Density Gradient Degassing Problem

name         Hsieh
status       student
grade        9-12
location     TX

Question -   In our experiment we trying to create a density
gradient with 2-propanol and H2O.  Our 2-propanol and H2O are mixed
and degassed with a vacuum pump, then transferred into a column via
2 peristaltic pumps. One for the 2-propanol and one for the
H2O.  The problem occurs during the process of filling up the
column. For some reason there seems to me a copious amount tiny air
bubbles, and the solution is milky colored.  The gradient does not
work because everything we throw in floats to the top because all
the tiny bubbles cling on to sample.  Where are the bubbles coming
from? We degassed both the 2-propanol and H2O.
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Hsieh,

Possible sources of the bubbles:
(1) when you uncouple the vacuum pump to hook up the peristaltic 
pump, air rushes in. Possible fix: do this very gently and very 
slowly (barely crack the seal and wait a few hours).
(2) your peristaltic pump is not hermetically sealed and air gets 
mixed in during the transfer. Possible fix: forget the pump, add the 
liquid very slowly and very gently (add the more dense liquid first, 
naturally) and then let it sit to form the gradient.
(3) the peristaltic pump is swirling the system too much and gases 
are being mixed into the system during the transfer. Possible fix: 
(see #2 above)
(4) you are somehow heating your system - although the biling point 
of propanol is reasonably high at 82C, it can still do some 
evaporation if you have local heating. Possible fix: make sure there 
are no pockets of heat, work somewhere air-conditioned :)
(5) Your column is not clean and proper wetting has not occurred, 
air bubbles form because your solution is not adhering evenly to the 
column surface. Possible fix: wash the column thoroughly and rinse 
it with your solvents, see if you get sheeting or droplets. If 
droplets form, clean the thing better.

Is it possible to hook-up your vacuum to the column and degas the 
solution there (rather than degas and then transfer)? You could also 
do the transfer, agitate the column to remove as much gas as 
possible, and then letting the column sit undisturbed overnight (or 
more). You could also combine the preceding action with actually 
bubbling in more gas into your column (fully impregnate the liquid 
with as much gas as it can hold) and then letting the column sit for 
overnight (this way the solution no longer spontaneously out gases).

Good luck, let us know how it turned out.

Greg (Roberto Gregorius)
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