MadSci Network: Chemistry

Re: Boiling point of glycol/water mixtures

Date: Thu Jul 22 08:12:13 1999
Posted By: Carlin Gregory, , Chemistry (BA and MS) in Synthetic , Williams Gas Pipelines - Texas Gas
Area of science: Chemistry
ID: 932568656.Ch

The boiling point of azeotropes of glycol/water increases with increasing 
concentration of glycol (ethylene, or common anti-freeze).  Here is a 
quote from Dow Chemical's Engineering and Operating Guide for Ambitrol 
(their trade name for anti-freezes):  "As liquids vaporize, pressure is 
exerted which increases as temperature increases.  Aqueous solutions of 
AMBITROL products have vapor pressures lower than water and boiling points 
above water.  However, aqueous solutions of AMBITROL products have vapor 
pressures close to that of water because of the water in the solution.  
Actually, the vapor pressure of the glycol itself is much less.  As a 
result, solutions of AMBITROL products will tend to lose water by 
evaporation as temperature rises above the dew point.  Because glycols are 
hygroscopic (attract water molecules), the fluids pick up water molecules 
from the air and dilute the solution, lowering the boiling point) as the 
temperature drops below the dew point."

So, you now have the information you requested.  I am not sure why you saw 
a boiling point depression, but you may need to take into account your 
elevation.  I sure hope this helps.  If you get really into the need for 
more information on glycol, contact Dow Chemical and request the 
referenced manual.  I keep one in my files as a source of excellent 

Good Luck
Carlin Gregory
Staff Chemist
Williams Gas Pipeline - Texas Gas

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