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I am using a rotary evaporator to separate DCM (dichloromethane) from some nasty tars. Before the evaporation is completed, I stop the process and collect the remaining slurry. The slurry is then positioned in a constant temperature oven, for a secondary evaporation. After cooling in a desiccator, the tar is measured.

The problem is that some of the tars remain deposited on the flask's wall. I can easily remove them with a soft brush, however this is a problem for 2 reasons:

  1. Some of the tar is lost, retained on the brush itself ( I require a very high precision since I work with samples of 100 or so mg)
  2. The amount of DCM for secondary evaporation is increased (I am trying to reduce that amount as much as possible)

Is there a round bottomed flask that has a TEFLON like coating on the inside, that can prevent the tar deposition on the walls?

There seem to be a lot of products that are referred as " Flask with Plastic Safety Coating". On which side is the coating applied (I assume is the outside)

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I've seen a couple of companies making flasks entirely from PTFE or PFA, which might be suitable. They may not hold up to heating with internal vacuum though, for example, check out the notes here.

And yes, the typical safety coatings are on the outside to hold the glass together on fracture.

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  • $\begingroup$ it says that the maximum temperature under vacuum is 50 C. Luckily for me, I only need 40. $\endgroup$ – Ivan P. May 23 '13 at 13:32
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Removing solvent from goo is very difficult. I had an oil that was a solid melting above 100 C. I pumped it under good vacuum as a thin film while doing all the usual tricks with ice then Dry Ice. It finally crystallized - and bubbled while doing it.

Siliconize the glass flask's inside. Cheap and easy is Rain-X for car windshields. Spray some inside the clean dry (air dry not oven dry) flask. Swirl for a couple of minutes, drain, dry. Couple of quick rinses with methylene chloride, set coating by baking at 110 C for 15 minutes. The coating is stripped by alkali and nucleophiles like NaOH or KOH in water, a base bath, aqueous ammonia, aqueous fluoride. Renew as needed.

If that is better, make it a formal procedure using Gelest Aquaphobe® CM or Aquaphobe® CF. Rain-X is $\ce{(RO)3Si}-$terminated polydimethylsiloxane oligomer in alcohol. It contains some silicone oil. The Gelest products are $\ce{Cl3Si}-$terminated oligomers. You do the 8% dilution in dry methylene chloride or toluene.

http://www.gelest.com/gelest/forms/GeneralPages/Applications/hydrophobic_materials.aspx

If you want to spend money, there are medical siliconizers for treating blood-handling glassware.

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