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Distillation Technique

  1. Jul 28, 2006 #1


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    I am planning on doing some distillation in the very near future; however, it has occurred to me that I have had no formal training in proper laboratory distillation techniques.
    Since I would very much like to avoid damaging my beautiful new glassware on my first attempt, I figured I would try to do it right the first time rather than having to learn the hard way by trial and error.

    My distillation apparatus uses 300 mm Graham condenser, a 1000 mL flat bottom, Florence, boiling flask, and a thermometer adapter built into the connecting tube (all 24/40 ground glass joints), plus a few joint clips.

    First of all, when putting it all together, how important is it to add some type of grease or other additional sealant to the joints?
    When attempting to distill under reduced pressure (i.e., the inside of the apparatus is at a partial vacuum compared to the surrounding atmosphere), I believe the grease is quite important in order to complete the seal. Also, adding some grease to the joints can help keep them from becoming stuck when trying to remove them afterward.
    Is this always a good idea to do? What type of grease is good to use (I assume “Vaseline” wont quite cut it). What about Teflon tape? Could a couple layers of Teflon tape be substituted or used in addition to the grease? Would it work as well?
    I have heard somewhere that the Teflon tape might cause extra stress on the joints due to a possible irregularity in the thickness of the layers.
    How important is it to use clip to help hold joints together? The safety clips I have are advertised as being safe up to 150° C, I assume beyond this, the difference in the expansion of the clips/glass will cause problems of some sort and put unneeded stress on the glass?

    What is the preferred method of heating the boiling flask? Although I have seen claims made that the borosilicate glass is safe for heating by a bare flame, I will not be so stupid as to put this to the test. I had planned on using a [lab] hot plate to heat the liquids, this seems pretty safe, but another method has come to my attention. Using a water bath, one could even more evenly heat the glass as well as provide a much more constant temperature (assuming one doesn’t want to distill at temperatures over 100° C, ideally somewhere a little less than that). One would place the boiling flask in a [metal?] pot of water and let the pot be directly heated by the hot plate. I am less inclined to use oil or sand baths to heat the flask.
    Placing some chemically inert, irregularly shaped, objects into the flask would also not be a bad idea either. If I were to get a few pieces of broken glass, for example, and drop them in with the liquid to be distilled, the glass help the liquid boil more evenly rather than “bumping”.
    How gradually should the heating process take (and for that matter, the cooling process)? Obviously one does not want to take a flask full of ice water and dunk it into a pre-boiling water bath, but at the same time, this type of glass is meant to be used with heat, so it can take a little abuse (but why put any more stress on it than necessary).

    What about the coolant used within the condenser?
    The most obvious coolant is air, but sometimes this may not be enough. Water is also another easy to come by coolant. All one would need to do is get a little aquarium/pond pump and some tubing, then one would have a pretty effective way at circulating water within the outer jacket of the condenser. But when distilling at higher temperatures, is using water appropriate? I have heard that at higher temperatures, you suppose to just use air as the coolant, using water would create too much of a temperature difference between the inner and outer sides of the glass. About at what temperature does this become an issue? As is my understanding, the lower the temperature one wants to distill at, the colder one’s coolant is allowed to be (somewhat counter-intuitive in a way).

    Due to the replacement cost, I am more than a little bit worried about breaking this stuff, how much of it is justified, how much is me just being paranoid?
  2. jcsd
  3. Jul 28, 2006 #2


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    Silicone grease, tapered teflon sleeves (you'll find 'em for all the tapers), vaseline, something, unless you're distilling diazomethane. The teflon sleeves are reusable, less contamination, and not nearly so messy.
    Guaranteed to break your joints. Teflon tape is for sealing pipe threads, only for pipe threads, and never to be used for anything but sealing pipe threads. It's also an excellent indicator that a complete moron has been at work --- when you see it used on anything besides pipe threads, step clear from the apparatus, and be certain to take no advice from anyone who uses it for other purposes.
    They keep the pieces from flying apart when things superheat and "bump."
    You don't get to change temperature rapidly --- so what?
    Ice point to normal water boiling point? No sweat.
    You're not being paranoid.
  4. Jul 28, 2006 #3
    You can get joint grease especially for the distillation but I wonder if using cork grease used for greasing musical instruments would work as well? Has anyone ever tried this? Either way you do need to grease it, if anything to make taking it apart easier, I know people who haven't bothered greasing it and ended up breaking some of their apparatus when taking it apart because it did not slide apart easily.

    And yup cold water is what you use as the coolant through the condenser.

    To heat the appartatus I always used a powermite which had an adjustable heating control which fit very nicely into the round bottom heating flask. Very handy way to do it. A hot place would work to but this is better. Add boiling stones to the round bottom flask before distilling so the liquid distills smoothly.

    Good luck!
  5. Jul 28, 2006 #4


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    I will likely buy some of those Teflon sleeves eventually, but for now I'll just get by with the silicone grease. I don’t suspect any silicone grease I get locally will be too different from the fancy grease sold specifically for lab use.

    OK, I should be using [cold/iced] water as the coolant for everything, even for the "higher" temperature distillations.

    Thanks Bystander and scorpa.
  6. Jul 28, 2006 #5


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    You want to keep the condenser temp low enough that your distillate condenses --- so, depends on what you're distilling.
  7. Jul 29, 2006 #6


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    It seems really straight forward but be damn sure to apply the vacuum before you start heating.
    One time I had a distillation going and it was just slow as hell, so I turned on a very low vacuum (vacuum on tap, open the valve just slightly) and hooked it up. My distillation pot immediately bumped really hard and it shot butanol through all of the tubes and into the collecting flask. Adding a vacuum is a lot more extreme than adding heat. If you hold a blow torch to the boiling pot you are still only affecting a small portion of the liquid, but if you apply a vacuum it immediately affects all of the liquid in the boiling pot.
  8. Jul 29, 2006 #7
    how much do you have to distill? nothing beats a kugelrohr for small scale stuff.
  9. Jul 29, 2006 #8


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    I do not plan on doing any vacuum distillation, but still good advice.

