Simple distillation
Organic chemistry I, 08/31/04
Distillations such as the one you will do today have been carried out for millennia in the search for more potently alcoholic beverages and potable water from sea water as well as an infinite array of chemical separations. The basic process is to separate one liquid from another by boiling the mixture and condensing the vapor above the boiling liquid.
It seems reasonable that distillation should work. Think of the present case: one liquid, ethanol, has a lower boiling point of 78ºC, while water has a higher boiling point of 100ºC. It would make sense that you could pick a temperature between the two, maybe 80ºC, and boil off only the ethanol. Unfortunately, this doesn’t work very well. It turns out that the difference between boiling points needs to be more than fifty degrees apart for simple distillation to work very well. There is also an added problem of the formation of an azeotrope in the case of ethanol and water mixtures.
First let’s look at why boiling points need to be more than fifty degrees apart for simple distillations to work very well. Figure 1a illustrates a sample temperature-composition diagram. One of the purposes of this diagram is to show the components of the vapor above a mixture of two liquids. Figure 1b illustrates how to use this type of diagram. The most important lesson to note is that although the vapor above a boiling liquid might be enriched in the lower boiling component, it also contains the higher boiling component. They are mixed to an unacceptable degree except when the boiling points are very different. One way to improve the separation is to use fractional distillation, and we will do that later on.
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Simple distillation
(a) (b) |
Figure 1. (a) Sample temperature-composition diagram and (b) directions for use
As I pointed out above, ethanol has an added trouble: it forms an azeotrope. In other words, it forms a mixture that boils at a constant temperature, at a given pressure, without change of composition. Today, you will distill a mixture of 95% ethanol and 5% water, which boils at about 78ºC.
Despite all these problems, careful simple distillations can lead to fairly pure products. Basically, as you boil your liquid, the vapor that forms first will be richest in the component with the lowest boiling point. You can tell that the lowest boiling point component has begun to distill off because the temperature will stabilize to within a few degrees. Eventually, the temperature will begin to increase again, indicating that the component with the lowest boiling point has distilled off and there is little left in the original flask. The temperature will increase and plateau a second time. At this point, the vapor will be richest in the component with the second lowest boiling point. You can collect this fraction of distillate, which will be primarily the compound that boils at the second lowest temperature. This process can be repeated until all the fractions of the original mixture have been separated.
Date: 8/5/04
Contact: rajameton@lcsc.edu
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