Iodide is often added to salt to prevent the salt from caking. The Morton marketing slogan “when it rains, it pours” refers to the ability of their salt not to cake in damp weather because iodide had been added. In addition to preventing caking, the presence of iodide in salt also meets an important nutritional requirement.
Iodide (I) oxidizes and becomes iodine (I2) when exposed to air as follows:
4H++ 4I- + O2 Ü 2I2 + 2 H2O
If this reaction occurs in salt, the iodine would discolor the salt and give it a bad taste. If you read the label of some salts, you may wonder why sugar is listed as an ingredient. Sugars are reducing agents that prevent iodide from being oxidized into iodine. In this experiment, you will be able to circumvent the presence of the sugar. If Iodide is present, it will be converted to iodine. Starch will react with the iodine by turning a dark blue. Once this happens, you will know whether iodide was present in the salt.
Why are oxidation and reduction reactions critical to the stability of the iodide in salt?
- Distilled water
- Different types of salt (table salt, rock salt, Himalayan or Celtic salt, kosher salt, etc.)
- Linet brand laundry starch (available in craft stores, some sewing stores and very good grocery stores)
- Hydrogen peroxide
- Disposal transparent cups (disposal party cups are good)
- Graduated cylinder
- Disposable plastic spoons
- White vinegar
- Tincture of iodine or povidone-iodine solution (available at the pharmacy or first aid section of the grocer’s).
Part I – Testing the Indicator
- Pour 120 ml of distilled water into a disposable cup. Add 0.5 teaspoons of the starch solution
- Using an eyedropper, add several drops of the tincture of iodine or povidone-iodine soution. A distinct reaction should occur. A more muted version of this reaction will occur whenever there is iodide in the salt samples you will be testing. Be careful to set your sample aside and inspect it five minutes after adding the starch because this reaction may occur more slowly.
Part II – Testing the Salt
- Measure four tablespoons of salt and place into a disposable cup. Add 250 ml of distilled water and stir well to dissolve as much of the salt as possible.
- Add one tablespoon of vinegar and one tablespoon of hydrogen peroxide to the salt water.
- Add 0.5 teaspoon starch solution. Stir well. Set aside for several minutes. Record what, if anything, happens.
- Repeat steps 1-3 with each type of salt.