Furch
Hazard to Others
Posts: 130
Registered: 8-10-2006
Member Is Offline
Mood: No Mood
|
|
Aspirator vacuum variation.
I've experienced something extremely odd. I vacuum distilled a mixture of substances which with my aspirator as the vacuum source usually come over in
the 105-135 °C range... But today, which is an extremely cold day (the tap water is 8 °C), the mixture came over at 70-115 °C. I was really
puzzled, to say the least!
I know these are my substances, they smell right, and I've confirmed with TLC references from previous runs.
This sounds ridiculous, but it's the only answer to the "problem", as I see it: the water is more dense this time of year, hence the aspirator can
create a better vacuum since more water (not from a volume point of view, though) flows through it per unit of time.
Yet this explanation would implicate that the vacuum is TEN FOLD better than when the water is 15-20 °C or so...
Is there anyone in the forum who was experienced a similar phenomena?
\"Those who say do not know, those who know do not say.\" -Lao Tzu
|
|
|
not_important
International Hazard
Posts: 3873
Registered: 21-7-2006
Member Is Offline
Mood: No Mood
|
|
That temperature difference drops the vapour pressure of water from around 15 mmHg to around 8 mmHg. While the actual vacuum generated is higher than
that, it tend to track the water vapour pressure pretty well. So while not a halving of pressure, perhaps a 1/3 drop. At that temperature and
pressure, I'd expect a 10 to 15 degree drop in boiling point, just guessing. There may be a little help from a lower pressure in the distillate
collector as well, if the room is cold; just a couple of mm but it helps.
So try looking up the pressure-boiling point information on what you're distilling and match the observed temperatures against pressure.
|
|
|
Furch
Hazard to Others
Posts: 130
Registered: 8-10-2006
Member Is Offline
Mood: No Mood
|
|
Thank you for your reply, not_important!
I've both checked Merck for the boiling points at different pressures, and it can roughly be estimated by a nomograph consisting of temperature
corrected at 760 mmHg, observed b.p., and pressure. According to this nomograph, the pressure I've been able to pull with my water aspirator was 1.9
mmHg.
Also, if there is a 10 °C b.p. drop from reducing the pressure to 1/2 of a given pressure, that means that from 110-135 °C @15-20 mmHg 10 70-115,
the pressure hals halved three times, i.e. the pressure oughta be roughly 2 mmHg. This of course in theory, but still frightingly close to
the result of the nomograph I consulted...
I'm still puzzled 
Perhaps I'll double check with a reference substance, just to be even more certain of the purity of what I'm distilling.
Is it possible to pull a 2 mmHg vacuum with a water aspirator? I think it sounds unreasonable.
Btw, here's a nomograph for you all, unless you already have one: http://designer-drugs.com/pte/12.162.180.114/dcd/chemistry/e...
[Edited on 22-2-2007 by Furch]
[Edited on 22-2-2007 by Furch]
\"Those who say do not know, those who know do not say.\" -Lao Tzu
|
|
|
not_important
International Hazard
Posts: 3873
Registered: 21-7-2006
Member Is Offline
Mood: No Mood
|
|
Can't get 2 mmHg with water, unless you pull some tricks - putting a water vapor trap inbetween the aspirator and whatever you're evacuating will let
you get pressures below the vapor pressure of water if there's not too much outgassing.
You can get that low using a glycol for the working fluid, but acid vapours make goop. Supersonic steam jet aspirators can go that low as well.
Even 15 to 20 mmHg sounds a little low to me, I'd expect expect about 10 mmHg higher at that water temperature range. Makes me wonder if the
thermometer is reading low at low pressures, the heat loss through the steam isn't being made up for by the input from the distilling vapours. Be
interesting to seal a bit of a reference material such as benzoic acid or sulfur in a small tube and wire it to the thermometer as a check.
|
|
|
Sauron
International Hazard
Posts: 5351
Registered: 22-12-2006
Location: Barad-Dur, Mordor
Member Is Offline
Mood: metastable
|
|
I run an aspirator pump with two high performance heads and a large motor that uses 10 liters of waters and even when I have mixed in ice so that the
water is <5 C I never see a vacuum much below 17 torr, I have both mechanical and electronic gauges on the thing so I know what it can pull.
So, there's something going on besides your change in tap water temperature.
I assume there has not been a huge change in barometric pressure at your locale, like, your city hasn't relocated to a mountaintop?
As S.Holmes used to say, when you have eliminated the impossible, whatever remains, no matter how improbable, is your answer.
|
|
|
|
![[*]](images/xpblue/default_icon.gif){kind=icon}
