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

©2001 CRC Press LLC
S i = n/V = P S /RT
Invaluable information about how a substance will behave in the environment
can be obtained by considering its three solubilities, namely those in air, water, and
octanol. These solubilities express the substance’s relative preferences for air, water,
and organic phases.
Returning to the definition of Z W as (1/v Wg if R), it is apparent that Z W also can
be expressed in terms of aqueous solubility, S W, and vapor pressure P S . For liquid
solutes, S W is 1/g iv W. For solid solutes it is F/g iv w. The reference fugacity f R is the
vapor pressure of the liquid, i.e., it is P S for a liquid and P S /F for a solid. Substituting
gives Z W as S W/P S in both cases, the F cancelling for solids. The ratio P S /S is the
Henry’s law constant H in units of Pa m 3 /mol, thus Z W is 1/H.
Polar solutes such as ethanol do not have measurable solubilities in water, because
they are miscible. This generally occurs when g is less than about 20. We can still
use the concept of solubility and call it a “hypothetical or pseudo-solubility” if it is
defined as 1/g iv S. For a liquid substance that behaves nearly ideally, i.e., g i is 1.0, the
solubility approaches 1/v S, which is the density of the solvent in units of mol/m 3 .
For water, this is about 55,500 mol/m 3 , i.e., 10 6 g/m 3 divided by 18 g/mol. For a
solid solute under ideal conditions, the solubility approaches F/v S mol/m 3 .
These equations are general and apply to a nonionizing chemical in solution in
any liquid solvent, including water and octanol. The solution molar volumes and the
activity coefficients vary from solvent to solvent. The Z value for a chemical in
octanol is, by anology, 1/v Og if R, where v O is the molar volume of octanol.
5.3.3 Solutions of Ionizing Substances
Certain substances, when present in solution, adopt an equilibrium distribution
between two or more chemical forms. Examples are acetic acid, ammonia, and
pentachlorophenol, which ionize by virtue of association with water releasing H +
(strictly H 3O + ) or OH – ions. Some substances dimerize or form hydrates. For ionizing
substances, the distribution is pH dependent, thus the solubility and activity are also
pH dependent. This could be accommodated by defining Z as being applicable to
the total concentration, but it then becomes pH dependent. A more rigorous approach
is to define Z for each chemical species, noting that, for ionic species, Z in air must
be zero under normal conditions, because ions as such do not evaporate. In any
event, it is useful to know the relative proportions of each species, because they will
partition differently. This issue is critical for metals in which only a small fraction
may be in free ionic form.
For acids, an acid dissociation constant Ka is defined as
Ka = H + A – /HA
where H + is hydrogen ion concentration, A – is the dissociated anionic form, and HA
is the parent undissociated acid. The ratio of ionic to nonionic forms I is thus
I = A – /HA = Ka/H + = 10 (pH – pKa)