When a solution of a solute in one solvent is shaken with a second solvent, which is immiscible with the first, the solute distributes itself between the two solvents in proportion to its solubility in the two pure solvents. Thus a constant ratio is set up between the concentrations of the solute in the two phases or immiscible solvent layers. This ratio or equilibrium can be represented by the equation:
K = ———
K is called the distribution or partition coefficient. C1 and C2 are the concentrations of the solute in the two solvents. These concentrations are expressed in grams/mL.
K is approximately constant for all concentrations of C1 and C2. However, K will vary with temperature as solubility is temperature dependent.
Properties of a Good Extracting Solvent
Properties desirable for a good extracting solvent are:
It should be immiscible, or very sparingly soluble in the liquid from which the solute is to be extracted.
It should readily dissolve the substance to be extracted.
It should be capable of being easily separated from the solute after the extraction. This is usually by distillation.
It should ideally be cheap and non-flammable.
Choice of Solvents
The choice of immiscible solvent pair can be generalized as follows. One solvent should be polar and the other solvent should be considerably less polar. The polar solvent is usually water and the low polarity solvent is diethyl ether or a volatile hydrocarbon.
A useful generalization in predicting solubility and hence partitioning between solvents is the following:
Salts and other compounds having an ionic structure are generally soluble in water and insoluble in ether and hydrocarbons. Conversely, organic compounds, including the unionized forms of organic acids and bases are generally insoluble in water and soluble in ether and organic solvents. However it must be remembered that most compounds have some solubility in both types of solvents.
When the analyte needs to be extracted from a solid material rather than a liquid, it is refrred to as a solid-liquid extraction. In this case, a solid sample is placed in the same container as the liquid and the analyte is separated from the solid because it dissolves in the liquid while the other sample components do not.
The extraction process can be carried out in two ways:
The sample and the liquid are shaken together in the same container. The resultant mixture is filtered, and the filtrate, which then contains the analyte, is collected.
The fresh extracting solvent is continously cycled over a period of hours through the solid sample via a continuous evaporation-condensation process. This maximizes the transfer of the analyte to the liquid phase and the filtrate is collected. This technique is known as a Soxhlet extraction. The advantages of this technique are:
fresh solvent is continuously in contact with the sample without having to introduce more solvent, which could dilute the extract.
the process is automated so that the extraction can be conveniently set to occur overnight if desired.