilute to 1000 mL with water.

Combine 200 mL of each solution in a 1 L Erlenmeyer flask with continuous gentle magnetic stirring.

References and Notes:

1. For an early example of this demonstration see Lippy, John D. Jr. Chemical Magic; A. L. Burt Co.: New York, 1930, pp 29.
2. Summerlin, Lee R.; Ealy, James L. Jr. Chemical Demonstrations: A Sourcebook for Teachers; American Chemical Society: Washington, DC, 1985; pp 34.
3. Tincture of iodine is prepared from 20 g of sodium iodide and 24 g of iodine in enough 50% (v/v) aqueous ethanol to afford one liter of solution.
4. This was published as Classroom Activity 92, see Wright, S. W. J. Chem. Educ., 2007, 84 (10), 1616A – 1616B. Note that not all of the table salt will dissolve. Tom Kuntzleman published a video on ChemEd X (Nov 2013) that shows readers how the activity is done. Tom Kuntzleman also published a demonstration on ChemEd X, An Orange to Black Halloween Chemistry Demonstration You Can Do with Household Items, (October 2015) including video that uses the same reactions.
5. See Shakashiri, Bassam A. Chemical Demonstrations: A Handbook for Teachers, Vol. 1; University of Wisconsin Press: Madison, WI, 1983; pp 49-50.
6. A similar experiment involving the precipitation of silver ion by iodide ion, and the dissolution of AgI in excess iodide, may be found in: Shakashiri, Bassam A. Chemical Demonstrations: A Handbook for Teachers, Vol. 1; University of Wisconsin Press: Madison, WI, 1983; pp 293-298. While we have not performed this experiment to our class, we see no reason why it should not be a satisfactory substitute for the mercuric iodide chemistry that we presented.
7. This was published as JCE Classroom Activity 41: Tick Tock, a Vitamin C Clock, see Wright, S. W. J. Chem. Educ., 2002, 79 (1), 40A-40B. See also Wright, S. W. J. Chem. Educ., 2002, 79 (1), 41-43. Alternately, the Landolt clock reaction between potassium iodate and sodium bisulfite may be presented, see: (a) Shakashiri, Bassam A. Chemical Demonstrations: A Handbook for Teachers, Vol. 4; University of Wisconsin Press: Madison, WI, 1992; pp 16-25; (b) Summerlin, Lee R.; Ealy, James L. Jr. Chemical Demonstrations: A Sourcebook for Teachers; American Chemical Society: Washington, DC, 1985; pp 75-76.
8. (a) Shakashiri, Bassam A. Chemical Demonstrations: A Handbook for Teachers, Vol. 2; University of Wisconsin Press: Madison, WI, 1985; pp 249-256; (b) Summerlin, Lee R.; Ealy, James L. Jr. Chemical Demonstrations: A Sourcebook for Teachers; American Chemical Society: Washington, DC, 1985; pp 82.

Editor's Note: Readers may be interested in reading Wright, S.W., My Favorite Element - Iodine, J. Chem. Educ., 2009, 86 (10), 1137.