chemrox
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Nitrogen atmosphere
I don't have a ot of sophisticated gear but I have glassware, heating and stirring equipment, a rotovap, etc. and I have to do a lengthy procedure
under N2. How can I minimize wasting gas? I used up 40# in two hours until I necked down my gas inlet tubing. Now I'm using a Mohr pipette for
this. Anyway, should I neck down more and try to use the smallest stream possible? Is there a way to maintain enough P to keep the air out without
using up 80# in 24 hrs? Is there a way to calculate, quantify this?
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Magpie
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I have never done this nor been trained in how to do it, but here's my naive thoughts:
Assuming that you have your nitrogen pressure reduced down to a few psi using a regulator, you could set the rate of N2 delivery using a rotameter
equipped with a needle valve. The meter would tell you the delivery rate in, say, mL/min.
The minimum rate that you need for your particular situation would seem to depend on working volumes, leak paths, etc. You might have to determine
this by using an O2 meter or such.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Sauron
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Does the procedure call for a stream of nitrogen or just a nitrogen atmosphere?
The former is what you are doing and is typical for sensitive distillations.
The latter, you use a balloon to maintain the slight overpressure and a Hg filled U-tube to seal off the system. Once the system is purged and the
balloon filled you can shut off the cylinder. If the balloon deflates because of a leak in system, reinflate it and try to locate and sel the leak
with Apiezon.
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Ozone
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The balloon method works well. Just beware if the balloon starts *inflating* whilst attached to your rig (happened once to me with HCl(g)). If there
is no other way (what I am doing now), build a glovebox! I'll post a link soon with an account-in-pictures of my progress.
Better than N2...Argon! It is heavier than air, so it blankets well, displaces other gasses, etc. If you can afford it, Ar is the way to go. Either
way, assemble your apparatus, I like to use 20 guage Luer-lock syringe needles with Suba-seals (septa). Connect 1 needle to a 1mL syringe (plastic,
BD, etc) which has been cut in half. Secure the assembly to the gas delivery hose with wire. Insert the needle into the septum closest to where your
reaction mixture will be. Insert a second needle into a septum placed as close to the top of the apparatus as possible (the top of a condenser usually
works well). Initiate a stream of gas in though the bottom needle, allow this to proceed for ~5 min. Fire up a bunsen burner (or propane torch) and
flame the outside of the glass--you will be surprised by the amount of water that is adsorbed onto the surface of otherwise "dry" glass. Continue
until no more water is evolved (this will be swept out by the gas). Do not locally overheat your glass (especially at ground glass joints!).
Then, remove the top needle about 0.5s before the bottom. Take the needle assembly from the gas hose and wire it to a balloon. Fill the balloon with
gas and insert the needle into the top-most septum (say, condenser), and you should be good-to-go.
Cheers,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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solo
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| Quote: | If you want your reaction to be under nitrogen only, in the first place you need to remove air or any gas that you have in the apparatus.
There are two main ways to achieve that: one is based on doing vacuum/nitrogen cycles (which has a specific name, but I don't remember it), the other
is based on diffusion.
In small scale reactions, people just seal the apparatus with a special rubber stopper that can be pierced by needles. Vacuum is applied, and then a
rubber balloon filled with nitrogen, connected to a needle, is inserted. Nitrogen substitutes the vacuum, and you have little or no air left. You can
repeat the procedure 2-3 times to be sure.
You can do the same in larger scale apparati, you just need to use a special piece of glassware (a gas inlet with stopcock). In this case, in order to
keep the pressure at about 1 atm, you use the system described in previous posts. The bubbler is just a Drechsel bottle filled with (little!) silicone
oil or similar stuff, and you need the oil only for detecting the flow. Basically you have your nitrogen source connected to a T joint. One arm is
connected to the bubbler, the other to your reaction vessel, and that's it. Hardly rocket science!
