The Salting-Out Effect: A Colorful Demonstration That Leads to Student - Teacher Activities

Co-Authored by James W. Webb and Otis Rothenberger

Department of Chemistry, Illinois State University, Normal, IL

Demonstrations are often used to illustrate chemical concepts. However, they can be made to be entertaining as well as educational. The entertaining aspect gets the attention of the audience so that the chemistry of the demonstration can be described and understood.

One example is the “salting-out effect” in which an ionic salt is added to a solution of water and an organic miscible solvent. The ions from the salt interact more strongly with the water molecules than with the solvent effectively causing two layers. This effect has applications in organic chemistry as well as in biochemistry to isolate large biomolecules¹. Salting-out demonstrations have been described by Shakashiri² and Flinn³.

Our interest in capturing audience attention leads to a question. How do we make this demonstration more visual to catch the attention of the audience?

Tom Kuntzleman suggested a solution in his ChemEd X blog, Chemical Mystery #8: Go Blue! followed by his Solution to Chemical Mystery #8: Go Blue!.⁴ He added food dyes that are more soluble in one of the layers making the layers more visible. The advantage to this approach is that the teacher or demonstrator can choose appropriate dyes that would make the demonstration more interesting, e.g. school or holiday colors.

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As an example, we offer a modification to the Kuntzleman’s isopropanol/water demonstration. We add potassium carbonate (K₂CO₃), phenolphthalein, and starch to a solution of isopropanol and water. The potassium carbonate salts out the two liquids and turns the phenolphthalein red which dissolves in the isopropanol layer. The starch forms a white-cloudy solution in water resulting in red and white layers. Try it yourself! Add 12 mL water, 12 mL isopropanol and 3 drops of 1% phenolphthalein to a 30 mL (15 cm) test tube followed by 3 grams potassium carbonate and 1 gram corn starch. Shake well and let stand for about 3 minutes or until the layers are observed.  

In our experience, a good visual demonstration can capture the interest of the students. This interest can lead questions regarding the chemistry of the demonstration. Student-teacher activities can then be designed to answer these questions.

We offer two examples in the form of questions:

Is sodium carbonate (washing soda, Na₂CO₃) as effective as potassium carbonate?

To answer this question, we start with equal volumes of isopropanol and water. We then add two drops of green dye and enough salt to form two layers or until un-dissolved salt is noted. When successful, the blue component of the green dye is dissolved in the upper isopropanol layer, and the yellow component remains in the lower water layer.

Using this procedure, the sodium salt is successful; however, the mixture needs to be warmed to about 30°C before the salting-out effect occurs (see figure 1- A & B).

sodium_carbonate_diff_temps.png

Figure 1: A - Isopropanol/water mixture with green dye and Na₂CO₃ at room temperature and B - Isopropanol/water mixture with green dye and Na₂CO₃ at 30°C.        

These results can be explained by the following facts and assumptions.

1. The salting-out effect increases with size and decreases with polarity of the solute molecule⁵. The potassium ion is larger (133 picometers vs 95 picometers) and more polar than sodium ion.
2. The solubility of potassium carbonate is greater than that of sodium carbonate (30.7 g/100 mL vs 111 g/100 ml).
3. Sodium carbonate works at higher temperature because its solubility increases with temperature; thus, there are more sodium ions available to interact with the solvents.
4. Hydrogen bonding is decreased at higher temperature⁶ allowing even sodium ions to break the interactions between water and isopropanol.

How does the type of alcohol affect salting out?

To answer this question, we tested methanol and ethanol in addition to isopropanol using both potassium and sodium carbonates. Table I summarizes the results.

Table 1: Conditions necessary for an effective salting-out.

salting_out_conditions.png

For salting-out to work, the salt must be able to overcome the polar attractions between the solvent and water. The more polar the solvent, the stronger these interactions.

In terms of the relative alcohol polarities: methanol > ethanol > isopropanol.

Thus, it is not surprising that the interactive forces between methanol and water cannot be broken even with the more polar and larger potassium ion and with a higher temperature.

Suggestions for further study

What is the effect of a less polar solvent such as acetone?

What is the effect of other sodium or potassium salts such as NaCl or KCl?

Conclusion

The entertainment power of a demonstration not only gets the attention of an audience; it also raises questions. These questions about the magic of a demonstration act as a catalyst that opens the doors to the science of chemistry.

Acknowledgement: Thanks to Tom Kuntzleman for his suggestions.

Citations

1. Hyde, A. M.; Zultanski, S. L.; Waldman, J. H.; Zhong, Y-Li; Shevlin, M.; Feng, F. General Principles and Strategies for Salting-Out Informed by the Hofmeister Series. Organic Process Research & Development 2017, 21, (9), 1355-1370.
2. Shakhashiri, B.Z. Chemical Demonstrations, Volume 3; UW press, 2011; pp 266 – 268.
3. Flinn Scientific. https://www.flinnsci.com/sorting-out-solutions/dc10104/
4. Kuntzleman, Tom, Chemical Mystery #8: Go Blue! followed by his Solution to Chemical Mystery #8: Go Blue!
5. Satoshi, E.; Pfenningsdorf, A.; Goss, K-Uwe. Salting-Out Effect in Aqueous Solutions: Trends with Size and Polarity of Solute Molecules. Environ. Sci. Technol. 2012, 46, (3), 1496–1503.
6. Mizan, T.I.; Savage, P. E,; Ziff, P. M. Temperature Dependence of Hydrogen Bonding in Supercritical Water. J. Phys. Chem. 1996, 100, (1), 403–408.