the problems with 100% pure ethanol

thunderfvck

I was reading that ethanol can only be purified up to something like 95% by distillation and further purification calls for CaO. Why is this? Is CaO a drying agent? Why is it that you can't seperate water completely from ethanol?
Is it just not feasible without CaO, like the tiny amount of water is nearly impossible to remove due to their being so much ethanol around it. Their structures are similar, they both are polar with the OH group and there are few carbons on the ethanol so they are pretty miscible (sp?). So I guess I can see how water would have trouble breaking free of its bond to ethanol in solution, at such a low concentration. I can see it, but is it right?

thunderfvck

At the concentration of 95% ethanol, water and ethanol form what is known as an azeotrope. This is a mixture in which the azeotrope distills off before the desired liquie (ethanol). Citing from my memory, ethanol boils at 78.3C and the azeotrope of 95% ethanol/water boils off at 78.15C. So you really neevr can get 100% ethanol from distillation because the azeotrope has a lower boiling point and so you have to use a dehydrating salt (CaO).
Thank you GOOGLE.

lavalamp

You should be aware that in any given substance (be it solid, liquid or a gas) that the atoms/molecules in it are all moving at different speeds (even if they all have the same Mr). This means that they all have different Kinetic Energies.

In a liquid some particles may have enough Kinetic Energy to break free of the surface of the liquid, that is why if you have a puddle of water, the water will evaporate over time. Of course, the higher the temperature of the liquid, the more likely it is that a particular atom/molecule will have the required Kinetic Energy to break free.

So in the case of the azeotrope you mentioned, when the mixture is heated up to evaporate the ethanol, some of the water molecules evaporate as well. As you continue heating, more water evaporates with the ethanol and eventually you end up condensing 5% water and 95% ethanol.

For more information, search for "Boltzman Distribution Curves" on google. Here is a pretty good Java applet for showing you the rms speeds of particles - http://comp.uark.edu/~jgeabana/mol_dyn/KinThI.html.

nautica

Pure ethanol can be prepared by adding benzene to a mix of 95% ethanol and water and then distillijng this solution. Benzene will for a different azeotrope with ethanol and water (7.5%). This boils at 64.9 C and allows removal of water along with some ethanol. Eventually pure "absolute alchol" is distilled over.

Nautica

Mr. Robin Parsons

Would a fractional distillation device allow, a closer approach to 100%, just slightly greater then 95%, or no real difference??

nautica

No, it would not. It takes the fractional distillation to get to the 95%. The problem is that the Boiling point of the azeotrope is lower than the ethanol, so you get both through you distillation.

Nautica

Mr. Robin Parsons

O.K. It is just that I had thought that that was the purpose of frational distillers was to separate products of differing boiling points...guess it jsut doesn't work all that well when they are that close, or azeotropic...right?

nautica

Yes, that is the purpose. The problem with this azeotrope, is that the azeotrope BP is slightly less than that of 100% ethanol, so the azeotrope becomes the product instead of what is left behind.

Nautica

Mr. Robin Parsons

Thank you...

temp_tsun

Hello, I have a question that is actually the same question as what was asked however. How do they know what the boiling point is of "pure" ethanol if they cannot produce "pure" ethanol?

chemisttree

One can produce pure ethanol. One way it is done by azeotropically distilling the water from the 95% azeotrope with something like benzene, for example.

temp_tsun

But what happens to the benzene? And will I get 100% pure alcohol or do I approach the 100%?

alxm

You can produce pure ethanol, just not by destillation in an environment where there's water around.

temp_tsun

You mean that it is 100% pure ethanol right? Can you give me a technique that can give us pure ethanol? And after producing pure ethanol what do I have to do to keep it pure?

alxm

Well you'd take your azeotropic, 95% ethanol, use a chemical drying agent, and then use destillation and extraction to remove the drying agent. Repeat as necessary until you get your desired purity.

To keep it at 100% you'd need to keep it away from water, including air that has water in it.

Borek

Put ethanol over some dehydrating agent.

Then just keep it tighlty closed.

chemisttree

The benzene will distill with the water. Collect it and discard. The excess benzene can be carefully fractionally distilled from the remaining ethanol. After that you will need to treat with a drying agent. You could try to dry 95% ethanol with some drying agent but the type you use would be important. Drying 5% is going to be messy and use a lot of drying agent. In the lab, I use sodium hydride to dry ethanol and distill from the slurry. Any water reacts with the hydride to produce hydrogen and sodium hydroxide. Distillation from that should give you the pure stuff. It is best to distill under an inert atmosphere like dry argon or dry nitrogen and distill directly into something like a schlenk. Don't try to distill from an agent like magnesium ribbon, sodium sulfate or molecular sieves (with ethanol you can use 5A, with methanol you need 3A) since they will release some of their water during the distillation. You need to filter before distillation with those agents and it isn't as easy as you might think (and keep anhydrous).

Of course, you're not going to drink this, right? Using benzene is dangerous and unless you have some sophisticated equipment to analyze the resulting ethanol, you never know if you are getting some cancer-causing benzene in your ethanol.