[Halma]

I'm having some trouble understanding azeotropes.

Ethanol and water, for example.  It's supposed to form an azeotrope at 96% ethanol.  At this point, water and ethanol boil together at 78 degrees.  This much I understand.

But what about at other concentrations?  What happens when you boil 98% ethanol?

At a lower concentration, say 50%, only the ethanol would boil?  If so, why is it so hard to get pure ethanol?

I know this is a very basic question, but I'm still learning and I'm a bit confused now.

Thanks

[Borek]

When mixture boils vapor has different composition then the mixture - but it is never one pure substance. So if your 50% ethanol boils vapor contains over 50% ethanol - thus distillate will have higher concentration. However, vapor over 98% ethanol contains more water then ethanol - in fact what is left after distillation will have slightly higher concentration of ethanol then the original mixture.

At 96% (azeotrope) composition of vapor is identical to the composition of solution - that's azeotrope definition.

[Halma]

I see, so the azeotrope point is just the point where the ratio of the compositions in the vapour is the same as in the liquid?

So if I have 98% ethanol, I could make it purer by just boiling it down?  (not distilling)

I also heard about people increasing the concentration of dilute nitric acid by boiling down.. If the boiling point of nitric acid is lower than water, how is that possible?  If you have a 20% nitric acid solution that's boiling, shouldn't most of the vapour be nitric acid?

Many thanks for the *delete me*

[plu]

So if I have 98% ethanol, I could make it purer by just boiling it down?  (not distilling)

Not exactly.  As Borek said, the composition of the vapour of an azeotrope is no different than the composition of the azeotrope itself.  The azeotrope is, in effect, acting like a unitary substance and not a mixture.  Therefore, distillation of 98% ethanol would not result in a purer mixture.  (You would just boil off a lot of solution  )

[Borek]

Not exactly.  As Borek said, the composition of the vapour of an azeotrope is no different than the composition of the azeotrope itself.  The azeotrope is, in effect, acting like a unitary substance and not a mixture.  Therefore, distillation of 98% ethanol would not result in a purer mixture.  (You would just boil off a lot of solution  )

But Halma is right - distillate will be no purer (contrary - it will contain more water), but what is left will be slightly more concentrated. That's simple mass balance, nothing more complicated.

This leads to the question - why we don't produce anhdyrous ethanol this way? To be honest I don't know. I suppose it all boils down to cost and efficiency Note, that you have to start with ethanol concentrated above 96%, so you must add some anhydrous ethanol to azeotropic solution before distillation.

That's reverted situation you have when you start distillation of diluted ethanol after fermentation - distilled solution has higher concentration of ethanol, but what is left has higher concentration of water.

Edit: just to clarify - plu - note that 98% is above azeotrope (96%).

[Borek]

I also heard about people increasing the concentration of dilute nitric acid by boiling down.. If the boiling point of nitric acid is lower than water, how is that possible?  If you have a 20% nitric acid solution that's boiling, shouldn't most of the vapour be nitric acid?

No idea where is this information from. 68% nitric acid forms an azeotrope.

Check wikipedia entry http://en.wikipedia.org/wiki/Nitric_acid.

[Donaldson Tan]

azeotropes are constant-boiling mixtures.

considering the case of 50% ethanol, boiling the mixture results in distillation. ethanol will first boil off, followed by water.

considering the cae of 98% ethanol, boilng the mixture will result in ethanol in boiling off, until its percentage composition is 96%, then it forms an azeotrope. Continue heating will only result in both water and ethanol being boiled off simultaneously.

In order to extract ethanol from the azeotrope, one has to add an immiscible organic solvent to the azeotrope whereby ethanol is more soluble. This results in the extraction of ethanol, ie. the mass transfer of ethanol from the aqueous to the organic layer. Next, seperate the organic layer and let the organic layer undergo distillation to obtain alcohol.

[Halma]

Thanks everyone, for your input.