Apr 30, 2010

RBF Teaches: Drying solvents

So, when you do real chemistry and are able to use a manifold appropriately you can start to look at some genuinely difficult chemistry requiring dry conditions. The dry (or anhydrous) reaction is quite commonplace in synthetic labs and you will most likely come across it in your career, probably very early on. Once again, I suggest reading APOC as it details the use and setup of a manifold, the ways in which to perform anhydrous reactions and also how to dry solvents. I will hopefully be able to outline a few key steps in how we dry solvents in the lab at present and the few tricks I have picked up along the way.

  1. So to start you need to select your solvent and identify the ideal way of drying it. A great companion for any lab monkey is Purification of Laboratory Chemicals. This book will also show you ways in which to purify any other chemical in your lab inventory (some are not there but a lot are). So, find your solvent in this book and it is generally ideal to look for the rapid purification and drying methods. In our case we will use THF as an example.
  2. The next step is to set up your equipment so that the solvent stays dry. First, wash your glassware with acetone to remove any water and also some pure diethyl ether to remove any crud from the acetone.  DO THIS IN A HOOD and remove as much solvent from the glassware as possible. Next, I suggest putting your glassware (Distilation apparatus, collecting RBF etc.) in a hot oven overnight to ensure any solvent or water is well and truly gone. When you assemble the equipment the next day, take it out of the oven while hot, assemble and put under vacuum to ensure no condensation occurs (You may have to use another empty RBF to get vacuum ;)). Once dry and cool put the glassware under argon. You should also use a second RBF to use initially to catch the first fractions of the distilation as some impurities or lighter solvents can come through. Next, our book tells us a number of ways to dry a solvent but in our lab we use a still to distill this solvent. However, the use of a distilation bridge can just as easily work when you have no space for a still.
  3. The use of activated molecular sieves or some other innocuous dried material (e.g. KOH for TEA) can be placed in your dry collection flask to prevent the solvent becoming wet over time. It is also a good idea to grease the farthest part of all joints lightly with silica grease to ensure an airtight seal.
  4. Now it is time to dry our solvent. The drying agent used is usually sodium metal (either wire or pressed) and benzophenone as an indicator (benzophenone turns blue when complexed with Na in an atmosphere free of oxygen and water). The benzophenone undergoes a reaction with sodium to give non-volatile products that appear blue (due to the nature of the complex) when the solvent is dry and the atmosphere free of oxygen. If the solution is wet the sodium forms NaOH and takes the water out of solution as this salt. As an option another still can be set up to pre-dry the THF using CaH2. This prevents excess consumption of the sodium and benzophenone. N.B. Not a good idea to dry chlorinated solvents with sodium. Always make sure you read the procedure.
  5. Once you have your solvent you wish to dry, add your drying agent. In the case of sodium it is a good idea to have your solvent under argon and to crush the benzophenone onto the sodium. Try to be quick as the sodium can catch fire in the air, ensure you have a lot of the mineral oil on the sodium so that it is not exposed to air. Place the sodium with crushed benzophenone on it into the solvent and attach this flask to your distilation apparatus.
  6. Begin your distilation at the required temperature making sure you have a sufficient cooling system in place (i.e. if your solvent boils at lower temperatures then cool it well with ice water, but if it boils high or freezes at low temps then r.t. water is not a problem). Do not boil the still dry, this is dangerous, simply leave enough solvent to cover the drying agent completely.
  7. Once the distilation is complete, disconnect your collection flask ensuring that it is kept under argon, stopper (using a small amount of grease at the top of the neck) and place to one side.
  8. Quenching: Scary stuff if you don't do it right. Initially to quench something like CaH or Na you want to use something that will react with the agent but in a controlled way, for these two agents iPrOH (isopropyl alcohol) is a good choice. iPrOH is hindered and will not react quickly with the agent. First, cool the remaining solution on ice. Next slowly add iPrOH (add a decent amount, usually around 50-100mL) and allow to stir for 15min or so. Now you can add EtOH or MeOH (DO NOT ADD WATER) and continue to stir it on ice for a time. Eventually, you can start to add water dropwise to completely quench the solution but be mindful of the temperature of the solution and the development of bubbles. This is a warning that the quench is occuring and usually occurs quite rapidly. Then you can dispose of your waste in an appropriate waste drum and glory in your dry solvent.
I hope this helps you I will add some pics next time I see someone doing a distilation. Keep safe and don't blow yourselves up in the meantime. Read APOC and get the purification book (don't download it via torrent ;) ). Next I hope to write about schlenk lines or what we call a manifold.


  1. YAY!! (to the solvents purification procedures)

    I have a problem with the manifold. One of the plastic or silicon screw stopcock (aid the opening and closing of the line) is ruined. Anyone knows how to fix this? What kind of super glue would work?

