greenyppols - 7-2-2006 at 18:28
What good is a Dry Ice/Alcohol bath? Some sites I've searched said the -78c or so it reaches is fine. Others say Liquid Nitrogen is the only way to
go. It seems even water cant be condensed at the -78c mark, I've read it needs at least -100c.
If most things cant be captured at that temperature, not even water, just what DOES it capture? Anything worthwhile? Liquid Nitrogen, I think after a
quick local search, may be out of reach. And after reading about condensing oxygen, it kinda scared me anyway. What determines the temp at which
something can be condensed at a given temperature? vapor pressure?
Magpie - 7-2-2006 at 19:18
The "non-condensable" carrier gas, say air, will still have a humidity even at -78C but it ain't going to be much. Just how dry do you have get your
"non-condensable" gas stream anyway?
Check a humidity chart. Air at -78C is going to be pretty dry I'm betting.
Or look up the vapor pressure of water at -78C. That pressure in mmHg divided by 760mmHg will be the mole fraction of water in the air exiting the
cold trap. This all presumes that the air stream has time to come to near equilibrium in the trap, of course. That is, your air flow rate is not too
high.
[Edited on 8-2-2006 by Magpie]
greenyppols - 8-2-2006 at 13:23
Sorry Magpie, I may have given wrong impression. I was merely seeking to protect my recent purchase of a vacuum pump, from another thread. It isnt any
particular need of a dry gas stream so much as protecting my pump. I've read contaminated pump oil will rapidly destroy the vanes in the pump. The
trap I acquired was intended for condensed solvents and any other what-not I thought I could reasonably trap.
The vacuum it pulls seems to be actually increasing a small bit, I hadnt mentioned a very,very minor bore sanding and cleaning the edges of the metal
vanes which I had thought didnt help anything, but the vanes must be "breaking in" a new seal, responsible for the better vacuum being pulled now, now
its past what I would say was 29.5, which according to a Vaccum conversion table I found translates to around 10 torr or so. A more accurate digital
gauge is somewhere down the line if I "need" one, which I dont see happening. I had intended to rebuild it but since it doesnt seem to NEED a rebuild
Im shying away from spending the $$$ on it for now. I set a limit of one year dedicated to an "acquisition" phase of equipment and chems and I have
one month left. Im rather proud of my small collection so far, next phase is actual construction of a lab area, which is in planning stages now. God
knows how long that would be and in the meantime Im still monkeying around with what I have, hooking it up and just experimenting to familiar myself
somewhat with procedure.
This is off topic but I havent taken an org chem course yet. I thought I read in a forum that someone offered an online chem degree (at your own pace,
I guess) by someone willing to teach it if you were willing to participate. I dont remember if it was free, but I had thought it was at the chemical
forums. I dont have time with other classes but Im in no real hurry anyway...yet. Im thinking this summer I'll squeeze in an intro course at least.
Damn I talk to damn much.
Of mice and men...
Fleaker - 8-2-2006 at 14:06
"It seems even water cant be condensed at the -78c mark, I've read it needs at least -100c. "
From what temperature, what flow rate, and what pressure? I see water condense all the time, it's doing so on the outside of my waterbottle right now.
It all depends on what you want to condense and the conditions you want to condense it in. As far as vacuum traps go, don't go spending $170 on a
beautiful dewar type glass trap! Find yourself some copper tubing, coil it, then get one of those disposable styrofoam drink coolers. It will stand up
to dry ice/MeOH (don't try it with acetone though!) or liquid nitrogen, and it's easily made, and it's not a big deal if you ruin it. Works well too.
I need to get a picture of my trap setup (glass ones)for cold water, dry ice, and LN2 (all in series) for my "cracking" system. Talk is cheap with me
though, I still owe Bromic a bunch of photos for his book :-(
Magpie - 8-2-2006 at 15:29
Greenyppols you are wise to protect your vacuum pump. I feel fortunate that I can use a water aspirator for my vacuum needs. I can get down to 27"Hg
vacuum in the winter when the water is cold. This will do fine for my occaisional vacuum distillation.
Vacuum
MadHatter - 9-2-2006 at 11:38
I consider my vacuum pump too valuable for anything other than air conditioning work. I dread
the thought of corrosive vapours coming into contact with any of the parts. Like you Magpie,
I use the hydroaspirator for vacuum distillation. Glass units are $11 and stainless-steel units
are $28 from http://www.macnanbio.com designed for low vacuum work. More than
sufficient for vacuum filtrations or distillations. By low vacuum they mean it doesn't pull
millitorr vacuums required in some physics appilcations. The aspirators also have the nice feature
in that noxious fumes get pulled down the drain as well.
neutrino - 9-2-2006 at 14:08
Aspirators can also be found on eBay. A good steel one is about $20. The occasional one also goes for a few dollars, keep an eye out for those.
Sorry to keep dragging this thread off topic.
greenyppols - 10-2-2006 at 04:48
I dont think thats off-topic, neutrino. Looks like I may end up constructing one of those water-jet aspirators after all. I was trying to avoid an
aspirator by killing two birds with one stone (vac pump) as I have well-water and did not want my expensively softened and treated house water to be
used for endless water aspirating. 
The lack of looking "further down the line" until I got there means I was blind-sided by the relative pain-in-the-ass needs of protecting my pump from
corrosive fumes and such. >>slaps self in forehead<<
But I still have a neat pump!
Magpie - 10-2-2006 at 09:12
If you anticipate using a lot of water then consider running a cold water unsoftened line to your sink. I did that at one place I lived to save money
and avoid drinking all that Na+.