Demonstration 2: Extraction of Iron from Cereal: The Oxidation States of Iron

Purpose

To extract iron from cereal and determine its oxidation state. Elemental iron in the form of minute iron filings can be extracted from cereal with a magnet. To dissolve the extracted iron in simulated stomach acid and determine its oxidation state. Whe dissolved in 0.1 M HCl, both the Fe(II) and Fe(III) oxidation states are obtained.

Materials

• Nabisco Instant Cream of Wheat (individual serving packets work best)
• Beakers
• Test-tubes
• Magnetic stir bar and stirrer
• 0.1 M HCl [simulated stomach acid; 4 mL conc. HCl (12 M) to 500 mL solution]
• 0.1 M KSCN Potassium thiocyanate [Add 0.97 g KSCN to water and dilute to 100 mL]
• 0.1 M K₄Fe(CN)₆ Potassium ferrocyanide [Add 3.7 g K₄Fe(CN)₆ to water and dilute to 100 mL]
• 0.1 M K₃Fe(CN)₆ Potassium ferricyanide [Add 3.3 g K₃Fe(CN)₆ to water and dilute to 100 mL
• 0.1 M Fe(II) Salt such as FeSO₄ or Fe(NH₄)₂(SO₄)₂ [Add 0.01 mole of either to water and dilute to 100 mL]
• 0.1 M Fe(III) Salt such as FeCl₃ or Fe(NO₃)₃ (Add 0.01 mole of either to water and dilute to 100 mL

Safety

Use normal safety precautions.

Procedure

Show boxes of cereals and note the ingredient listing for iron. This listing could read "iron," "elemental iron," "reduced iron," or some iron compound [usually Fe(II)]. "Reduced iron" is elemental iron. Explain to students that in order for iron to be absorbed in the intestines, it must be in the Fe(II) state. Ask them how we could get iron out of the cereal. What forms of iron could be removed with a magnet?

Extract the iron by crushing and stirring the dry cereal with the stir bar for about 5-10 min. Extract the stir bar from the cereal. Let students examine it. They will note the iron "whiskers" clinging to the bar. Place the stir bar in a beaker with 100 mL 0.1 M HCl. Explain that the sample has to stand several days in order for the iron to dissolve. Compare the pH of the HCl to that of the stomach. Compare the conditions of the demonstration with that of iron in the stomach, e.g., temperature, agitation, and time in the HCl solution. The stomach empties about every 2-3 hr.

Cover the beaker and examine it every day for a week. Point out that the iron has dissolved. Ask what compound is present. Students will say iron chloride. But which one?

In test-tubes, show the reactions of known iron salts with the reagents, making a table as shown. Using these results, determine the oxidation state of iron dissolved in the acid.

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