Bumping is a phenomenon in chemistry where homogenous liquids boiled in a test tube or other container will superheat and, upon nucleation, rapid boiling will expel the liquid from the container. In extreme cases, the container may be broken.
Cause
Bumping occurs when liquids are heated or has its pressure reduced very rapidly, typically in smooth, clean glassware. The hardest part of bubble formation is the formation of the small bubble; once a bubble has formed, it can grow quickly. Because the liquid is typically above its boiling point, when the liquid finally starts to boil, a large vapor bubble is formed that pushes the liquid out of the test tube, typically at high speed. This rapid expulsion of boiling liquid poses a serious hazard to others and oneself in the lab. Furthermore, if a liquid is boiled and cooled back down, the chance of bumping increases on each subsequent boil, because each heating cycle progressively de-gasses the liquid, reducing the number of remaining nucleation sites.
Bumping and foaming are problems that commonly arise during evaporation and can lead to sample loss and contamination. As Jim Dawson, president of Heidolph North America (Elk Grove Village, IL), explains, the primary culprits are applying “too much heat, too much rotational speed, or too much vacuum” to a particular application.
Bumping occurs when the sample boils and bubbles rapidly, causing the product that is being concentrated to splash out of the vessel. Roland Anderson, laboratory products manager at KNF Neuberger (Trenton, NJ), gives the following analogy: “If you had sauce in a pan that you were looking to concentrate, and you were stirring it at a very low temperature, you would be able to concentrate it without splashing the sauce everywhere. If you were simply to turn the heat up on that saucepan and let it start bubbling, you would splash that sauce around the outside of the pot.”
Prevention
The most common way of preventing bumping is by adding one or two boiling chips to the reaction vessel. However, these alone may not prevent bumping and for this reason it is advisable to boil liquids in a boiling tube, a boiling flask, or an Erlenmeyer flask. In addition, heating test tubes should never be pointed towards any person, just in case bumping does occur. Whenever a liquid is cooled below its boiling point and re-heated to a boil, a new boiling chip will be needed, as the pores in the old boiling chip tend to fill with solvent, rendering it ineffective.
A sealed capillary tube can also be placed in a boiling solution to provide a nucleation site, reducing the bumping risk and allowing its easy removal from a system.
Stirring a liquid also lessens the chances of bumping, as the resulting vortex breaks up any large bubbles that might form, and the stirring itself creates bubbles.
Automation advances
Vacuum controllers get closer to the root of the problem. By controlling the speed of the pump, these devices can drastically reduce or eliminate the likelihood of bumping.
KNF Neuberger’s new vacuum controller has an automatic feature that senses the vapor pressure of the solvent and uses that as a set point. This feature allows the user to optimize the speed of evaporation without bumping. “When using that controller, we don’t ever recommend people put a bump trap in line,” says Anderson.
Then there are foam sensors, which “detect the foam inside the evaporating flask and aerate the system slightly to make sure that the foam collapses,” Hertz explains. “The foam sensor allows automatic, unattended distillation of foaming samples.”
In Dawson’s opinion, automation is the ideal solution to bumping and foaming. “We want to manage the conditions by automatically adjusting the vacuum,” he says, pointing out that Heidolph’s AUTOaccurate program supports precise automatic pressure control for evaporation.
“But what’s interesting,” says Dawson, “is that despite all these advances in automation, chemists tend to prefer to adjust manually.” Budgetary constraints may be part of the reason, but then there are those seasoned and patient chemists who simply feel more comfortable with the manual methods they have always used, he explains.
