Same Aggregation State
If a compound is added to a mixture of two immiscible liquids, it will
become distributed between the two layers in a definite concentration ratio.
If C1 and C2 are the equilibrium concentrations of
the compound in solvents 1 and 2 respectively,
then where K is the equilibrium constant or partition coefficient.
This equation applies only when the molecules in each phase are in the
same state of aggregation. When association and dissociation occurs, more
complex equations must be applied.
No convention exists regarding whether the concentration in the organic
phase or aqueous phase should be placed in the numerator to determine K.
One should always specify which way the partition coefficient is being
The concept of partition coefficient is applied in the pharmaceutical
and health care industries in a variety of ways, including solvent extractions
of drugs from dosage forms (e.g. extraction of aspirin from tablets for
quality assurance tests), drugs from biological fluids (e.g. clinical laboratory
analyses of drugs in blood/plasma or urine), and medicinal agents from
plants (e.g. the extraction of vinca alkaloids in the development of cancer
chemotherapy agents). Solvent extraction is the process of removing a constituent
from one liquid phase by bringing this phase into contact with a second,
immiscible, liquid phase. The number of extractions required to isolate
or purify a substance is determined by the partition coefficient (K) and
the relative volumes of the two phases.
At equilibrium, the weight of solute remaining in the original solvent
after a single extraction (W) or after multiple extractions (Wn) is:
e.g. A 25 ml sample of an aqueous solution containing 0.1 g of acetanilid
was extracted with three 10 ml portions of ether. The combined ether extracts
were evaporated to dryness and the residue weighed. The ether/water partition
coefficient for acetanilid is 3.0. What is the weight of the residue (i.e.
how much is extracted)?
The most efficient extraction results when a large number of extractions
are carried out with small portions of extracting liquid.