Abstract

Objective

The goal of this project is to test different types of salt to see if they contain iodine, an essential micronutrient.

Introduction

"Iodine is an element that is critical for normal function of the thyroid gland, which is a key regulator of the body's basic metabolic rate. Iodine is a micronutrient, meaning we require only small amounts of it. For example, adults need about 150 micrograms of iodine in the form of iodide ion per day. However, an insufficient supply of iodide via diet and drinking water causes the non-cancerous enlargement of the thyroid gland known as goiter. Prolonged lack of iodide can lead to loss of thyroid function and the birth defect known as cretinism, which has been recognized since the Middle Ages. Iodide ion, in the form of potassium iodide, may be added to table salt to produce "iodized salt" in order to easily provide the population with a sufficient dietary supply of this essential nutrient. One difficulty with this is that iodide ion is easily oxidized to iodine by atmospheric oxygen" (Wright, 2007). The chemical equation below shows the oxidation of iodide to iodine:

"An accumulation of iodine in a box of table salt would result in the salt's becoming yellow to red in color and the development of a very noticeable bad taste. To avoid this problem, a reducing agent, typically dextrose (C₆H₁₂O₆) is added to reduce back to colorless iodide any iodine that may be formed" (Wright, 2007). The chemical equation below shows the reduction of iodine to iodide by dextrose:

In this project, you will test various samples of salt to determine whether they contain this essential micronutrient. In the test, hydrogen peroxide (H₂O₂) reacts with iodide ion present in the salt sample:

Starch is also added to the testing mixture, so that any iodine produced will form a blue starch-iodine complex. You will use the colored starch-iodine complex as an indicator, identifying the presence (or absence) of iodine in various types of salt.

Terms, Concepts, and Questions to Start Background Research

To do this project, you should do research that enables you to understand the following terms and concepts:

• Oxidation
• Reduction
• Iodine
• Iodide ion
• Hydrogen peroxide
• Vinegar
• Dextrose
• Soluble starch

Questions

• Look at the ingredients list on a container of plain (non-iodized) salt. What other ingredients are included besides salt? What do these ingredients have to do with the Morton Salt® slogan "When it rains, it pours?"
• Compare the ingredients on a container of plain table salt to the ingredients in iodized table salt. Which ingredients are included only in iodized table salt? What is the purpose of the ingredients found only in iodized salt?
• What is the purpose of the hydrogen peroxide in the test for iodide?
• What is the purpose of the vinegar in the test for iodide?
• Assuming iodized table salt contains 0.006% iodide by mass, how much iodized salt would a person need to consume each day in order to get the recommended 150 micrograms of iodide?
• Seawater contains approximately 32 g of total dissolved salts per liter, including about 60 micrograms of iodide. Why is it undesirable to evaporate seawater completely to dryness and use the salt to supply our daily 150 micrograms of iodide?

Bibliography

• This project is from Classroom Activity #92, from the Journal of Chemical Education, and was written by Stephen W. Wright of Pfizer Global Research and Development:
  Wright, S.W., 2007. "Testing for Iodide in Table Salt," Journal of Chemical Education 84 (No. 10, October): 1616A–1617A.
• Linus Pauling Institute. (2003, April; updated 2010, March). Micronutrient Information Center: Iodine. Retrieved June 30, 2010, from http://lpi.oregonstate.edu/infocenter/minerals/iodine/index.html
• UNICEF. (2008, October 1). Micronutrients - Iodine, Iron and Vitamin A. Retrieved June 30, 2010, from http://www.unicef.org/nutrition/index_iodine.html
• Salt Institute. "Salt Frequently Asked Questions," [accessed June 24, 2010] http://www.saltinstitute.org/About-salt/Salt-FAQs.

Materials and Equipment

To do this experiment you will need the following materials and equipment:

• Distilled water
  • Tip: using tap water can produce erratic results.
• Iodine antiseptic solution
  • Use either tincture of iodine or povidone-iodine solution, found in the first aid section of grocery stores and drugstores.
• 3% hydrogen peroxide (H₂O₂) solution
• White vinegar
• Laundry starch solution (e.g. Linit® brand laundry starch)
• Plastic measuring spoons (or graduated cylinders)
• 7 disposable plastic cups (10 ounce or larger)
  • Tip: In order to see the iodine-starch reaction, the cups should be clear plastic or colored plastic with white interior.
• Disposable plastic spoons
• Different types of salt, for example:
  • Plain (non-iodized) table salt
  • Iodized table salt
  • Pickling salt
  • Rock salt (filter any dirt from the solution before testing)
  • Kosher salt
  • "Lite" salt
  • Sea salt
  • Tip: some nations permit the use of potassium iodate (KIO₃) as an iodine supplement in iodized salt. The advantage of potassium iodate over iodide is that the iodate ion is indefinitely stable in air. The procedure used in this project will not detect iodine added as potassium iodate. The U.S. Food and Drug Administration (FDA) has not approved the use of potassium iodate as a food additive in the U.S.

Experimental Procedure

1. Do your background research so that you are knowledgeable about the terms, concepts, and questions, above.

   Positive Control: Iodine-Starch Reaction

2. Pour 1/2 cup (120 mL) of distilled water in 10 ounce (or larger) plastic cup.
3. Add 1/2 teaspoon (2.5 mL) of starch solution.
4. Add several drops of iodine antiseptic solution and stir well with a clean, disposable plastic spoon.
5. What do you observe?

   Testing Various Types of Salt for the Presence of Iodide

6. Place 4 tablespoons (about 80 g) of salt in a 10 ounce or larger plastic cup.
7. Add 1 cup (240 mL) of distilled water and stir well for about a minute with a clean, disposable plastic spoon. Not all of the salt will dissolve, but any iodide present in the salt will dissolve.
8. Add 1 tablespoon (15 mL) of white vinegar.
9. Add 1 tablespoon (15 mL) of 3% hydrogen peroxide (H₂O₂).
10. Add 1/2 teaspoon (2.5 mL) starch solution.
11. Stir the mixture well with the disposable plastic spoon, and then let it stand for a few minutes. Does a color form?
12. Repeat steps 6–11 using different types of salt. Which types have detectable amounts of iodide?

Variations

• The effect of pH on the rate of the reaction may be shown by setting up a mixture with iodized salt in which the vinegar is omitted. The starch-iodine color develops very slowly, or not at all, and can require 12–24 hours or more.
• The effect of added reducing agents upon the rate of the oxidation reaction may be shown by adding 1–2 teaspoons of corn syrup to the iodized salt solution. Corn syrup is primarily a solution of dextrose (glucose) and fructose in water. The starch-iodine color develops more slowly in this case.
• For another Science Buddies project involving the exciting color-forming interactions of iodine and starch, see "Investigate the Kinetics of the Amazing Iodine Clock Reaction."

• For more science project ideas in this area of science, see Cooking & Food Science Project Ideas.

Credits

This project is from Classroom Activity #92, from the Journal of Chemical Education, and was written by Stephen W. Wright of Pfizer Global Research and Development:
Wright, S.W., 2007. "Testing for Iodide in Table Salt," Journal of Chemical Education 84 (No. 10, October): 1616A–1617A.