Anhydrous Solvents Part 2: The THF Still - Do's and Don'ts

The THF still is one of the most common reasons for serious fires at chemistry departments around the world. As solvent purification systems (SPS) become more and more common this problem will gradually disappear. However for now SPS is still pretty expensive so the THF stills will linger around for some time to come. There are many reasonable ways to construct a THF still. All you need for a basic THF still is:

• A round bottom flask
  
• A still head
  
• A reflux condenser
  
• An inert gas inlet and gas bubbler
  
• A heating mantle
  
• Sodium, benzophenone and some THF

The main reason why THF stills blow up is incompetent users. It only takes one sloppy chemists to screw things up in a big way. However, even with competent, careful chemists THF stills occasionally blow up. So what can you do to avoid the THF still disaster?

First and foremost train all users and/or limit the number of people allowed to turn it on and off. Have an on/off sheet where people record their name, time etc.

• Get a flow-stop switch - This clever little gizmo shuts the power off if the water stops running. This will stop most accidents before they happen. Your electrician will know what to do.
  
• Get a "hat" for your heating mantle (see picture) to avoid water dripping in and setting the whole thing on fire.
  
• Use Teflon joints or Teflon sleeves to protect joints from fusing (see picture)
  
• Don't use excessive amounts of sodium. Ideally use a sodium press to make thin sodium thread. This will give you a large surface area and ensure that you don't end up with a big fat ball of sodium when you eventually have to quench the still.
  
• Never turn the inert gas off.
  
• Have enough THF in the system. Always ensure that the round bottom flask (RBF) is filled sufficiently with THF to allow the still head to fill up and leave the RBF at least on third full.
  
• ONLY fill the THF still with dry THF. Don't just grab any old THF bottle from the solvent cabinet. Have THF bottles specifically for the still only.
  
• Don't put a stirring bar in the RBF. This will only result in a big fat ball of sodium down the track. Sodium wire is generally enough for controlled boiling. If you must add something throw some boiling chips in.
  
• BE PATIENT! Don't turn the heating mantle to the max because you are in a hurry. It take 30-60 minutes to get a THF still up and running in a controlled fashion.
  
• Place your heating mantle on a lab jack. When the shit hits the fan you can lower the heating mantle and stop heat transfer immediately.
  

I think that is enough for today. There are many things to consider and I have by no means covered everything but this should be a good start. D!