Sciencemadness Discussion Board

% of azeotrope

shadow - 15-3-2008 at 18:39

My future experiments require anhydrous ethanol, so I thought I would distill some 40 proof store bought ethanol down to 96%, and then clean it up from there.
So I look in my rank ametuer literature, and it says that the azeotrope distills over first, and I see drops coming over at a little under 80C, and then the temp rises to around 85 or so, and then continues until I get tired and turn it off.
The literature says that the vapour pressure of the water increases with temperature, so is more than 4% water coming over?
To determine the quantity of components of the distillate, I can do density tests with my scale, excel, and my volumetric flasks, or somebody can say, "Hey dude, it's all 96/4, GO TO THE NEXT STEP!!!
Thanks ahead,

not_important - 15-3-2008 at 19:03

If you used only a simple distillation rig, you'll only get approximately the azeotropic mix. increasing in water content as you distill more off. to get a distillate that's really close to the azeotrope you need to do fractional distillation.

JohnWW - 15-3-2008 at 20:50

You can buy a pycnometer from an home-brewing shop, to measure the density of the distillate. Then, using tables of densities of ethanol-water mixtures, given in, for example, the International Critical Tables, Perry's Chemical Engineers' Handbook, and the Handbook of Chemistry & Physics, one can obtain the ethanol content from the density.

microcosmicus - 15-3-2008 at 21:45

Alternatively, measure boiling point --- after all, you already have a thermometer
and a means to heat the alcohol at hand from the distillation. If it's close to 78.1 you
go on to the next step, if not, you distill it again.

You can find a density-concentration table here

Look here for all you wanted to know about the thermodynamics
of distilling ethanol but were afraid to ask:

[Edited on 16-3-2008 by microcosmicus]

shadow - 16-3-2008 at 01:23

Just as I expected.
When I first read of the procedure, I imagined that the ratio coming over was static.
Well I was wrong.
Thank you so much for the help.
And a word to JohnWW, the USofA really is a wonderful place, even with all the BS.