Five Colors from One Solution
- Page ID
- 128813
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Required Training |
Required PPE |
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UC Lab Safety Fundamentals |
Lab coat, safety glasses/goggles, nitrile gloves |
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Performers Required: 1 |
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Equipment |
Chemicals |
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Five 100 mL beakers |
Phenolphthalein in ethanol solution |
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Disposable pipettes |
Sodium carbonate (Na2CO3), 5% solution |
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Iron (III) chloride (FeCl3), 50% solution |
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Ammonium thiocyanate (NH4SCN), 30% solution |
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Potassium ferrocyanide (K4[Fe(CN)6]), 5% solution |
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Procedure:
- To prepare this demonstration, add the following to five separate beakers:
- 50 mL water and 1 drop phenolphthalein solution
- 1 drop of Na2CO3 solution
- 1 drop of FeCl3 solution
- 1 drop of NH4SCN solution
- 1 dro of K4[Fe(CN)6] solution
- Pour the contents of beaker #1 into #2, then from #2 into #3, then from #3 into #4, and finally from #4 into #5. There will be a distinct color change with each successive step; #1 is colorless, #2 is fuchsia (basic phenolphthalein), #3 is yellow (acidic Fe3+(aq)), #4 is red (Fe(SCN)2+ species), and #5 is deep blue (Prussian blue).
Clean-up: The contents of beaker #5 can be diluted with water and rinsed down the drain.
Hazards: Ethanol solutions are flammable, and should be kept away from ignition sources. Solutions of FeCl3 are corrosive to metals. Phenolphthalein, FeCl3, and NH4SCN are toxic if swallowed, and phenolphthalein is a potential carcinogen and reproductive hazard. The Prussian blue present in beaker #5 will stain skin and clothing.
Principle: This demonstration uses an indicator and several different reaction products to produce five different colors from a single solution. The indicator phenolphthalein is colorless in neutral solution (beaker #1), but turns fuchsia in the presence of the basic Na2CO3 solution in the second beaker. In the third beaker the H3O+ ions produced by the hydrolysis of the iron(III) salt bind the OH- ions from the Na2CO3 solution, leading to decolorization of the phenolphthalein; at the same time the solution turns yellow due to the presence of the hydrolyzed iron(III) species:
[Fe(H2O)6]3+ + H2O → [Fe(H2O)5(OH)]2+ + H3O+
[Fe(H2O)5(OH)]2+ + H2O → [Fe(H2O)4(OH)2]+ + H3O+ etc
In the fourth beaker iron (III) salts form deep red complexes such as [Fe(SCN)(H2O)5]2+ with the SCN- ions. The extreme stability of colloidal Prussian blue (KFeIII[FeII(CN)6]) dominates in the fifth beaker, so that the deep blue color brings the series to a close. Deviations from the given concentrations can lead to slight differences in the effects due to the formation of precipitates or mixed colors.
Notes: Ensure the thiocyanate solution is fresh (< 3weeks old), as it slowly decomposes and gives muddy brown precipitates instead of the desired deep red color.