I was a bit bored today and thought I'd try to make Coca-Cola flavored simple syrup. The smart way to do this would be to, I don't know, buy fountain syrup
. I decided to boil off the water under vacuum.
I heated a small sample of Coke to about 75 C in the microwave; my thought was that this sort of gentle heat wouldn't alter the taste/cook sugar/etc, but would be the boiling point in my chamber vac (which reaches about .1 to .15 atm; so boiling somewhere in the 40-55 range.) Put it in the chamber and set the vacuum. It started to bubble heavily (probably the remaining CO2 disappearing) and the chamber fogged up heavily, but quickly all the activity stopped. Leaving it under the vacuum for a minute, nothing happened, and we saw next to no reduction.
Testing temperature again, it was about 30C. I think the issue is that I forgot evaporative cooling. Specific heat of vaporization depends on temperature and pressure, but at the normal boiling point water's is ~2.23 kJ/g, and I don't think it changes by much under normal lab conditions. Meanwhile the specific heat capacity of water is about 4.184 J/(g*K). We see immediately that each percent
of a sample of water that vaporizes is enough to cool it (assuming no heat input from the outside) by ~5 degrees. So before I saw any significant boil-off, the sample would rapidly cool below the boiling point. Drat.
This technique might be recoverable; I could use my Anova to produce a water bath and draw a vacuum over a (large!) flask; basically we're looking at a rotovap without the distillation apparatus. My chamber vac even has an attachment to pull from a container, but probably not the right hookup for standard lab equipment. Well, you can't win them all.#mineileavetochanceandchemistry