lysander
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What is up with my Graham condenser?
I was preparing HCN testing a new recipe today. Evidently a Graham condenser is overkill: I was running chilled water through it and the vapor
(boiling point 25C) was condensing in the mouth.
But I don't understand what's going on here: http://youtu.be/SqLJU862yMQ
The condensate sits in the mouth. It looks like it's winding its way down the condenser, but there's no siphon effect. Instead, something is
bubbling back up through the condenser.
I ran the reaction for over half an hour and captured a grand total of 5mL of HCN. But the reaction was bubbling aggressively in a Florence flask on
my mixer/heater the entire time. I know the smell of HCN, and there was only a little leakage initially until I set all the glass joints.
The only hypothesis I can come up with consistent with the observations is that the HCN is boiling before reaching the drip point into the
distillation flask.
But this hypothesis seems improbable given that the condenser was almost freezing, the ambient was right about the boiling point of the condensate (so
it should have emerged needing some heat to vaporize), and I'm not dead from inhaling leaked HCN. (Or am I?  ) Furthermore, I don't know why any vapor would get sucked back through the Graham condenser given that the joints had
sealed with liquid from the condenser through the kjeldahl bulb back to the boiling flask.
I'm going to get a Liebig condenser for the next attempt, but if anyone has insight on what's going on with this one I'd appreciate it!
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Crowfjord
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I think that part of the problem is the orientation of the condenser - this type operates best in a vertical orientation. Perhaps the reaction flask
was not boiling hard enough to generate the needed over-pressure to push condensate through the condenser. It looks like you have the appropriate
connectors, just rearrange the setup with the take-off adapter and condenser switched so that the condenser is vertical an leading straight into the
collection flask.
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Praxichys
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I can only hope you are trolling, but I'll take the bait.
If you don't know what you're doing practice with something else before you kill yourself.
That video does not demonstrate the correct use of a graham in your situation. Use it vertically or there will be a significant amount of
back-pressure. As the liquid bubbles through the tube the fluctuations in pressure cause periods of calm followed by periods of violent boiling. The
pressure can cause anything from joint leakage to an explosion. The inertia of a fluid-filled graham is quite a lot. One big bump and you can pop a
flask/joint.
As for the back-flow you were experiencing, it will be helpful to know what "recipe" you were using and all reagents loaded into the setup, including
any scrubbers.
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Bert
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Thread Moved
26-8-2015 at 18:48 |
lysander
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OK, I could swear I've seen Graham condensers used in that orientation, but if I try that condenser again I'll set it vertically.
As for safety: I was using a full face shield, lab coat, nitrile gloves, and cross ventilation. I always assume there is some possibility of
explosive glass failure in this kind of reaction (and prepare accordingly), but I did not have the drip joint secured so at least I gave any
overpressure one exit point.
Now if anyone can address the backflow: The 1L boiling flask was filled with 100mL distilled water, then saturated with K3[Fe(CN)6], then added 50mL
H2SO4, then continued stirring and added heat up to about 60C.
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aga
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Might help if you had a vac adapter on the collection flask to let the pressure out ...
The video shows a completely sealed system.
Air pressure in the flask is preventing the condensate travelling down the graham.
Lucky it didn't blow off the glass joint with all that HCN in there.
[Edited on 27-8-2015 by aga]
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lysander
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Yes, I was worried about the drip adapter sealing with the collection flask. It's not fully seated, but just to make sure at several points I lifted
it completely off. Next run I'll use a flask with a tail to ensure the system can't pressurize.
Still wondering where all the HCN could have gone, though I haven't verified the efficiency or rate of this reaction, so maybe most of the condensate
was just water....
[Edited on 27-8-2015 by lysander]
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aga
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Higher pressure = higher BP ...
It didn't go anywhere.
The joints look pretty much seated (therefore sealed) in the video.
Edit :
Re-viewing the video, there does seem to be a gap, so not sealed.
Guess i have to switch to 'incorrect apparatus' as stated above by others.
[Edited on 27-8-2015 by aga]
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lysander
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Meaning the only correct use of a Graham condenser is in the (near-) fully-vertical orientation?
(Regarding the pressure/bubble-up through the condenser: The collection gap could have gotten "liquid sealed," but I assume that wouldn't be enough to
maintain a significant pressure differential through the collection flask.)
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aga
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if you want stuff to condense and drip Down, best not give a whole series of Up to get past, i guess.
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lysander
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Just found another thread confirming the Graham condenser should be vertical.
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chemrox
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Of course Graham condenser are used in a vertical position .. that's what they're designed for. Get a Liebeg off ebay for $20.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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aga
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Why 'Of Course' ?
An explanation would be helpful to people who do not know, like me.
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lysander
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Not to mention all the pictures that show them setup at shallow angles. Just do a Google image search for "graham condenser." The vast majority
depicting a complete apparatus in what I just reviewed show the condenser in a more horizontal configuration.
[Edited on 27-8-2015 by lysander]
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cyanureeves
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i think pressure gets past the condensate but no vapors are following and no vapors are cooling down because they cant reach the coils.it is all
equalled balanced pressure from the boiling flask to the point where the condensate is collecting.more heat will probably push the joint apart right
at the flask or the graham.more heat will probably ruin your end product too and there is no room for mistakes with what you are doing.
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zed
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Ummm. Are you still alive?
Never attempted a distillation in a totally closed system. Generally, one attaches an adapter onto the receiving flask, to relieve back pressure.
Then, the exhaust vented through that adapter, is channeled through a trap, or a series of traps, to render it harmless.
https://www.google.com/search?q=distillation+adapter+functio...
Something like that vacuum adapter is just fine. Hook some tubing up to it, and attach it to a trap. Bubbling escaping gases though a strong NaOH
solution should capture escaped HCN, in the form of NaCN. There may be other reagents that would simply destroy it.
[Edited on 28-8-2015 by zed]
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