Tests for Iron(II) and Iron(III) Ions
Introduction
A number of experiments are done by mixing drops of two or more reagents and recording the results. At the end of this lab, you will be able to identify the iron(II), Fe2+, and iron(III), Fe3+ ions in solutions.
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Procedure
- Put five drops of 0.1 M Fe(NH4)2(SO4)2, iron(II) ammonium sulfate, on an acetate sheet. Add one drop of 0.1 M K4Fe(CN)6, potassium ferrocyanide. Observe the color of the precipitate that first forms. Allow the precipitate to stand a few minutes and observe the color once more. Record observations.
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- Observations:
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- Put five drops of 0.1M Fe(NH4)2(SO4)2, iron(II) ammonium sulfate, on an acetate sheet. Add three or four drops of 0.01 M solution of KSCN, potassium thiocyanate. Record observations.
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- Observations:
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- Put five drops of 0.1M Fe(NH4)2(SO4)2, iron(II) ammonium sulfate, on an acetate sheet. Add one drop of 0.1M K3Fe(CN)6, potassium ferricyanide. Record observations.
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- Observations:
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- Put five drops of 0.1 M FeCl3, iron(III) chloride, on an acetate sheet. Add one drop of 0.1 M K3Fe(CN)6, potassium ferricyanide. Record observations.
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- Observations:
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- Put five drops of 0.1 M FeCl3, iron(III) chloride, on an acetate sheet, add one drop of 0.1 M K4Fe(CN)6, potassium ferrocyanide. Record observations.
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- Observations:
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- Put five drops of 0.1M FeCl3, iron(III) chloride, on an acetate sheet. Add two or three drops of 0.01 M KSCN, potassium thiocyanate. Record observations.
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- Observations:
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Questions
- A solution prepared by dissolving ferrous sulfate in water does not give a red color with KSCN when tested immediately. The solution does give a red color after standing two weeks. Explain this observation.
- Suppose you were only permitted to use two reagents instead of three to test for these iron ions. Which reagent would you eliminate from your kit? Explain.
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Safety
Wear safety goggles and aprons in the lab at all times. Do not ingest any chemicals and avoid contact with the skin.
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TG Answers
- The iron(II) air oxidized to iron(III). (Air oxidation is a common problem with solutions of Hg(I), Sn(II), Cu(I), etc.)
- Since the thiocyanate and the ferrocyanide both test for iron(III), eliminating one of these will still permit testing for both of the iron ions.
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TG Lab Hints
- You may have the students use acetate sheets or 24 well plates.
- Put the five solutions in pulled and labeled Beral pipets.
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TG Materials
- 0.1 M FeCl3 (weigh 5.4 g of FeCl3•6 H2O and dissolve in 25 mL distilled water to which 4 mL of concentrated HCl has been added. Dilute to 200 mL.
- 0.1 M K4Fe(CN)6 (dissolve 8.4 g of K4Fe(CN)6•6H2O in 200 mL of distilled water.)
- 0.1 M K3Fe(CN)6 (dissolve 6.6 g of K3Fe(CN)6 in 200 mL of distilled water.)
- 0.01 M KSCN (dissolve 0.19 g of KSCN in 200 mL of distilled water.)
- 0.1 M Fe(NH4)2(SO4)2 (dissolve 7.6 g of Fe(NH4)2(SO4)2•6H2O in 25 mL of distilled water to which 2 mL of concentrated sulfuric acid has been added. Dilute to 200 mL.)
- (Iron (II) sulfate may be substituted for iron(II) ammonium sulfate. Both must be made fresh because Fe2+ will oxidize to Fe3+.)
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