whupharm_xu - 26-9-2004 at 18:21
during the distillation, b.p.of the required distillate should be 80 degrees, when Ar gas flow is used as protected gas, the b.p is 40. Ar has so
great effect on b.p???is it possible?
Organikum - 26-9-2004 at 18:46
It seems you dont distill anymore but evaporate by a forced stream of gas.
Of course you dont get a distillate but a evaporate - you dont transform the liquid into gas by heating but transport it mainly in form of moisture -
little drops of liquid.
What is the sense of a stream of Argon btw? If you want to exclude air you purge ONCE with Argon or another intert gas, you may leave a minimal flow
for antibump (capillary) later but thats it. The boiling liquids take all space anyways, once the air is removed its done, where shall fresh air come
from?
A capillary as antibump device is to be so small in diameter that for example a usual childsplay balloon full of nitrogen suffices for a distillation
of several hours.
[Edited on 27-9-2004 by Organikum]
Marvin - 26-9-2004 at 19:04
Organikum's post is almost entirly correct but an evaporating liquid is being turned into gas, not transported as droplets.
There is nothing particulally special about a liquids boiling point, its just the point where a liquids vapour pressure exceeds atmospheric pressure.
If the pressure of the air was lower, the boiling point would be to. The key thing here though, is that at boiling point the pure gas has enough
pressure to displace all other gas from the system, ie its not blocked from reacing the condensor, below this point it has to diffuse, which is very
slow.
Using a moving stream of gas is just another way of making vapour move from the surface of the liquid to the condensor where some of it will turn back
to liquid.
BromicAcid - 26-9-2004 at 19:41
You're just shifting the equilibrium:
X(l) <----> X(g)
Be removing the gas from the area above the liquid causing it to form more vapor.
Occasionally I've used this technique in class to evaporate off volatile solvents that contain organics which are too sensitive to heat.
Magpie - 27-9-2004 at 18:51
I don't see how you could lower the boiling point by that much unless you lowered the pressure of the atmosphere over the liquid drastically. If
your thermometer bulb is in the neck of the flask my guess is that your too rapid stream of Ar is cooling your thermometer bulb to 40 but the liquid
is actually more like 80.
Tacho - 28-9-2004 at 03:38
I agree with Magpie. Is the liquid really boiling? I mean, bubles coming from below the surface? Or the temperature is just remaining stable at 40?
vulture - 28-9-2004 at 09:47
The liquid is being "torn" away by the constant gas flow. The vapor will have almost the same composition as the liquid and you'll end
up with more distillate as without gas at the same temperature, because condensating droplets are blown through the column before they can fall down.
Therefore your thermometer reading will be of no use.
I observed this effect myself when I was distilling alcohol with a vigreux and left an air entrance at the distilling flask whilst pulling vacuum.
Marvin - 28-9-2004 at 21:44
Hmm, I dont like the term 'torn away'.
Above the surface of the liquid the gas is saturated in the liquids vapour. To get furthur the liquid vapour (gas) has to diffuse through the air to
reach the condensor.
By blowing a stream of gas you are mechanically moving the vapour laden mixture towards the condensor. This is vastly quicker than diffusion. The
gas over the surface of the liquid quickly gets saturated with vapour and the process continues.
Unlike with distillation you lose whatever the vapour pressure of the liquid is at the temperature of the condensor, unless you use a closed cycle of
gas.