|MadSci Network: Chemistry|
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 information. Good Luck Carlin Gregory Staff Chemist Williams Gas Pipeline - Texas Gas
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