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

66 principles of extraction
In this instance, the value for D would be substituted for KD in equation
(2.29).
The formula for expressing repeated extractions is
% RX ¼ 1
1
1 þ KDðVE=VOÞ
n
100 ð2:32Þ
Applying equation (2.32) to the previous calculation having three successive
multiple extractions where KD ¼ 5, VE ¼ 50 mL, VO ¼ 1 L, and n ¼ 3, the
cumulative recovery is calculated to be 48.8% (Table 2.4).
Repeated extractions may be required to recover the analyte su‰ciently
from the aqueous phase. Neutral compounds can have substantial values of
KD. However, organic compounds that form hydrogen bonds with water,
are partially soluble in water, or are ionogenic (weak acid or bases) may have
lower distribution coe‰cients and/or pH-dependent distribution coe‰cients.
Additionally, the sample matrix itself (i.e., blood, urine, or wastewater) may
contain impurities that shift the value of the distribution coe‰cient relative
to that observed in purified water.
Investigation of the principle of repeated extractions demonstrates that:
The net amount of analyte extracted depends on the value of the distribution
coe‰cient.
The net amount of analyte extracted depends on the ratio of the volumes
of the two phases used.
More analyte is extracted with multiple portions of extracting solvent
than with a single portion of an equivalent volume of the extracting
phase.
Recovery is independent of the concentration of the original aqueous
sample.
2.2.2. Methodology
The LLE process can be accomplished by shaking the aqueous and organic
phases together in a separatory funnel (Figure 2.13a). Following mixing, the
layers are allowed to separate. Flow from the bottom of the separatory funnel
is controlled by a glass or Teflon stopcock and the top of the separatory
funnel is sealed with a stopper. The stopper and stopcock must fit tightly
and be leakproof. Commonly, separatory funnels are globe, pear, or cylindrically
shaped. They may be shaken mechanically, but are often shaken
manually.
With the stopcock closed, both phases are added to the separatory funnel.
The stopper is added, and the funnel is inverted without shaking. The stop-