Re: [Phys-l] Triple point pressure

[John Denker <jsd@av8n.com>]

On 02/14/2007 03:48 AM, Savinainen Antti wrote:
> ... whether it is possible to have a triple point with higher total pressure than indicated above
> as long as the partial pressure of water is that 611.73 pascals?


1a) Consider heavy water, D2O.  It has its own triple-point, with its own triple-
point temperature (about 0.102 C)  and its own triple-point pressure.

1b) Next consider a mixture of H2O and D2O.  I haven't checked, but I believe
nothing tricky (see item (2) happens in this system, so as a first approximation I
would expect the properties of the mixture to smoothly interpolate between the
properties of the pure ingredients.

In this scenario, the triple-point pressure (P3) of the 50/50 mixture will not be
the plain /sum/ of the P3 values of the ingredients;  it will be closer to /half/
of the sum.

Dalton's law of partial pressure applies to the vapor /only/.  It tells us that
we can ignore one gas when measuring the partial-pressure of the other gas.  It
does *not* tell us that we can ignore one ingredient when defining the triple-
point properties of the other ingredient.  There is no Dalton-like law for the
solid or liquid, both of which participate in defining the triple point.

2) Things get even trickier if we consider a water/octane mixture.

The vapor will be a simple mixture of water vapor and octane vapor, and Dalton's
law of partial pressures will apply in the usual way.

The liquid will not be so simple.  I can choose conditions such that there
will be a phase separation, where one phase is water with some dissolved
octane, while the other phase is octane with some dissolved water.  In
equilibrium, there are also two different solids, separated by their own
phase boundary (although this equilibrium may be hard to obtain).

In this scenario, you have to be careful what you mean by "triple point".
For pure water it means "the" meeting point of "the" three phases ... but
what does it mean now that there are five distinct phases?