  2. I think you would be better off buying a new screw end. If I read the post correctly you are talkin about the screw cap part not the stopcock itself. I would just take it to a glassblower and they will give you a new one (so much faster than gluing it.

  3. its plastic screw cap.. not glass stopcock. Sorry for the misinformation.

    How do I purify ACN? I tried distill off with CaH2 and it produced too much gas, build up pressure and push my distillation thermometer. I'm afraid I did not distill this solvent properly at the end ie still wet. Any suggestion?

  4. I dried ACN over pottasium carbonate for 24hr, then molecular sieve for 24hr and distilled with CaH2 (where the problem arose with build up pressure due to gas formation, H2).

  5. Did u have the system connected through your manifold via a hose attached to this piece of glassware?:
    under inert gas so the hydrogen gas could escape? Otherwise u have constructed a rather dangerous closed system that will relieve the pressure at some stage
    With respect to your manifold problem im thinking u have Young's type adapters similar to the ones in this photo?

  6. Rotary Valves:


    Glass taps

  7. Yes, I did. I connected the hose from manifold for inert gas to fill the whole system. It is strange that the pressure tend to build up faster and pushed thermometer rather than traveling down the end of the condenser and released through that hose. I heated up the oil bath to the solvent at 300C, otherwise it won't distill. I reckon, despite the low bp, ACN is very polar and that might contribute to the difficulty to vaporise.

    Yes, I am using the Young's type adapter in the manifold, well spot on. My problem is the black cap snapped off from the white inside. Somebody turn the cap too tight to close the line and broke it *sigh*

    I am working in a brand new Research Institute, unlike a proper chemistry lab in our School of Chemistry, I always have to figure out some crude ways to fix stuff.

  8. This comment has been removed by the author.

  9. Couple of things u can try. Stir the acetonitrile with CaH2 without refluxing for a few hours in the distillation apparatus to quench any remaining water. What kind of bubbler system do you have? Perhaps your N2 or Ar was turned up to high? Any blockages in the system? Was there inert gas venting through your check bubbler/valve? You could attach a y-tube to your distillation setup one with inert gas going in and another bubbler at the other end of the Y tube. If you are sure everything is unblocked you could try using Keck clips on the joints. Too much grease?

    I'm assuming you have a postdoc or senior PhD student to assist with such matters? Probably not a good idea for a new student to be left to their own devices ;)

    As for your schlenk line. If you use too much glue u will end up gluing the tap to the glass and then u will be screwed (no pun intended). If you get the right amount of glue and if u think u can stick plastic onto teflon, good luck with that. Otherwise i would take that manifold to the glassblower to get him to sort that out. If thats not an option test to see if the schlenk line has a leak at that spot. If not then maybe u can live without one of the lines. Probably not a good idea to be turning on the high vac with a Liquid N2 trap if you have a leak there.

    300C seems excessive to distill acetonitrile which boils at 82C. Maybe the thermosat controller is borked. I hate those controllers you dip into the oil.

  10. I use an improvised bubbler like this one:


    with metal rod attached to the host-manifold. Since the oil tend to flow out and blocked the host and contaminate the manifold, I use rubber stopper vial (syringes on each end of the host into the vial) ie. as the oil trapper. Not very impressive, I think it makes more sense now that I might turned up the gas too high because of this.

    I will try redo the ACN purification with CaH2 for few hours (or overnight?) before continue with distillation.

    OMG I feel so pressure that I only have 2 schlenk lines working for anhydrous reactions. I have lots of them to go :(

    Yes, we have a Posdoc here and few very-noob PhDs, no senior PhD. I usually approach her about chemistry and try attempt technical problems like this myself.

  11. With your bubbler system how do you evacuate your flask then back-purge with inert gas? Surely you get silicon oil being sucked back all the time into your make-do-traps?

    If you are lucky enough to have a postdoc in your group you need to take full advantage of him/her. Sure you will make mistakes and you will learn from your mistakes. But do you want to learn from your mistakes by losing an eye (or two) or by being exposed to some nasty chemical? They essentially have a duty of care for you, so if you ask them "is this setup correctly" and they say get fracked and you end up in hospital because Acetonitrile/HF/HMPA or whatnot boiled in your face, it's not your fault it's theirs. However, on the other hand there is common sense which they cant fix.

    end of rant

    To cut this long story short you need to drain as much/all information from your postdoc as possible. It will greatly assist you to develop your experimental skills.

  12. Ahh Lab Chimp, you are my hero. Except for those few times you sent me to Andrei you gave me so much knowledge. I am so much better at chemistry thanks to you.

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