Solid Phase Extraction Technique
Extraction is a separation process of removing a substance from one phase to another phase. The two phases must be immiscible but should be in contact. Eg. organic compounds can be separated from inorganic compounds with the help of an apparatus called a separating funnel.
When both phases are liquid then this is called liquid-liquid extraction (Solvent extraction)
When one phase is solid and the other is liquid that is called solid-liquid extraction.
Liquid-Liquid extraction
It involves the transfer of solutes from one liquid phase to another liquid phase by contact, this process is called Distribution of Partitioning.
Solutes are distributed based on their relative solubility in two different liquids one liquid is generally aqueous and the other is organic.
Partition Coefficient-: When the compound is shaken in the separating funnel with two immiscible solvents, it will distribute itself between two solvents. Normally one of the solvents is water and another solvent is organic. Most organic compounds are soluble in organic solvents while some are in water.
As per Nernest distribution law, the ratio of concentration of a given solvent in a pair of two immiscible liquids and in equation with each other is constant provided the temperature and pressure are constant. It is denoted by “k”.
Let the solute be x and the solvents be A&B
Then
k=concentration of x in A/concentration of x in B
Nernest distribution law is applicable only when
i) Temperature is constant
ii) When dilute solutions are employed:- the ratio c1/c2 will not remain constant if the concentration of a distributed substance is high in the solvents. Generally the higher the concentration the larger the deviation. In case both solutions are saturated with solute then k is given by
k=c1/c2=s1/s2
s1&s2 represents the solubility of solute as solvent. so one can say that it is applicable if the solute is sparingly soluble in both phases.
iii)When the molecular state of the solute is the same in two solvents. If there is an association of molecules the law is modified as c1/n(c2)1/2=k
where n = no. of molecular associating
iv) When the two solvents are mutually insoluble and or very sparingly soluble and their mutual miscibility is not altered by the addition of solute to them.
Types of Liquid-Liquid Extraction-:
- Single Extraction-: Solute is extracted from one layer of liquid by distributing it with the second immiscible liquid in a separating funnel. The solute is extracted only once with extracting solvent. After vigorous shaking the liquid phases are separated. This method is used for solutes having large distribution coefficients in extracting solvent. If the larger volume of extraction solvent is used then more solute will be extracted.
Effect of Volume on Extraction
If there are 300 particles of compound and the distribution ratio is observed in solvent as 1:2
If we double the volume of the solvent i.e. 200 ml &100 ml of solvent then 300 particle distribution
If we use a larger amount of extraction solvent more solute is added.
2. Multiple Extraction-: Single Extraction is suitable only if the distribution confection is larger. If the distribution coeffection is less than a single step extraction then the significant amount of solute will remain extracted. For complete extraction portions of fresh extracting solvent are added and extraction is carried out multiple times to ensure complete removal of solute. 95 to 97% solute can be extracted. But this is very tedious & time-consuming.
3. Counter Current Extraction:- This method is used to separate a mixture containing the substance with a close distribution ratio. It is a multistage solvent Extraction process. Countercurrent means two immiscible liquids move in opposite directions. Actually, one liquid is stationary, and the other moves. Two liquids are brought into contact, mixed & allowed to separate. The upper layer is transferred off in 6the direction of the lower in another, this cycle can be separated again & again to achieve the maximum separation.
This apparatus was invented by scientist Lymer C Breg. It consists of approx 50 tubes and a series of individual extrasions is performed in individual tubes.
This involves two steps
i) Extraction
ii) Transfer
4. Continous Extraction-: When the distribution coeffection is very low multiple extraction tedious, time consuming and large volume of extracting solvent is required. In this method there is a continuous flow of immiscible solvent through the solution to be extracted. The time of contact between the two phases is very less and equation is never attained but some solute is transferred to the continuously flowing solvent.