Red, White and Blue Density Column

Introduction

Create a beautiful, red, white and blue density column to demonstrate the salting-out effect, relative density and miscibility of organic solvents with water. The density column contains three liquid layers consisting of toluene, methyl alcohol and water. Methyl alcohol is “salted out” and separated from water by adding potassium carbonate. The water is dyed blue with copper(II) sulfate, and the toluene is colored red by adding Sudan III. Why don’t the methyl alcohol and toluene mix?

Concepts

• Miscible and immiscible liquids
• Salting-out effect
• Density
• Hydrogen bonding

Materials

Safety Precautions

Methyl alcohol is a flammable solvent and a dangerous fire risk. It is toxic by ingestion and may cause blindness. Keep away from flames and heat. Toluene is a flammable liquid and is moderately toxic by ingestion. Potassium carbonate is a corrosive solid. Perform this demonstration in a hood or in a well-ventilated lab only. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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 three-layered liquid may be stored for several weeks in a sealed bottle. To dispose of the liquids, pour the top-most organic layer (toluene) into a shallow container and allow the solvent to evaporate according to Flinn Suggested Disposal Method #18b. The remaining liquid layers may be disposed of down the drain with plenty of cold running water according to Flinn Suggested Disposal Method #26b.

Procedure

1. Obtain 75 mL of distilled water in a 250-mL beaker and add 75 mL of methyl alcohol. There should be one liquid layer because methyl alcohol is miscible with water.
2. Weigh approximately 50 g of potassium carbonate into a large weighing dish or cup.
3. Gradually add the potassium carbonate to the methyl alcohol–water solution. Stir the mixture with a stirring rod to dissolve the solid.
4. As the potassium carbonate dissolves in solution, two layers will begin to separate in the liquid. Continue adding potassium carbonate until the lower aqueous layer is saturated (undissolved solid remains).
5. Using a pipet, remove the upper layer of methyl alcohol and place it in a clean, 150-mL beaker. Repeat as needed to remove all of the methyl alcohol.
6. Decant the aqueous potassium carbonate solution into a clean, 150-mL beaker. (Do not transfer any solid.)
7. Add about 3 microspatulas of copper(II) sulfate pentahydrate crystals to the aqueous potassium carbonate solution. Stir to dissolve—the final solution should be clear and blue. (There may be some undissolved solid in the bottom of the beaker.)
8. Obtain 75 mL of dry toluene in a 150-mL beaker. Add about 2 microspatulas of solid Sudan III dye and stir to dissolve. The solution should be clear and red.
9. Slowly pour the three solutions into the 240-mL glass bottle in the following order:

• • Blue aqueous solution (contains potassium carbonate and copper sulfate)
  • Colorless methyl alcohol solution
  • Red toluene solution (contains Sudan III)

10. The result is a three-layer liquid. The water layer at the bottom is blue (CuSO₄ and K₂CO₃), the middle layer is “water white” or colorless (methyl alcohol), and the top layer is red (toluene and Sudan III).

Teacher Tips

• This kit contains enough materials to prepare at least one demonstration bottle. (There is at least a 20% excess of each reagent.) The three-layered liquid density column will keep for several weeks. Remove the polyethylene inner liner from the polypropylene cap supplied with the kit. Cap the bottle to prevent leakage and store the sealed bottle for display in a secure location where the bottle will not accidentally be tipped over.
• Removing the methyl alcohol from the aqueous solution by pipet in step 5 rather than simply decanting gives better separation of the aqueous and alcohol layers.
• Additional dyes may be used to achieve a red–yellow–green “traffic light” density column. Add one microspatula of methyl red indicator to the methyl alcohol layer after it has been salted out and separated (step 5). This will give a clear yellow solution for the middle layer. Add about one-half microspatula of potassium ferrocyanide [K₄Fe(CN)₆] as needed to the blue aqueous layer to give it a nice green color. The result is red–yellow–green from top to bottom.
• The density of pure methyl alcohol (d = 0.791 g/mL) is less than that of toluene (d = 0.866 g/mL). In this three-layer density column, the toluene is the topmost (least dense) layer because the methyl alcohol contains dissolved potassium carbonate, which increases the density of the solution.
• Replacing methyl alcohol with ethyl alcohol does not give a three-layer liquid because toluene and ethyl alcohol are miscible (step 9). Ethyl alcohol can be used, however, to demonstrate the “salting out” effect with potassium carbonate (steps 1–4). Two layers will form when potassium carbonate is added to 50% ethyl alcohol.

Discussion

Making a three-layered liquid demonstrates (1) the miscibility of methyl alcohol and water; (2) the “salting out” effect that results when a salt is added to an aqueous organic solution; (3) the immiscibility of toluene and methyl alcohol because toluene is a nonpolar solvent; and (4) the relative densities of the three solutions. Adding suitable dyes or indicators to the different liquid layers makes the phase boundaries clearly visible and creates a beautiful red, white and blue density column.

The ability of an organic compound to dissolve in water is drastically reduced when an inorganic salt (many different salts will work) is added to the water. This effect, called salting out, occurs because the water molecules bind strongly to the inorganic cations and anions in the salt solution, and are thus unavailable for solvating the organic compound. Salting out thus decreases the solubility of organic substances in the aqueous phase. This principle is used in two different ways to isolate or purify an organic compound from a reaction mixture. If the reaction mixture is an aqueous solution, salt is generally added to extract or separate the organic component into a separate layer. (This is similar to what happens in step 4 of the Procedure.) Alternatively, an organic solution obtained by preliminary extraction of a reaction mixture with water is generally “washed” with saturated sodium chloride solution in the last step. Washing the organic solution with saturated salt solution will remove most of the water dissolved in the organic solvent.

References

This activity was adapted from Chemistry of Organic Compounds, Flinn ChemTopic™ Labs, Volume 19; Cesa, I., Editor; Flinn Scientific Inc.: Batavia, IL (2006).