When it's not practical (or not possible) to apply vacuum, the usual procedure consists in having a flow of nitrogen going through the apparatus (i.e.
you need an inlet and an outlet). Based on the principle of diffusion, you can assume that after some time you have almost no air left, because
nitrogen carried it all away.
The major drawback of this method (especially for small scale reaction) is that nitrogen carries away your solvent too, so you may end up with a
different solvent composition, different concentration, or even with a solid (I say that from personal experience...).
What you can do then is to maintain the flow at the beginning, to remove air, and then switch to the static nitrogen system as above.
To answer beheada, yes, you do need a pressure gauge, if you connected directly the big cylinder to the line you would just blow everything up; reflux
doesn't make a huge difference, you just need to use a good condenser and connect the nitrogen line to the top of the condenser rather than to a
sidearm of the flask (or you will see the solvent condensing in the N2 line!). |
....................Lavoisier, a member of chemical forum.
http://www.chemicalforums.com/index.php?topic=13071.msg60543...
I had to do a methylation of an amine under nitrogen and found after purging the system with argon I placed a ballon full of nitrogen on top of my
refluxing column and was able to do a reflux under nitrogen for an hour....the above pic and comments are the result of my inquiry at that
forum........solo
It's better to die on your feet, than live on your knees....Emiliano Zapata.
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Ozone
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Yes!
The manifold is the way to go (if you have one ).
Cheers,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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chemrox
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Thanks to you all! I only need an atmosphere of N2 and I can evacuate with a pump before starting. I have 3-way stopcocks but no septa. However I
can improvise with rubber hosing and a baloon can be attached by way of a marie on the gas line. I need to stir and that may be my worst gas leak
area...don't have a power supply for the geared motor so have to use the cone drive beast. Assuming that leak can be plugged the rest of the system
should be gas tight with some stopcock grease and joint clamps.
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chemrox
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Even better, I have a Florence flask with a stopper to fit the top. I can flame it, evacuate it and fill it up with N2 or Ar (same price locally) and
attach a balloon with a "T" or marie as they somtimes call it. With this simple device I can easily blow out all the air, drop in a strirring bar cap
it, pull a vacuum, let the N2 in again and keep pressure with a baloon. I was also thinking of Hg in a small cylinder but this is really cool. I
hadn't realized the Hg was so I could cut the N2 flow and maintain atmosphere. Balloon is so much cleaner! My solvent is DCE. Will it mix with the
N2 back to the balloon and destroy the balloon?
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solo
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How about some leg work......... ballons are made of polyethylene, so look up reaction of DCE with polyethylene.....somehow I don't think they will
react........solo
It's better to die on your feet, than live on your knees....Emiliano Zapata.
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Ozone
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I thought that most "party" balloons were made from latex, viz. uncrosslinked rubber (which is why your balloons go all sticky and make a horrible
mess when stored for a long time--especially in a hot garage). In my experience, chlorinated solvents like DCM, DCE, etc. go right through latex
gloves to defat the skin below . Usually, aluminized mylar gloves are used when
handling these solvents (at least by those who care about it--I just shake it off and it evaporates harmlessly). At the least, the material would
swell and become far more permeable to gas.
This would depend on the vapor pressure of your solvent, which depends on the solvent and the temperature. how hot do you want to run this thing--Hmm,
if you have your balloon "plugged in" on top of an efficient condenser, DCE should not be too big a problem!
Methinks some sort of empirical test be in order, Arrr...
Got a cume tomorrow, so a study I go. I'll address this more a little later.
Cheers,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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solo
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| Quote: | Originally posted by Ozone
I thought that most "party" balloons were made from latex, viz. uncrosslinked rubber (which is why your balloons go all sticky and make a horrible
mess when stored for a long time--especially in a hot garage). |
Yes, rubber , latex, and polyethylene films (scientific ballons)....I guess the most common and available type ballon are those of latex, and
rubber......solo
It's better to die on your feet, than live on your knees....Emiliano Zapata.
