All liquids will undergo some degree of evaporation if they sit in an open container. Evaporation occurs when molecules at the surface of the liquid gain enough energy to detach themselves from the bulk fluid and leave as vapor. Several factors will increase the rate of evaporation, including raising the liquid temperature, lowering the ambient atmospheric pressure and increasing the surface area of the liquid. You can speed up the evaporation rate of a liquid, such as a lab solvent, by employing a combination of heat and vacuum.
Things You'll Need
- Side-arm vacuum flask
- Rubber stopper
- Rubber hoses
- Vacuum source
- Cold trap with isopropanol/dry ice
- Ring stand and clamp
- Hot water bath
- Boiling chips
Pour the liquid to be evaporated into the side-arm vacuum flask, taking care to only fill it 1/3 full or less. This prevents the liquid from accidentally being drawn into the vacuum line if it bubbles or foams during evaporation. Add a few boiling chips to prevent bumps or splashes in the event the liquid boils.
Seal the opening of the flask with a rubber stopper and clamp the flask in place, attached to the ring stand, to prevent it from tipping over.
Attach a thick-walled rubber hose to the vacuum inlet of the flask, then attach the other end of this hose to the inlet of a cold trap clamped to a ring stand. Attach a second hose between the outlet of the cold trap and a vacuum source, such as a pump. Immerse the cold trap in isopropanol/dry ice to cool it.
Turn on the vacuum source so that vacuum is applied to the flask. Depending on the liquid, it may begin to bubble or foam. You may slightly loosen the clamp and gently swirl the flask to promote more even or more efficient evaporation.
Observe the flask to see the rate at which the liquid is evaporating. If, after several minutes, there is no appreciable evaporation, loosen the clamp on the flask and slide the hot water bath underneath, then insert the flask in the water bath and re-tighten the clamp. The applied heat will speed up the evaporation further.
Observe the flask, with some intermittent swirling, until all the liquid has evaporated. Turn off the vacuum source, then disconnect the rubber hoses. Empty the cold trap into a suitable hazardous waste container, taking care when handling the extremely cold trap components.
Tips & Warnings
- The purpose of the cold trap is to condense and trap the evaporated liquid vapors before they reach the vacuum source. Some vapors can harm vacuum pumps or may be hazardous in other ways.
- You can also speed up evaporation using a device known as a rotary evaporator, which applies a vacuum to a solution contained in a round bottom flask while heating and rotating the flask. However, this type of apparatus is not available in every lab.
- Use caution when heating any chemicals. Heating may cause chemical reactions, high pressures, toxic fumes or flammable vapors. Read the Material Safety Data Sheets for the solvent you are evaporating and any chemicals it contains before you begin.
- The vapors of many liquids are harmful if inhaled. If you are performing this procedure with any chemical other than water, work in a fume hood.
- Wear appropriate protective equipment including lab coat, gloves and goggles.
- Inspect the vacuum flask for cracks or other imperfections before you begin. A damaged flask can implode and shatter under vacuum.
- Use caution when working with dry ice. It is extremely cold and evaporates to produce large quantities of carbon dioxide gas which can be hazardous in confined spaces.
- Isopropanol is harmful when ingested or by inhaling its vapors.
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