principles of extraction and the extraction of semivolatile organics ...

50 principles of extraction
the list of key chemical properties? In general, there is a trend toward an
inverse relationship between these parameters such that high water solubility
is generally accompanied by low hydrophobicity, and vice versa.
Many authors use this relationship to estimate one of these parameters from
the other. However, it is this author’s opinion that the n-octanol/water partition
coe‰cient and water solubility are not interchangeable (via inverse
relationships) because they measure di¤erent phenomena. Water solubility
is a property measured at maximum capacity or saturation. The n-octanol/
water partition coe‰cient measures distribution across an interface. While
the relationship between water solubility and the n-octanol/water partition
coe‰cient may be highly correlated for closely related families of congeners
(Figure 2.5), as the diversity of the compounds compared increases, the correlation
between these two parameters decreases (Figure 2.6). However, solubility
should remain on the list of essential chemical properties because if the
value of the octanol–water partition coe‰cient is unavailable, water solubility
can be used as a surrogate. Also, solubility is used to estimate the
Henry’s law constant.
2.1.3. Acid–Base Equilibria
The acid–base character of a chemical and the pH of the aqueous phase
determine the distribution of ionized–nonionized species in solution. Starting
from the equilibrium dissociation of a weak acid, HA,
HA Ð H þ þ A ð2:9Þ
the equilibrium constant for dissociation of a weak acid can be written as
Ka ¼ ½Hþ Š½A Š
½HAŠ
ð2:10Þ
Analogously, the dissociation of the conjugate acid, BH þ , of a base, B, is
described as
and the related constant is
BH þ Ð H þ þ B ð2:11Þ
Ka ¼ ½HþŠ½BŠ ½BH þ ð2:12Þ
Š
Ionizable compounds’ Ka values (Figure 2.7) have an orders-of-magnitude