The process of a solute dissolved in one solvent being
pulled out, or “extracted” into a new solvent actually involves an equilibrium process. At the time of initial contact,
the solute will move from the original solvent to the extracting
solvent at a particular rate, but, after a time, it will begin to move back to the original solvent at a
particular rate. When the two rates are equal, we have equilibrium. We can thus write the following:
Aorig ⇌ Aext
in which A refers to analyte and orig and ext refer to
original solvent and extracting solvent, respectively. If the analyte is more soluble in the extracting solvent
than in the original solvent, then, at equilibrium, a greater percentage will be found in the extracting
solvent and less in the original solvent. If the analyte is more soluble in the original solvent, then the greater
percentage of analyte will be found in the original solvent. Thus, the amount that gets extracted depends on the
relative distribution between the two layers, which, in turn, depends on the solubilities in the two layers. A distribution coefficient
analogous to an equilibrium constant (also called
the partition
coefficient) can be defined as
follows:
Often, the value of K is approximately equal to the
ratio of the solubilities of A in the two
solvents. If the value of K is very
large, the transfer of solute to the extracting solvent is considered to be
quantitative.
A value around 1.0
would indicate equal distribution and a small value would indicate very little
transfer. Uses of the distribution
coefficient include:
1.the calculation of the
amount of a solute that is extracted in a single extraction step,
2.the determination of the
weight of the solute in the original solute (important if you
are quantitating the solute in this
solvent),
3.the calculation of the optimum
volumes of both the
extracting solvent and the
original solution to be used,
4.the number of extractions
needed to obtain a
particular quantity or
concentration in the extracting solvent, and
5.the percent extracted.
The following
expansion of the previous equation is useful for these:
