The Chemical Kidney

Introduction

Mix water from a large beaker back and forth into five other beakers. Your students will observe a variety of color changes until at the very end the large beaker contains “lemonade.”

Concepts

• Complex ions
• Precipitates

Materials

Safety Precautions

Ferric chloride is a skin/tissue irritant. Ammonium thiocyanate is toxic by ingestion and dangerous when heated to decomposition (170 °C) or when in contact with acids since fumes containing cyanides may be produced. Tannic acid is toxic by ingestion and inhalation, and is a suspected carcinogen. Oxalic acid is a skin and eye irritant, and is toxic by ingestion with an LD₅₀ of 375 mg/kg. Wear chemical-resistant gloves, a chemical-resistant apron and chemical splash goggles. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The resulting solution may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Procedure

1. Arrange five 1000-mL beakers in a row across the demonstration table. Set a sixth beaker behind the row.
2. Place 25 drops of ferric chloride solution into beaker 1.
3. Place 2 drops of ammonium thiocyanate solution into beaker 2 and 10 drops of ammonium thiocyanate solution into beaker 3.
4. Place 12 drops of tannic acid solution into beaker 4.
5. Using a 10-mL graduated cylinder, measure out 10 mL of oxalic acid solution and transfer it to beaker 5.
6. Fill the last beaker (beaker 6) with 600 mL of distilled or deionized water.
7. Pour the water from beaker 6 into beaker 1. Swirl to mix the contents. Pour the solution back into beaker 6. The solution should be pale yellow, like the color of lemonade.
8. Pour the solution from beaker 6 into beaker 2. Swirl to mix the contents. Pour the solution back into beaker 6. The solution should be orange-red, like the color of iced tea.
9. Pour the solution from beaker 6 into beaker 3. Swirl to mix the contents. Pour the solution back into beaker 6. The solution should be dark red, like the color of wine.
10. Pour the solution from beaker 6 into beaker 4. Swirl to mix the contents. Pour the solution back into beaker 6. The solution should be blue-black, like the color of grape juice.
11. Pour the solution from beaker 6 into beaker 5. Swirl to mix the contents. Pour the solution back into beaker 6. The solution should be yellow. Use your imagination to describe the final yellow solution!

Student Worksheet PDF

13021_Student1.pdf

Teacher Tips

• If the solution in beaker 1 is too pale, add a few more drops of iron(III) chloride solution.
• Be creative when doing this demonstration and make up a funny story to go along with each of the colors. Your story may include a very thirsty person ordering various drinks (lemonade, iced tea, red wine, grape juice) and then because he/she drank too much he/she had to go to the bathroom.
• Use only distilled or deionized water in order to obtain the best results.
• An alternate idea is to use various types of glasses (i.e., wine glass, beer mug) in place of the large beakers.

Answers to Questions

1. Note the initial contents of each beaker as well as the final color.

   Beaker 1—Contents: Ferric chloride solution. Color: Pale yellow
   Beaker 2—Contents: Ammonium thiocyanate solution. Color: Orange-red
   Beaker 3—Contents: Ammonium thiocyanate solution. Color: Dark red
   Beaker 4—Contents: Tannic acid solution. Color: Bluish-black
   Beaker 5—Contents: Oxalic acid solution. Color: Yellow

2. Write the following balanced chemical equations for the reactions that occurred in this demonstration. Note the formation of ferric tannate in beaker 4 and the iron(III) oxalate complex in beaker 5 are not included, due to their unknown compositions.
   1. The combination of iron(III) ions with water (beaker 1)

      Fe³⁺(aq) + H₂O(l) → [Fe(H₂O)₆]³⁺(aq)

   2. Hydrated iron(III) ions reacting with thicocyanate ions (beaker 2)

      [Fe(H₂O)₆]³⁺(aq) + SCN^–(aq) → [Fe(H₂O)₅SCN]²⁺(aq)

3. The same reaction occurs in beaker 2 as in beaker 3—beaker 3 reacts with the excess hydrated iron(III) ions from beaker 2. Why, then, is beaker 3 a different color?

   Beaker 3 is a different color than beaker 2 because of the higher concentration of iron(III) thiocyanate complex ions. The ions are reddish in color, but because there are more in beaker 3, the color of beaker 3 is a darker, deeper red than beaker 2.

Discussion

Beaker 1: Hydrated iron(III) ions, [Fe(H₂O)₆]³⁺, are present in the solution in beaker 1. The initial solution is orange at a concentration of 2 M. Upon dilution with 600 mL of water from beaker 6, the solution becomes pale yellow.

Beaker 2: Thiocyanate ions, SCN^–, are present in beaker 2 in a limited quantity. When mixed with an excess of hydrated iron(III) ions from beaker 1, a small amount of iron(III) thiocyanate complex ions are formed. Beaker 3: When additional thiocyanate ions from beaker 3 are added to beaker 2, they react with the excess hydrated iron(III) ions from beaker 2 which did not react in the previous step. The color of the solution becomes a deeper red as more of the iron(III) thiocyanate complex ions are formed. Beaker 4: When the mixture of iron(III) ions in beaker 3 is mixed with the tannic acid solution in beaker 4, ferric tannate is formed. The chemical formula of ferric tannate is not given because the composition is variable. The formation of ferric tannate imparts a bluish-black color to the final solution in beaker 4. Beaker 5: An iron(III) oxalate complex is formed when the iron(III) tannate solution in beaker 4 is mixed with the oxalic acid solution in beaker 5. This final complex is yellow in color. Note: The actual formula of this final complex is unknown, but the formula given above is likely. The iron(III) ions may also have waters surrounding them. In fact, by this point, so many components are present in solution that several of them may be contributing to the final yellow color.

References

Rohr, W., Eastchester High School, personal communication.

Shakhashiri, B. Z. Chemical Demonstrations, A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison, WI, 1983; Vol. 1, pp 341–343.

Summerlin, L. R., Ealy J. L. Chemical Demonstrations, A Sourcebook for Teachers, 1st Ed., American Chemical Society, 1985, Vol. 1.