McKay, Donald. "Front matter" Multimedia Environmental Models ...

experimentally, but not directly, using gas chromatographic retention times. It is
possible to measure the vapor pressure above the boiling point by operating at high
pressures. Beyond the critical point, the vapor pressure cannot be measured, but it
can be estimated.
The triple-point temperature at which solid, liquid, and vapor phases coexist is
usually very close to the melting point at atmospheric pressure, because the solidliquid
equilibrium line is nearly vertical; i.e., pressure has a negligible effect on
melting point. Melting point is easily measured for stable substances, and estimation
methods are available as reviewed by Tesconi and Yalkowsky (2000). High
melting points result from strong intermolecular bonds in the solid state and
symmetry of the molecule. Ice (H20)
has a high melting point compared to H2S
because of strong hydrogen bonding. The symmetrical three-ring compound
anthracene has a higher melting point (216°C) than the similar but unsymmetrical
phenanthrene (101°C).
The critical point temperature is of environmental interest only for gases, since
it is usually well above environmental temperatures. For example, it is 305 K for
ethane and 562 K for benzene. Its principal interest lies in its being the upper limit
for measurement of vapor pressure.
The location of the liquid-vapor equilibrium or vapor pressure line is very
important, since it establishes the volatility of the substances, as does the boiling
point, which is the temperature at which the vapor pressure equals 1 atmosphere.
Methods of estimating boiling point have been reviewed by Lyman (2000), and
methods of using boiling point to estimate vapor pressures at other temperatures
have been reviewed by Sage and Sage (2000). For many substances, correlations
exist for vapor pressure as a function of temperature. The simplest correlation is the
two-parameter Clapeyron equation,
©2001 CRC Press LLC
ln P = A – B/T
A and B are constants, and T is absolute temperature (K). B is DH/R,
where DH
is
the enthalpy of vaporization (J/mol), and R is the gas constant. A better fit is obtained
with the three-parameter Antoine equation,
lnP = A – B/(T + C)
Care must be taken to check the units of P, whether base e or base 10 logs are used,
and whether T is K or °C in the Antoine equation. Several other equations are used
as reviewed by Reid et al. (1987).
Correlations also exist for the vapor pressure of solids and supercooled liquids.
Of particular environmental interest is the relationship between these vapor pressures,
which can be used to calculate the unmeasurable supercooled liquid vapor
pressure from that of the solid. The reason for this is that, when a solid such as
naphthalene is present in a dilute, subsaturated, dissolved, or sorbed state at 25°C,
the molecules do not encounter each other with sufficient frequency to form a
crystal. Thus, the low-energy crystal state is not accessible. The molecule thus
behaves as if it were a liquid at 25°C. It “thinks” it is a liquid, because it has no