Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Babcock_Hall on February 17, 2016, 09:40:08 AM
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When someone asked me about this, I said that they did not work, which is something I was told a long time ago. We also found one textbook that said not to use them. However, it seems to me that it depends upon their mode of action. From the Colorado web site, "Boiling chips are small, insoluble, porous stones made of calcium carbonate or silicon carbide. These stones have pores inside which provide cavities both to trap air and to provide spaces where bubbles of solvent vapor can form. These bubbles ensure even boiling and prevent bumping and boiling over and loss of the solution." The air would be lost in a vacuum distillation, but the bubbles of solvent might still form. So, why don't boiling chips work for vacuum distillations?
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From what I remember the way it works is that the saturated pressure over a curved surface is different (either smaller or higher, depending on the curvature) than the saturated pressure over a flat surface. Porous stone makes plenty of curved surfaces, where the boiling point is lower, these supply nucleation sites for boiling. Perhaps when the pressure above the liquid is already zero lowering locally the saturated pressure stops to change anything?
Just thinking out loud.
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Boiling chips are used to prevent bĪ¼mping. The nucleation sites, curvature, stored air to form bubbles, etc and whatever are meant to avoid foaming out of the flask by sudden boiling.
This, in many ways, goes out the window in a vacuum distillation. First you're going to heat way more gently, because you expect a lower boiling point. With constant vacuum, large bubbles, even of the solvent, won't form. Or at least will collapse sooner. If it were me,I'd use smooth glass balls,there's no need for pores under constant vacuum. But again, like Borek: said, no one really knows why, and its really not that important in any case if you're careful.
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I guess that the 64-dollar question is, "What is the best way to minimize bumping in a vacuum distillation?" I sometimes think that a stir bar helps, and it sounds as if you think glass beads help.
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I used to like stirring too... I never liked anti bumping granules really but do help. You still might get bumping but they should reduce it. A broken splint works the same, iirc from school days, but again, I do not like the idea of putting wood into the mix with what I am trying to purify.
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There are anti-bumping granules especially made for vacuum distillations. I don't know what's the difference in theory, but they are much larger than regular ones.
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As I understand distillation, one seeks to get an equilibrium between the gas and liquid phases. Bumping can occur if more heat is applied than can enter the gas phase. If a gas bubble does form, this can release superheated molecules into the gas phase. A solution to avoid superheating and bumping is to introduce or form gas bubbles. At atmospheric pressure, boiling chips or sticks can release a small amount of gas and promote smooth boiling. If boiling chips are used in a vacuum distillation, the gas is released too quickly. For those of you that are old school, a nitrogen gas capillary ebulliator may be used. This works well at reduced pressure as a capillary reduces the rate that a gas can enter the heated liquid.
As an undergraduate, I had used a spinning band column to separate a mixture of diastereomers. In this case a spiral column mixes the gas and liquid up the distillation column, but it also generates gas bubbles in the distillation pot. The spinning band was run at an amazing 14,000 rpm (as I recall). Although I have used boiling chips, a capillary ebulliator, and a spinning band column, I have found I could get acceptable results by simply using a magnetic stir bar. I always ran the stirrer at the maximum rate I could. My objective was to generate enough turbulence to increase the gas-liquid interface. You can do this at atmospheric or vacuum distillations.