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Newsgroups: sci.chem,
From: (Bruce Hamilton)
Date: Tue, 1 Jul 1997 12:54:08 +12

In article <>
"W. R. Smith" <> writes:
>  In sci.chem, altavoz <> wrote:
> >1 liter of gas slightly below it's condense prees' put into a
> >1 liter bottle which allready has another gas at pressure X,
> >  The partial press' will be the sum . BUT ! Will compressing
> >the 1st gas into the bottle allready filled with another condense
> >the 1st gas ?
>If the individual gases and their mixture obey the ideal-gas EOS, and if
>the temperatures of all the gases (before and after mixing) are the
>same, then the partial pressure of the gas that was originally slightly
>below its condensation pressure will be equal to its original pressure
>in the final mixture.  Thus, if was below its condensation pressure
>originally, then its partial pressure will remain so in the final
>mixture (i.e. no condensation).

Most gas mixtures do not follow the ideal gas equation of state, for example
Glasstone has a whole section devoted to " Real Gases and Dalton's Law".
[ begin extract ]
 " As might be anticipated, real gases do not obey Dalton's law, which was
   enunciated on the basis of what would now be regarded as crude
   experiments. Unless the gases interact chemically, the total pressure is
   usually greater than the sum of the partial pressures, except for very
   highly compressed gases when there is strong interermolecular
   attraction.  Some idea of the extent of the deviation from strict additivity
   will be obtained from the data in Table 40; they are for mixtures
   containing approximately equimolar amounts of thge two constituents
   in which the deviations are approximately maximal.

   Table 40  Positive Deviations from Dalton's Law
   Total Pressure  Argon-Ethylene  Oxygen-Ethylene
     ( atm. )        ( atm. )         ( atm. )
        30             0.85              -
        40             1.6               -
        50             2.7              2.5
        75             6.2              5.2
       100             8.0              7.2
[ end extract ]

Now, without bothering to find a text book to ascertain the cumulative
effects of all the differing behaviors of ideal gases and real gases in
gas mixtures, they apparently are very significant.

When we have calibration gas mixtures prepared, the total pressue is
kept very low if any component partial pressure is near to its saturation
pressure - which makes such standards very expensive on a $/mole
basis. The effects are quite complex, because mixtures containing
small concentrations of low volatility HCs like pentane and hexane
that appear OK by our partial pressure calculations have been rejected
by the supplier - who has significantly reduced the concentration and
also provided a fairly low pressure cylinder.

>It seems to me, tho, that the actual PvT properties of the original
>gases and of their mixture (i.e. the EOS) are a crucial factor here,
>since the original gas was _almost_ at saturation conditions.  I'd say
>that the answer depends on the properties of the gases and their

That was the assumption that I made - that the component was
very close to saturation, and that various real gas effects would
push it over the limit - based on our experience of obtaining
standard mixtures. The effect is apparently greatest for gas
mixtures where the component gases have different properties,
rather than mixing members of the same family, like HCs.
The laws are apparently good approximations at ambient
pressures, but soon deviate from reality with increasing pressure.

          Bruce Hamilton

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