Runningbear
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Maximum boiling azeotrope
Can someone help me out with the difference between maximum and minimum boiling azeotropes.
For example, Nitric acid is a maximum boiling azeoptrope. I know that if I distill a nitric/ water azeotrope that 50% water, the azeotrope will come
across as 68% nitric, but will the azeotrope come across first at 68% then leave the excess water or will the water come across first?
thanks in advance,
Runningbear
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gsd
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A maximum boiling azeotrope (also called as negative azeotrope) means the boiling point of azeotrope is higher than that of either of pure
components.
e.g. 1) HCl / Water system
B.P. of pure HCl is -85 Deg C. and that of pure Water is 100 Deg. C. However their azeotrope at 20.2% HCl concentration has a B.P. of 108.6 Deg. C.
e.g. 2) HNO3 / water system
B.P. of HNO3 is 86 Deg. C. and that of azeotrope at 67.4% HNO3 concentration is 120.7 Deg. C.
A minium boiling azeotrope (also called as positive azeotrope) means the boiling point of azeptrope is lower than that of either of pure components.
e.g. Acetonitrile / water system.
B.P. of CH3CN is 81.6 Deg.C. and that of azeotrope at 17.1 % Acetonitrile concentration is 76.2 Deg. C.
To answer your specific question, if you start distilling 50 -50 mixture of H2O and HNO3, then the distillate will be reacher in water till
the concentration of mixture remaining in the pot reaches to 67.4% HNO3 and 32.6 % water. There after the distillate and the residue will have the
same composition.
gsd
[Edited on 15-1-2010 by gsd]
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Runningbear
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thanks GSD,
So if I kept the distillation temp above 86C and under 100C, then nothing would come over because the maximium boiling azeotrope is 120C?
Is that correct?
Runningbear
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watson.fawkes
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Quote: Originally posted by Runningbear  | | So if I kept the distillation temp above 86C and under 100C, then nothing would come over because the maximium boiling azeotrope is 120C? [...] Is
that correct? |
No. If that were true, the vapor pressure of the mixture would be zero between those
temperatures. So that's obviously not true. Phase diagrams for azeotrope have both a liquid composition and a vapor composition curve which give,
respectively, the boiling point of a given mixture and the composition of the vapor that comes off of it. See this page for a treatment of this topic that discusses nitric acid as its last example.
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gsd
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To add few more comments:
Just like any other liquid, mixture of water and Nitric acid (whether dilute, concentrated or azeotropic) has finite vapour pressure even at low
temperatures. This has got nothing to do with it's azeotropicity. So if you maintain the pot temperature between 86 and 100 then the mixture in it may
not boil but it will surely lose vapours; the rate of loss corresponding to the vapour pressure of the mixture at that temperature.
You can condense those vapours and obtain the distillate. However the rate of distillation will be very slow. This process is akin to obtaining pure
water from salt / brakish water in a solar still wherein the water temperature never reaches its boiling point.
And lastly if the mixture in the pot is azeotropic then the composition of the vapour coming out of the pot will be exactly equal to the compostion of
the liquid in the pot.
gsd
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