absolute ethanol

[David F. Spencer]

In article <39D496C8.CF8877AE at bristol.ac.uk>, Jonathan Warawa 
<J.Warawa at bristol.ac.uk> wrote:

> I need to keep a stock of 100% ethanol absolutely absolute for use in
> critically drying tissues for scanning electron microscopy.  Does anyone
> know if there is some moisture absorbing reagent which I can keep in the
> ethanol (ie CaCl2 or activated charcoal) that will absorb out the water
> which is sure to contaminate my ethanol due to moisture in the air.
> Thanks,
>   Jon Warawa
>   J.Warawa at bristol.ac.uk

As the other poster suggested the most effective way to do this is with 
4 Angstrom molecular sieves. I believe I've also used 3 A for this 
purpose but the 4 A is the norm. They are a good way to keep all types 
of solvents dry and have been used to purify solvents as well.  If you 
need to dry the molecular sieves before use you need quite high 
temperatures (300C +) because water sticks quite tenaciously to them. 
They are available in several formats: the small pellets (cylinders) are 
quite effective but you can get them in spheres.

CaCl2 wouldn't be a good choice because it of course dissolves in 
ethanol. Activated charcoal isn't likely to accomplish anything useful. 
Anhydrous CaSO4 is also slightly ethanol soluble.

Here is a electron microscopy reference which includes the description 
of using molecular sieves for keeping ethanol dry:

http://borisy.bocklabs.wisc.edu/pages/protocols/electmicrosctext.html

Dave

-- 
David F. Spencer, PhD
Dept. of Biochemistry and
       Molecular Biology
Dalhousie University
Halifax, Nova Scotia, Canada

dspencer at is.dal.ca