Vol. 1, No. 1
The Chemical Educator
© 1996 Springer-Verlag New York, Inc.		                              

ISSN 1430-4171
http://journals.springer-ny.com/chedr
S 1430-4171(96)01009-6

The Salting Out of Ethanol and Water: A Colorful Illustration of Intermolecular Forces

Eugene T. Smith
Department of Chemistry
Florida Institute of Technology
Melborne, FL 32901-6988, USA
Smithg@roo.FIT.edu

This demonstration can be used to introduce or illustrate the concept of intermolecular forces.

Water and ethanol are made immiscible by the addition of potassium carbonate. This effect can be visualized in a large lecture hall through the initial addition of a few drops of bromothymol blue and a drop of acid to a water/ethanol solution. After addition of potassium carbonate, the yellow solution separates into two phases, a colorless aqueous and blue organic phase. This demonstration can be used to introduce or illustrate the concept of intermolecular forces.

The process of salting out, the separation of an organic phase from an aqueous phase by the addition of a salt has been known for nearly a century [1]. This method is commonly used by biochemists in the purification of proteins [2, 3]. Weak intermolecular forces (e.g., hydrogen bonds) between organic molecules or nonelectrolytes and water are easily disrupted by the hydration of electrolytes. Water and ethanol, which are soluble in all proportions (i.e., miscible), are made immiscible by the addition of potassium carbonate in a variation of a demonstration previously described [1].

Two test robes with stoppers, l0 mL each of ethanol and water, a few grams of potassium carbonate, a few drops of bromothymol blue indicator, and a drop of 1 M HCI are required. Ethanol, water, pH indicator and hydrochloric acid are combined in the first test robe to form a single-phase yellow solution. Potassium carbonate is added to the second test tube. The contents of the first test robe are added to the second test robe, and the mixture is stoppered and shaken. The solution will turn blue and separate into two phases, a colorless aqueous and blue organic phase. A number of test robes containing the reactants are distributed throughout a lecture hall, and the students are allowed to mix the reactants themselves. In this manner, the students get a better view of the salting-out process. An alternative approach is to scale up the reaction in two large graduated cylinders.

This demonstration, which takes only a few minutes to set up, can be used to illustrate or introduce the concept of intermolecular forces, as well as to illustrate an acid/base reaction. Some questions to be pondered by the students include the following:

1. Which phase is water, which is alcohol? Why?
2. Is the separation of water and alcohol complete?
3. Could other liquid pairs or salts be used?
4. Could a dye be found that would wind up in the water rich phase? Why?

The intermolecular forces between water and ionic compounds (e.g., ion-dipole) in the aqueous phase, and the intermolecular forces between the organic dye and ethanol (e.g., dispersion) in the organic phase are discussed. It is important to note that the color change of the pH indicator is attributed to the fact that potassium carbonate acts as a base in water, and has nothing to do with the salting out effect. The pH indicator may also be substituted with food coloring.

Handling and Disposal: The waste mixture from this demonstration can be safely flushed down the drain with water.

REFERENCES

  1. Shakhashiri, B. Z. Chemical Demonstrations; University of Wisconsin Press: Madison, WI., 1983; Vol. 3, p. 266.
  2. Lehninger, A. L. Principles of Biochemistry; Worth: New York, 1982; p. 144.
  3. Zubay, G. Biochemistry; Addison Wesley: Reading, MA, 1983; p. 51.