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defined by the equilibrium concentration ratio: BCF c /c . Measurement of Section 12.8
organism water
a BCF (which varies with species of fish) is time-consuming and costly ($30000), and Two-Component
Solid–Liquid Equilibrium
one can roughly estimate BCF of an organic compound with low polarity from K
ow
using log BCF log K 1.32 if 1.5 log K 6.5 [D. Mackay, Environ. Sci.
10 10 ow ow
Technol., 16, 274 (1982); for better equations see W. M. Meylan et al., Environ.
Toxicol. Chem., 18, 664 (1999); for a review of BCFs, see J. A. Arnot and F. Gobas,
Environ. Rev., 14, 257 (2006)]. High K values are also correlated with high values
ow
of preferential absorption of organic pollutants in soil. The main substances in soil that
absorb organic pollutants are mixtures of organic compounds.

12.8 TWO-COMPONENT SOLID–LIQUID EQUILIBRIUM
We now discuss binary solid–liquid diagrams. The effect of pressure on solids and liq-
uids is slight, and unless one is interested in high-pressure phenomena, one holds P
fixed at 1 atm and examines the T-x solid–liquid phase diagram.
B
Liquid-Phase Miscibility and Solid-Phase Immiscibility
Let substances B and C be miscible in all proportions in the liquid phase and com-
pletely immiscible in the solid phase. Mixing any amounts of liquids B and C will pro-
duce a single-phase system that is a solution of B plus C. Since solids B and C are
completely insoluble in each other, cooling a liquid solution of B and C will cause
either pure B or pure C to freeze out of the solution.
The typical appearance of the solid–liquid phase diagram for this case is shown in
Fig. 12.19. T* and T* are the freezing points of pure B and pure C.
B C
The origin of the regions on this diagram is as follows. In the low-temperature
limit, we have a two-phase mixture of pure solid B plus pure solid C, since the solids
are immiscible. In the high-T limit, we have a one-phase liquid solution of B plus C,
since the liquids are miscible. Now consider cooling a liquid solution of B and C that
l
has x close to 1 (the right side of the diagram). Eventually, we reach a temperature
B
where the solvent B begins to freeze out, giving a two-phase region with solid B in
equilibrium with a liquid solution of B and C. The curve DE thus gives the depression
of the freezing point of B due to solute C. Likewise, if we cool a liquid solution of B
l
plus C that has x close to 1 (the left side of the diagram), we eventually get pure C
C
freezing out and AFGE is the freezing-point-depression curve of C due to solute B. If
we cool a two-phase mixture of solution plus either solid, the solution will eventually
all freeze, giving a mixture of solid B and solid C.

T
R
T *
C
Liquid solution (l.s.)
F of B C
T 1
D
I H T *
B
T 2 G
l.s. solid C
E l.s. solid B
K
T 3
Figure 12.19
S Solid B solid C
Solid–liquid phase diagram for
0 x′ B x′′ x ′′′ 1 complete liquid miscibility and
B
B
solid immiscibility. P is held fixed.
x B