tom haggen
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Suck Back
I've seen this term mentioned around the forum a few times, and haven't really been able to understand it. I tried browsing through my
O-chem lab book, and it made no mention of this term. I was just wondering if anyone could explain what it is and what causes it?
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The_Davster
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Suck back occurs when a gas of high solubility is bubbled into aqueous solution. It causes the solution to rapidly rise up the bubbling tube, and
possibly into the reaction flask that is producing the aforementioned gas. This could be disastrous depending on the nature of the gas generating mix
and what the gas is being bubbled into.
Preventing suckback is quite easy to do, there are two common ways. One is to have a inverted funnel for the exit of the gas into what it is being
bubbled into. This works by reducing the height the solution would be sucked into the bubbling tube, preventing it from contacting the gas generating
mix.
Another way to prevent suckback is to place a sealed flask along the line of the gas transport tube. This stops any suckback from getting into the
gas generating mix by causing any suckback that does occur to be left in the aforementioned flask.
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tom haggen
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Oh I see, so this only occurs when you are running a gas generation reaction. So with the second way of preventing suck back, it sounds like it can be
set up almost like a water trap.
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BromicAcid
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Not only can it occur in a gas generation setup where the gas generation is the main goal, but it can also occur during a normal distillation, if for
example you were running the exit gasses from a closed vessel distillation into something to scrub them to remove hazardous components, well, during
distillation the pressure inside the vessel is not constant, as the liquid is distilling there is a good chance there is a positive pressure and
bubbles will go through your liquid, but eventually there comes a point where the distillation slacks off and when it does so the gasses inside the
vessel contract and this will pull the liquid into the vessel and into the receiving flask.
It occurs in a number of other situations as well, if suckback is expected a trap is something that is worth taking the time to set up.
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neutrino
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I have also seen this term in conjunction with vacuum distillation with an aspirator. Occasionally, some of the aspirator’s water will suck into the
distilling apparatus. Again, a trap (BromicAcid’s flask) is used here.
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Magpie
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I think suckback refers to any situation where you have created a vacuum in a vessel and that vacuum "sucks back" liquid from another
vessel, inadvertantly.
As rogue chemist has indicated the inverted funnel (an air gap) or an intermediate isolation vessel (trap) should do the job for the chemist.
In engineering applications suckback is prevented by "vacuum breakers," "backflow preventers," "air gaps," etc. I have
a vacuum breaker to prevent my lawn irrigation water from entering my household drinking water.
[Edited on 3-5-2005 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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tom haggen
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Hows this picture look? I believe the guy is using an Erlenmeyer flask as a water trap.
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BromicAcid
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Looks good for distilling under reduced pressure. I've had water come in through the aspirator when using it to evacuate my apparatus partially,
little drips of it even though there is a vacuum. That is a good setup for that particular purpose.
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garage chemist
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An inverted washing bottle in the gas line is especially well suited for reactions with gases, since if suckback occurs, the liquid will be pushed
back into the reaction vessel by the increasing gas pressure.
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azaleaemerson
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You don't even need a vac for suck back, just a pressure difference will do. I was once helping a (Post hole. Digger.) chemist at a company
where I was a tech. He was using nitrogen gas to slowly volatilize an alcohol and carry it into a solution of aluminum alkyl. Well, the nitrogen
got cut off at one point so that the pressure in the Alalk pot was now higher than the pressure in the alcohol for just an instant. In that time a
slug of Alalk pushed back into the alcohol. There was a delay as we saw the mixing lines of the two solutions. We jumped back and he slammed down
the hood sash. A few seconds later the flask shattered.
I do very few reactions at home and I do them only after very careful safety consideration. In my short time as a tech I've seen some bad
accidents.
azalea
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