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How to Recover the Dissolved Solid in a Stock Solution

Although rare, there may be cases where a lab wants to recover the chemical dissolved in a solution. A lab stock solution is typically very concentrated, since it is diluted to generate calibration solutions of lower concentration. Thus the stock solution will have a relatively large amount of dissolved chemical and it might be worth trying to recover this. To do so, you can use a common piece of lab equipment known as a rotary evaporator.

Things You'll Need

Rotary evaporator
Round bottom flask
Holding clip
Water bath
Hot plate
Cold water source
Vacuum source
Rubber hoses
Cold trap with isopropanol/dry ice bath(optional)

Instructions

- 1
  Fill a round bottom flask no more than half full with the stock solution. Do not overfill the flask, since you will have to tilt it on an angle to attach it to the evaporator.
- 2
  Attach the flask to the inlet spout of the rotary evaporator. Use a clip to ensure the flask does not slip off.
- 3
  Place a water bath on a hot plate underneath the sample flask and heat the water to a temperature slightly below the boiling point of the solvent in the stock solution. If the chemical you are recovering is heat sensitive, you can use an even lower temperature.
- 4
  Connect the rotary evaporator cooling coil inlet and outlet points to a cold water source and a drain, respectively, using rubber hoses, then start the water flow.
- 5
  Attach a cold trap to the rotary evaporator vacuum inlet using a rubber hose. Use a very cold solution such as a mixture of dry ice (solid carbon dioxide) and isopropanol to cool the cold trap. This is optional but is recommended if you are removing a very volatile solvent that may be pulled out of the evaporator collection flask and into the vacuum pump. Connect the other end of the cold trap to the vacuum source. If you are not using a cold trap, connect the evaporator directly to the vacuum.
- 6
  Start the sample flask slowly rotating (about one revolution per second) using the evaporator control pad and also start up the vacuum source.
- 7
  Slowly lower the entire evaporator assembly so that the sample flask is partly immersed in the water bath. The stock solution is now being gently heated while under vacuum in a spinning flask. This combination of heat, vacuum and increased surface area will quickly evaporate most solvents.
- 8
  Observe the evaporation apparatus. You should see solvent vapors travel from the sample flask into the condensing coil area where they will condense into liquid and drip into the collection flask. Some vapors may be pulled past and caught in the cold trap.
- 9
  Stop the rotation when all solvent has been removed and only dry chemical remains in the sample flask. Raise the apparatus to remove the flask from the water bath, turn off the vacuum and break the vacuum in the apparatus with the upper stopcock.

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