ssdd
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Bioling Point of Water at Different pressure
What equation can I use to find the boiling point of water at different pressures?
-ssdd
Yes I know this is a remedial question, but I can't seem to find the answer.... 
All that glitters may not be gold, but at least it contains free electrons.
-- John Desmond Baernal
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S.C. Wack
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http://www.chemsoc.org/exemplarchem/entries/pkirby/exemchem/...
There are equations for correction of boiling points but these have variable constants AFAIK.
BTW when using official barometers in the US (not having a home barometer) for the pressure for bp determination, one must also correct for altitude
as these are calibrated for the altitude that they are stationed at. This caused me some perplexity until I figued this out, here at 1400' with stated
pressure at 29.9" but bp's strangely low. (i.e. The Denver Post says that the current pressure there is 30.22". I don't think so.)
[Edited on 9-9-2007 by S.C. Wack]
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ssdd
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Thanks
-ssdd
All that glitters may not be gold, but at least it contains free electrons.
-- John Desmond Baernal
http://deepnorth.info/ |
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gsd
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The good old Antoine's Equation for water holds good for considerable range :
Log (Ps) = 7.96681 - (1668.2/(228+T))
Where Ps is Vapour Pressure of water in mmHg
T is temperature in Deg. C.
and Log is to the base 10
gsd
[Edited on by gsd]
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16MillionEyes
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What I think would be more interesting is understand how those guys came up with those equations.
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gsd
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To the best of my understanding, Antoine's Equation is an excercise in curve fitting.
You have a basic equation : Log (Ps) = A - (B/(C+T))
This is an equation with 2 variables and 3 constants.
Now Take known vapour pressure data of any pure conpound.
Plot it on a graph paper. From sufficient number of data points find out value of A B C.
What you have is an impirical formula for vapour pressure as a function of temperature.
Here are few web-pages pertaining to Vapour Pressure and Antoine's Equation
http://www.msstate.edu/dept/Chemistry/scf2/4411/vaporP.htm
http://www.ece.umd.edu/~nsw/ench250/antoine.htm
http://imartinez.etsin.upm.es/dat1/ePv.htm
gsd
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Quibbler
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Just to add a bit of substance to these approximate equations.
Water (or anything) boils when G(liquid)=G(vapour)
[G = Gibbs free energy]
G(liquid) changes very little with presssure, but for the vapour dG=VdP=RT dP/P (ideal gas)
So fiddling around and integrating
ln(P) = -/\H/RT +/\S/R
where T is the boiling point (in kelvin) at P (in atm)
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Magpie
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S.C. Wack says:
| Quote: |
BTW when using official barometers in the US (not having a home barometer) for the pressure for bp determination, one must also correct for altitude
as these are calibrated for the altitude that they are stationed at. This caused me some perplexity until I figued this out, here at 1400' with stated
pressure at 29.9" but bp's strangely low. (i.e. The Denver Post says that the current pressure there is 30.22". I don't think so.)
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This is an important observation; thanks for pointing it out. I have been mistakenly using airport values.
Researching this I find that when you call your local airport tower to get a barometric pressure what you actually get is a barometric pressure
adjusted to a sea level condition. This is for use by pilots flying in that locality. By setting their altimeter to this adjusted value it will
indicate the correct altitude above mean sea level.
For example, I live about 400 feet above sea level. The airport gave me 29.84"Hg. A local weather station gave me a true barometric pressure of
29.41"Hg.
[Edited on by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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S.C. Wack
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Local weather station can mean a couple of different things...what I was trying to say is that local NWS stations give pressure adjusted for sea
level...for the purpose of nice isobars instead of isobars that follow altitude contours more than anything else.
[Edited on 12-9-2007 by S.C. Wack]
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Magpie
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Yes, there is that too. I did make sure that the pressure my "weather station" was giving me was a true barometric pressure.
The single most important condition for a successful synthesis is good mixing - Nicodem
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