Water is an important constituent of the fermentation broth and with a boiling point of 100 deg. C. lies intermediate between the least and the most volatile components of the mixture. It has one important difference from the other components, however, in that it forms an azeotrope with ethanol. An azeotrope is a mixture of two liquids with a boiling point lower than either constituent. In the case of ethanol and water the azeotrope occurs at a mixture of about 96** percent ethanol (v/v) and 4 percent water. The boiling point of this azeotrope is 78.1 °C. whereas the B.P. of 100% ethanol is 78.4 °C. As far as the system is concerned it "thinks" that this mixture of ethanol and water is a single liquid with the lower boiling point of 78.1 °C. and proceeds to separate it on that basis. The ethanol which is purified by a fractionating column is not, therefore, pure 100 percent ethanol but pure 96 percent, the "impurity" being pure water. No amount of re-distillation under the conditions we are using will influence this percentage.
If it is absolutely essential to remove all the water, for example if it is to be mixed with gasoline to produce gasohol, then special methods are available to accomplish this. For our purposes, however, where we are going to dilute the alcohol with water to 40 percent anyway, the presence of 4 percent water is of no consequence.
**Footnote. In the literature you will find slightly different values for the azeotrope composition, all hovering around 96%. One reason for this is that the percentage can be expressed either volumetrically (v/v) or by weight (w/w). There is a difference because ethyl alcohol has a specific gravity of 0.8 compared to 1.0 for water. For example, 96% ethanol v/v works out to 95% w/w. If so inclined you may worry about this, but a more important question is — should a good martini be shaken or stirred!
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