Distilling Isopropyl Alcohol

As a follow-up to last time, when we separated a mixture of sand, salt, iron filings, and rice, today we separated a mixture of two liquids: water and isopropyl alcohol. We took 10 mL from a bottle of rubbing alcohol, which is 70% isopropyl alcohol in water, and ran a distillation in order to separate it into its pure components. Distillation is a separation technique which takes advantage of liquids having different boiling points. Because IPA boils at a lower temperature than water, we were able to boil the mixture and collect the vapor using a condenser.

From 10 mL of the mixture, we were able to recover about 3 mL of isopropyl alcohol. Because no separation technique ever recovers 100% of the components, the students were not surprised to learn that we did not get back all 7 mL of alcohol. What was surprising was that the recovered alcohol was only about 91% instead of 100% pure. This is because some of the water evaporated with the alcohol, so the vapor we condensed was actually a mixture of the two. In addition, it turns out that isopropyl alcohol and water form an “azeotrope,” which is a mixture that, when it boils, produces a vapor with the same composition as the mixture. Thus, it is impossible to completely separate isopropyl alcohol from water using simple distillation; the best we can ever get is a more concentrated alcohol at 91% instead of 70%.

Author

Dr. Maureen Griffin

Maureen earned a Ph.D. in Chemical and Biomolecular Engineering from the University of Pennsylvania in 2003. While at Penn, she developed a novel micro-mechanical technique called micropipette peeling to investigate the role of muscle cell adhesion in normal and diseased skeletal muscle cells. After graduating, Maureen worked full time as a post-doctoral researcher and then a staff scientist a SelectX Pharmaceuticals. She joined the teaching staff in 2008 and was excited to be made an executive staff member in 2009. Maureen also continued to consult part time for SelectX until her daughter's birth in 2009; now she is focused on Science from Scientists and, of course, her children. Maureen uses her spare time to read, blog, cook, and renovate her house.

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