Latsabb
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Lab technique question.
I go to a college which has a relatively limited chemistry lab, especially in the organic chemistry lab. All distillations that we perform are with a
Bunsen burner, as no oil bath's are available. As many of you can likely imagine, this makes temperature control quite difficult, especially when
distilling small amounts. Many of our labs have us using 50mL pointed flasks with between 15-20mL of solution, and a Bunsen burner makes this
situation very tricky. Especially if you are trying to balance a temperature within a certain range.
It should be noted that boiling stones are used, rather than magnetic stirrers, and a mesh ring for dispersing the flame is not available either. I am
not sure how it is other places, as I dont have any chemistry experience from other institutions, but I have been led to believe that this is not
exactly the preferred method of doing things, lol.
Now then, my questions. Being that an oil bath is not an option for heating, I often find my distillations being ruined by a sudden boiling, which
travels into the neck, and then into the condenser. I then have to stop everything, dump it all back into the flask, and put on a new distillation
set. This is incredibly aggravating, and I want a way to avoid this. I was thinking that if I had a beaker filled with water below the pointed flask,
I could submerge the pointed flask into that, then use the Bunsen burner to heat the water in the beaker. With a thermometer in the beaker, I could
more easily keep the temperature within a range, and the larger volume of liquid would remove the temperature spikes from direct heating of the
solution.
Is that something that can be done? Is there a reason that this would not be recommended? It seems logical enough from my side, but I am afraid that I
may be overlooking something, and I dont want my yield or the glassware to suffer the consequences here.
If anyone knows of a better and/or safer way of doing this, please let me know! Thank you.
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aga
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Knowing the temperature Range that you want to achieve would be useful.
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Latsabb
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All distillations to date have been sub 100 C. Off the top of my head, I remember 40, 53-55, and one around 83. So all below the boiling point of
water.
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Zombie
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What you asked, and answered is absolutely correct. Using either water or oil in a beaker is what I have seen recommended time, and time again. For
the very reasons you expressed.
That puking into the condenser is "Bumping", and the boil stones are supposed to prevent this.
I can only assume the volume you are dealing with is so low the liquid super heats spontaneously, and Bumps.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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Latsabb
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To be more specific on the most recent lab, which caused the most problems, it was a lab making 1-bromobutane from 1-butanol. The distillation was
done in the around 85 degrees C, and temperature control was a major problem. If I let it simmer, it wasnt distilling well at all, but as soon as I
moved the flame closer, or turned up the gas, it would boil vigorously, and shoot into the neck. As soon as I saw it start to foam a bit, I would drop
the flame, but it was almost always too late. The solution contained 10mL of water, 7mL of 1-butanol, and 7mL of sulfuric acid, as well as 9 grams of
sodium bromide. So there wasnt very much liquid, and it made for a very difficult process.
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aga
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If 100C isn't too high, just use a water bath.
For your description (50ml flasks) it would be simple to add a 250ml beaker full of water, and put the flask in that.
Heat the beaker.
Temperature will never rise above 100C so long as there is water in there.
Obviously you can add a thermometer to the beaker, and if the temperature gets too high, add cold water.
In any event, 100C will be the absolute upper limit.
What kind of Chemistry College is this when what you need to know comes from the Forum Drunkard on a website instead of your Tutor ?
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gdflp
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Then, yes a water bath would be very useful for this type of situation. The only concern I would have is of the beaker breaking due to uneven heating
from an open flame with no wire gauze. To answer your other question, I believe that the standard heating method in undergraduate labs is to use
heating mantles with traditional round bottoms flasks, using a magnetic stirrer only when necessary for the reaction to proceed well.
EDIT: Beat to it by our very own Forum Drunkard
[Edited on 2-21-2015 by gdflp]
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Latsabb
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I am studying chemical engineering, and the focus does not seem to be on the lab side. Our first lab involved a reflux and a distillation, and the
theory and mechanisms of both were never discussed in advance. We were told how to construct the apparatus, and what to do, but not how or why. I
spend several days reading up on both in advance of the lab, which is why I had the idea that how we are being told to do it is not exactly
conventional.
The school has very limited funds in their chemistry department, and most of those funds went to their analytical side last year when they brought in
a new HPLC and ICP-OES. We are so low on glassware that we are not even allowed to use traps during vacuum drying, because then there wouldnt be
enough Buchner funnels and flasks to go around to all the groups. We are not graded on our yield, luckily, but I usually just dont have the time to
dump into constant overboiling and such, because I need to get back home at a reasonable time to reach picking up my kids from school, which is why I
am after alternatives here.
The labs are being instructed by 3rd year students to keep the costs down, and the two in charge of our group straight up said that they didnt know if
this would be good for the glassware or not, which is what left me concerned. Especially since we are already at a lack of glassware, and I dont want
to end up on the bad end of that discussion should something break when I am doing something that isnt in the lab guide.
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aga
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Find a metal can in the garbage, like a food tin or a coke tin with the top cut off.
That will work, and will be free.
Edit:
all you need is something you can boil water in - it does not need to be borosilicate 3.3 glass.
[Edited on 21-2-2015 by aga]
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aga
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Where in the world are you Latsabb ?
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Latsabb
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Ok, thank you very much. So long as this will work without problems, it is fine. The reason that I wanted to stick to glassware was so that I could
see what was going on in the pointed flask, as I need to keep a journal for color changes and such.
Edit: I live in Norway.
[Edited on 21-2-2015 by Latsabb]
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aga
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If the Glass is not totally in the bean can full of water, you can still see what is happening ....
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subsecret
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@aga: Good idea.
You could put a piece of foil on top of the can and around the flask to get a steam bath, or even something like a steam-warmed heating mantle.
If you need to go hotter, you could use sand as the bath material, though it will heat more slowly. Cooking oil will also get well above 100C if you
aren't opposed to the smell.
Fear is what you get when caution wasn't enough.
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Magpie
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I think using a water (or oil) bath in a beaker would be fine, if: You place a screen on a ring stand and support the beaker on that screen. I think
this was standard before we got these fancy mantles.
I would recommend a round bottom flask over a pointed flask, which I believe is designed as a receiver, not a boiling flask. But, as you say, you may
not have a choice.
Edit: I really don't know the purpose of a pointed bottom flask.
It is usual to assume that the bath needs to be ~20 °C higher than the flask contents.
Ideally you would place the thermometer in the stillhead. Using boiling stones is highly recommended.
I use mantles because they are convenient. But if you look back to the posts of garage chemist you'll see that he mostly used his bunsen burner. And
he's no slouch.
[Edited on 22-2-2015 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Latsabb
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We were instructed to use pointed flasks rather than round bottom flasks due to the fact that we are using Bunsen burners. They said that with the
flame directly beneath the pointed flask, the point spreads the flame out more, so that we dont need to move the flame around the bottom of a round
bottomed flask. Someone else mentioned that it was easier to see when there were only a few drops of liquid left in a pointed flask, assuming the
distillation was going for full boiloff.
There will still be a thermometer in the stillhead, but I was thinking about having two. One in the water so that I could see where it was as an early
warning to avoid overheating in my flask. But thank you very much for the tips. I will take everything that I can get.
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