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Levi
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| Quote: | Originally posted by solo
How about some leg work......... ballons are made of polyethylene, so look up reaction of DCE with polyethylene.....somehow I don't think they will
react........solo |
I think the question chemrox was asking is "is DCE miscible with the N<sub>2</sub>" rather than "will DCE destroy the ballon."
Chemcrime does not entail death. Chemcrime is death.
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chemrox
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Well .. yes you're right. Actually it could be regarded as two questions: 1st is will DCE or DCM diffuse through N2 atmosphere?
2nd is, if DCE/DCM diffuses through N2, will it attack latex? They are made of Latex and one has to get the ones rated for Helium.
I've got two choices for appartus;
one is a Florence flask that once did duty as a coffee carafe but is pyrex .. like those cork coated one we use for boling water sometimes (?) anyway
that has a one hole stopper that fits it and I can use it with my heater/stirrer but no condenser
The other is a 3L, 3-neck rb for which I have a stirring shaft and blade but the bearing is new, tight .. the motor has to be set just exactly right
or it stops .. this has been a problem .. i got it to run all night once so far ...this setuo I could use with a condenser and the balloon could go
atop the condenser as O3 said .. a thermometer adapter minus the little rubber part would do nicely for that.
As you see, my biggest p[roblem of the moment is the stirrer motor setup.. I'm just not making it work as it should. The bearing is Teflon and the
upper section is tighter than the lower .. also I've noticed my stirring rod shaft is of slightly varying diameter .. close to 10mm but in places more
like 10.05-10.20 ..
The motor is mounted on one upright of a stand that is braced to another stand with two horizontal rods. The pot is supported from below with a ring
.. with or without heating mantle ..and at two of the joints with clamps. There is flexibility in this setup I wish I could get rid of as the weight
of the motor causes some bending that is greater higher up the shaft than lower where the pot is .. any thoughts on how to make the stirring motor
setup more reliable would help a lot.
And there's this concern: if the bearing is set loose enough for the motor to turn without working too hard , will gas escape through the bearing and
deflate the balloon causing the atmosphere to be lost?
No such dilemma attend the simpler, makeshift setup but no condenser .. so maybe a bubbler is essential to that apparatus?
[Edited on 16-3-2007 by chemrox]
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Ozone
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Wow!
How stout (viscosity) is the solution you plan to handle? Unless I am doing a polymerization (excepting solution or suspension), I use a magnetic
stirrer placed directly underneath the heating mantle (or, use a heating-stirring plate, if you are using a sand bath,etc.).
If you wish to use a bushing with an over-head stirrer, you are going to need constant positive pressure--*or*--you could fill the joint wiht
something heavy, like Hg (which works well, but is not generally recommended).
A glove box is sounding like a better idea .
Cheers,
O3
-Anyone who never made a mistake never tried anything new.
--Albert Einstein
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chemrox
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I don't have a curved stirring bar small enough to fir through my 24/40's. I'm afraid my straight one might flop around too much to stir well but I
might give it a whirl (sorry) .. I could fill the bushing gap with silicone grease ...on the other hand I'm not using heat so I really don't need a
condenser either do I? I wish I had a glove box. I might do a Gatterman one of these days. Have you used a straight stir bar with a rb flask? I
will call around and see if I can get a curved one (egg shaped) locally .. but the only thing about the Florence I don't like is its pretty thin
walled .. I sue a secondary containment when I stir it hard the way I will have to for this. I could put Hg in the bushing I have but it's Teflon and
wasn't made for it. Probably would work .. but if it didn't it might leak slowly overnight so the next day my rod is silver colored and my solution
is poisoned ...
[Edited on 16-3-2007 by chemrox]
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chemrox
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I wanted to tell you all that I got a 1000 ml flat bottom boiling flask to avoid the overhead strirrer complications.
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