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5.6B: Step-by-Step Procedures for Rotary Evaporation

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    95730
  • Rotary Evaporation Procedure

    Figure 5.67: a) Sample to be evaporated, b) Attachment of sample to bump trap on rotary evaporator, c) Lowering of sample into water bath.
    1. If your institution uses a water circulator connected to the condenser to minimize water usage (open cabinet of Figure 5.65), be sure that there is ice in the water reservoir.
    2. Pre-weigh a round bottomed flask, and fill it no more than half-full with the solution to be evaporated (Figure 5.67a).
    3. Connect the flask to the evaporator's "bump trap" using a plastic clip ("Keck clip", Figure 5.67b).
      The bump trap prevents foaming or splashing solutions ("bumping") from dirtying the condenser or collecting in the receiving flask where components cannot be recovered. A bump trap should be kept clean so that if solutions do collect there, they can be rinsed back into the original flask without worry about contamination from previously evaporated samples.
    4. Use the joystick knob on the apparatus to lower the flask into the water (Figure 5.67c) so that the flask is partially submerged. Be sure the flask is not positioned so low that the joint with the plastic clip is in the water.
    Figure 5.68: Student uses the rotary evaporator.
    Figure 5.69: a) Turning on the water aspirator, b) Adjustment of the flask's rotation, c) Closing the stopcock to further reduce the pressure.
    1. Turn on the vacuum source (Figure 5.69a), which may be a water aspirator or vacuum pump. A hissing sound should be heard from air being pulled through the stopcock. The partial vacuum will help hold the flask securely onto the bump trap.
    2. Begin rotating the flask at a medium rate by adjusting the rotation notch (to roughly \(110 \: \text{rpm}\), or to one-third of the maximum rotation value, Figure 5.69b).
    3. Close the stopcock on the evaporator by turning it perpendicular to the bleed valve (Figure 5.69c). The hissing sound should stop, and the pressure inside the apparatus will decrease further.
    Figure 5.70: a) Evaporating a solution (with arrow pointing to the solvent reservoir), b) Formation of solid in the flask.
    1. Allow the solution to evaporate. Solvent should collect in the large round bottomed flask reservoir (indicated in Figure 5.70a). There is no set amount of time required for complete evaporation.
      1. If the expected compound is a solid, keep evaporating until a solid or thin film appears (Figure 5.70b). A film may form if the temperature of the bath is above the solid's melting point, if crystallization is slow, or if the warmth of the bath prevents crystallization (Figure 5.71b shows a film that crystallized into Figure 5.71c after a day).
      2. If the expected compound is a liquid, evaporate until the approximate expected volume is seen and it appears that the liquid level is no longer changing (Figure 5.71a). Evaporation can also be tested for by lifting the flask out of the water bath, drying the outside with paper towels, and feeling the flask with your hand as it rotates. If solvent continues to evaporate, the flask will feel cool.
      3. If solvent bumps from the flask into the bump trap (Figure 5.67b), raise the flask slightly out of the water bath, and/or allow small amounts of air into the apparatus by partially turning the stopcock (Figure 5.69c). If large amounts of liquid have bumped, stop the evaporation and rinse the trap with solvent such that the rinsing is added back to the flask. Evaporate the solvent.
      4. It is acceptable if solvent pools in the bump trap during normal evaporation (not bumping). It will sometimes later vaporize, and if it doesn't, it's still been removed from the flask. Pooling often happens if the solvent has a relatively high boiling point.
    2. When it appears that the sample has completed evaporating, allow it to remain in the reduced pressure system an extra few minutes to remove any final solvent residue.
    Figure 5.71: a) Residual liquid after evaporation, b) A sample after evaporation that left a film on the flask, c) The sample sample that crystallized after allowing the flask to dry overnight.
    1. To stop the evaporation, reverse all the previous steps: open the stopcock, stop the rotation, turn off the vacuum source, lift the flask from the water bath, and remove the flask. Turn the evaporator off completely if you are the final user of the day.
    2. The residue in the round bottomed flask should contain the desired compound, but may still contain trace amounts of solvent. If the compound is not particularly volatile (or if it's a non-volatile solid), the flask can be left open in a locker until a future lab session. After fully dry, the mass can be obtained and the compound analyzed.

    Rotary Evaporation Summary

    Rotary evaporation setup components: water aspirator, condenser, rotary evaporator, ice water circulator to condenser
    Figure 5.72: Rotary evaporation setup.

    Table 5.12: Procedural summary of rotary evaporation.

    Be sure there is ice in the water circulator (if used).

    Fill a round bottomed flask no greater than half-full.

    Connect to the bump trap with a plastic clip.

    Lower the flask into the water bath to submerge the liquid (don't submerge the plastic clip).

    Turn on the vacuum source (vacuum will hiss).

    Rotate the flask at a moderate rate (one-third the maximum value).

    Close the stopcock in the apparatus (hissing will stop).

    Evaporate until solid forms or liquid level doesn't appear to change anymore, then evaporate an extra few minutes for good measure.

    To stop evaporation, reverse all steps:

    • Open the stopcock
    • Stop the rotation
    • Turn off the vacuum
    • Lift the flask from the water bath
    • Remove the flask

    Contributor

    • Lisa Nichols (Butte Community College). Organic Chemistry Laboratory Techniques is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Complete text is available online.