    I only have a 1 Liter boiling flask, so I cant do too much at a time, but I will never-the-less be putting every mL of space to good use each time.
  10. Jul 29, 2006 #9


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    Uhhhh --- no more than half-filled, and better between half- to one third-filled for a distillation; nor, do you boil everything through. You wind up sweeping a mixture aerosol through the vapor to the condenser when overfilled, losing your separation.
  11. Aug 7, 2006 #10
  12. Aug 7, 2006 #11


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    Yes, that does indeed look like a useful resource, Thanks.
  13. Aug 7, 2006 #12
    Copper Distillation

    I'm not too sure if all of this should be devoted to an entirely new thread, but it was this thread that got me wishing I had my own glass distillation apparatus and indirectly motivated me towards this project. Anyways, since I cannot afford $100 or more for a nice glass setup, I decided to build myself one out of some scrap copper. (Some day, I’ll build a glass one but I need to learn the art of glassblowing first.) It features a 2.5 foot copper fractioning column and long condenser hooked up to an outer PVC water jacket fueled by a drill pump that circulates ice-water. The metal-metal joints are silver soldered and I used Teflon tape to make the glass-metal connections.

    I want to ask you all what I can distill using this copper apparatus. Although glass is clearly more versatile, the major advantage I see in using copper is its higher working temperature. My initial thoughts were to just use it for distilling ethanol and diethyl ether. It is actually advantageous to distill ether using copper because the copper helps hinder the formation of explosive peroxides. I suppose there is no reason I could not use cooper to distill aromatic compounds like benzene and toluene. Perhaps glacial acetic acid production would be suitable as well. Obviously, hot strong acids like nitric will destroy the vessel.

    I am still leery about distilling bromine using my new copper setup. I recently distilled another couple milliliters of bromine from BCDMH using my old PVC apparatus, which is now heavily corroded red-orange with bromine. I wanted to test bromine’s reactivity toward copper. I took a very thin gauge of copper wire and placed it in a small beaker filled with bromine and when nothing seemed to happen I heated the beaker up with a candle to get some nice smelly bromine vapor. After everything cooled down, it looked as if part of the copper wire turned blue, but I could not really tell too well. From what I can find out on the internet, cupric bromide is black, not blue. So I guess that entire test was pretty much inconclusive.

    My next question is in regards to the determination of the concentration of a distillate, specifically ethanol. I distilled some ethanol using my new setup and want to see how close I can get to the 95.6% azeotrope of ethanol and water. The ethanol burns when a match is held to it which I think I remember reading means it is at least 40% ethanol. My first thought was to burn a known quantity of the ethanol and measure the volume of water left over (taking into account that 4.4% water also burned along with the ethanol). I realized that this would be a rather crude method of determining the concentration, however, because a lot of water would evaporate due to the heat of the reaction. I thought about drying my ethanol with calcium oxide or sodium hydroxide and then measuring how much water these salts absorbed, but I realize that by the time they absorb all the water from the ethanol, they will also absorb much water from the atmosphere. Is there another fairly accurate procedure for determining the concentration of ethanol that does not require a photospectrometer or other fancy equipment?

    Lastly, and probably the only question that is relevant to this thread…where exactly does one place the thermometer during distillation? I am guessing that the bulb of the thermometer should go right at the head of the apparatus where the vapors that are going to be condensed come over, no?
    Last edited: Aug 8, 2006
  14. Aug 10, 2006 #13
    To Cesium:
    Your thermometer should have the top of it's bulb just below the joint where your condesnser attaches. To get a good reading of ethanol content you can use a device called a hydrometer. You can find them that go up to 200 proof in 2 proof (1% ethanol) increments. You should be able to find these on plenty of moonshine sites or any site that deals with ethanol stills for the purpose of auto fuel. Some sites also sell zeolite as a molecular sieve to dry the ethanol. Another way to reach near 100% ethanol is by azeotropic distillation with benzene. However you would neep some special apparatus to do this and if you plan on drinking any of your distillate you will want to stay away from the benzene. Actually even if you don't plan on drinking it, i would stay away from benzene. It is a carcinogen and a molecular sieve will be a much easier method of reaching 100%. The higher the concentration of ethanol you get from your still though, the easier using a sieve will be.

    I guess all i meant to answer was put the top of the thermometer bulb below the condenser joint and a hydrometer is what you need to check your ethanol concentration.
  15. Aug 10, 2006 #14
    Thank you for your response bw800402.

    I think you are right that using a hydrometer is the best way to measure the concentration of my distillate. If I understand it correctly, a hydrometer basically measures the density of a liquid. I have resorted to measuring the density of my ethanol by hand and then referencing to a density table.

    After two careful distillations, I have a liquid with a density of about 0.84g/mL. This corresponds to about 86% ethanol (172 proof) by volume. I plan on drying the ethanol with copper sulfate. I am making some right now by electrolyzing copper in some dilute sulfuric acid.

    I am familar with the concept of azeotropic distillation with benzene that you mentioned. If I remember correctly, some toluene also does the trick (and poses less of a health risk). I don't really see any advantage that this method has over drying, however.

    Although I did ferment this ethanol with sugar, I don't want to drink it. Chances are some methanol was also generated during fermentation and I don't want to risk going blind. 200 proof would be a little too strong for drinking anyways :wink:.
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