Environmental Engineering Reference
In-Depth Information
Analyte Volatility If an analyte is volatile, then clearly L-L extraction cannot be
used. The separatory funnel and continuous extraction can only be used for
semivolatile or nonvolatile compounds. In Chapter 2, we gave an approximate
definition of what are VOCs and SVOCs. The volatility of chemical from a liquid
can be evaluated from its Henry's law constant (H). Mackay and Shiu (1981)
provided the following useful guidelines for organic solutes in water.
10
3
atm
m
3
/mol
Volatile: volatilization is significant if
H
is in the range of 10
3
Highly volatile: volatilization is rapid if
H
>
to
10
5
atm
m
3
/mol
Semivolatile: volatilization is slow if
H
is in the range of 10
5
-
3
10
7
atm
m
3
/mol
Nonvolatile: volatilization is not important if
H <
3
10
7
atm
m
3
/mol
Henry's constant used in defining volatility should not be confused with vapor
pressure. Vapor pressure relates to the volatility from the pure substance into the
atmosphere, whereas H refers to the volatility from liquid to the air. A compound
with high vapor pressure (such as ethanol) could have very low volatility if ethanol is
present in water, but it evaporates quickly when a drop of ethanol is spilled on to a
table.
Analyte Partition Coefficient The partition coefficient of an analyte is defined
as: D ¼ C
s
/C
w
, where C
s
and C
w
are the equilibrium concentration in solvent and
water, respectively. The extraction efficiency (E) is independent of the initial analyte
concentration, but is a function of the partition coefficient and water-to-solvent ratio
(V
w
/V
s
):
C
s
V
s
C
s
V
s
þC
w
V
w
¼
1
E ¼
ð7
:
9Þ
V
w
V
s
1
D
1þ
The residual concentration of analyte in the aqueous phase after a single extraction
can be expressed as:
V
w
1
D
V
s
C
w
¼ C
w
DV
s
þV
w
¼ C
w
ð7
:
10Þ
V
w
þ1
For nth successive extractions, the residual concentration of an analyte in an aqueous
phase is:
2
4
3
5
n
n
V
w
DV
s
þV
w
1
D
V
s
C
w
¼ C
w
¼ C
w
ð7
:
11Þ
V
w
þ1
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