Environmental Fate and Transport of Solvent-Stabilizer Compounds - Environmental Investigation and Remediation - page 132

TABLE 3.8
Acid Dissociation Constants for Stabilizer Compounds

Percent Dissociated
at pH 7 Chemical Name p K a Temperature (°C) References

Pyridine 5.25 25 Complete dissociation Perrin (1972)

Nitroethane 8.46 25 3.47 Perrin (1972)

Phenol 9.994 20 0.13 Perrin (1972)

Nitromethane 10.21 25 0.062 Perrin (1972)

p - tert -Amyl phenol 10.43 25 0.0167 Schultz (1986)

Triethylamine 10.778 25 0.017 Riddick et al. (1985)

Nonylphenol 10.778 25 0.017 Lipnick et al. (1986)

Diisopropylamine 11.05 25 0.009 Perrin (1972)

2,6-di- tert -Butyl- p -cresol 12.23 25 Serjeant and Dempsey (1979)

5.89 × 10 −6

tert -Butyl alcohol 19.2 25 Serjeant and Dempsey (1979)

6.31 × 10 −13

Note: The majority of stabilizer compounds, including 1,4-dioxane, do not contain functional groups capable of
dissociation.

dissociation constants is similar to pH; p K a is the negative log of the acid dissociation constant.
The chlorinated solvents and most of the solvent-stabilizer compounds are stable with respect to
acid-base reactions, and few have literature values for the acid dissociation constant.
Weak organic acids in aqueous solutions are strongly affected by the pH of the aquatic system:

M

-

log

A

=- pH p K . (3.23)

M

a

HA

In the normal pH range of aquatic systems, the relevant range of p K a is 3-10; p K a values less than
3 are expected to be 99% dissociated, whereas p K a values greater than 10 are expected to remain
completely undissociated (Lyman et al., 1990). The higher the pH, the more a compound with a p K a
between 3 and 10 will dissociate (Schultz, 1986). Most of the p K a measurements available are made
at room temperature (25°C), which is much warmer than temperatures expected in most surface
waters and groundwaters. Therefore, laboratory values of p K a will typically overestimate the ten-
dency for an organic compound to ionize under ambient subsurface conditions. Table 3.8 provides
acid dissociation constants for the few stabilizer compounds whose acid dissociation constants have
been measured and reported in the literature. The majority of stabilizer compounds do not contain
functional groups capable of dissociation.

3.3 SUBSURFACE FATE AND TRANSPORT PROCESSES
Contaminants in the subsurface migrate slowly. The most rapid rates of migration in the unsaturated
zone occur by vapor transport through relatively dry soils, particularly when vapors are denser than
air. The most rapid migration rates in the saturated zone are equal to the groundwater l ow velocity.
Contaminant migration is retarded by chemical, physical, and biological processes contributing to
the contaminant's elimination, transformation, or otherwise delayed arrival at the groundwater
interface or at an observation well down-gradient from the point of release. The following sections
focus on the subsurface fate and transport processes in groundwater that favor migration of some
stabilizer compounds while eliminating others.