Single extraction solution/extractants

Let us consider a specific example, viz., the extraction of a solution of 4 Og. of n-butyric acid in 100 ml. of water at 15° with 100 ml. of benzene at 15°. The partition coefficient of the acid between benzene and water may be taken as 3 (or J between water and benzene) at 15°. For a single extraction with benzene, we have ... 

In Example 7.14 a single extraction results in an extraction efficiency of only 60%. If a second extraction is carried out, the fraction of solute remaining in the aqueous phase, ( is given by... 

Thus, a single extraction improves the separation of the solutes by a factor of 2.5. As shown in Figure 12.1, a second extraction actually leads to a poorer separation. After combining the two portions of the extracting phase, the concentration ratio decreases to... 

One of the attractive features of SFE with CO2 as the extracting fluid is the ability to directly couple the extraction method with subsequent analytical methods (both chromatographic and spectroscopic). Various modes of on-line analyses have been reported, and include continuous monitoring of the total SFE effluent by MS [6,7], SFE-GC [8-11], SFE-HPLC [12,13], SFE-SFC [14,15] and SFE-TLC [16]. However, interfacing of SFE with other techniques is not without problems. The required purity of the CO2 for extraction depends entirely on the analytical technique used. In the off-line mode SFE takes place as a separate and isolated process to chromatography extracted solutes are trapped or collected, often in a suitable solvent for later injection on to chromatographic instrumentation. Off-line SFE is inherently simpler to perform, since only the extraction parameters need to be understood, and several analyses can be performed on a single extract. Off-line SFE still dominates over on-line determinations of additives-an... 

If D is large, i.e. > 102, a single extraction may effect virtually quantitative transfer of the solute, whereas with smaller values of I) several extractions will be required. The amount of solute remaining in the aqueous phase is readily calculated for any number of extractions with equal volumes of organic solvent from the equation... 

If the value of > is known, equation (4.5) is useful for determining the optimum conditions for quantitative transfer. Suppose, for example, that the complete removal of 0.1 g of iodine from 50 cm3 of an aqueous solution of iodine and sodium chloride is required. Assuming the value of D for carbon tetrachloride/water is 85, then for a single extraction with 25 cm3 of CC14,... 

Fig. 3.25 Singl e-solute batch extraction between immiscible liquid phases.

Receptor models 71a and 71b for dinucleotides (dinucleoside monophosphates) have been developed out of 69 by replacing the naphthoyl residue, which is not involved in any host-guest interactions in the complexes, by a second carbazolyl cleft. The adenine selectivity of the two pincers was reflected in single extraction experiments dissolved in CH2CI2,71a removes one full equivalent of both adenylyl(3 -+ 5 )adenosine (ApA) and 2 -deoxyadenylyl(3 ->5 )-2 -deoxyadenosine [d(ApA)] 72 out of aqueous solutions of guest (8-fold excess) but only half the amount of d(ApG) [103]. 

Clearly if log D> 3, a single extraction will effect an almost quantitative extraction when the phase volumes are equal. If the aqueous phase were as much as 100 times larger than the extracting organic phase, the same efficiency would be achieved if log D> 5 successive extractions with fresh portions of solvent is another solution. 

Metabolites are mostly extracted into aqueous or methanolic solutions. Subsequently, they are separated and analyzed as separate fractions, either aqueous or lipophilic. Here again, it should be emphasized that there is no single extraction... 

EXAMPLE 23-1 For a system with D = 2, compare the fraction of solute remaining unextracted for the following situations (a) single extraction with an equal volume of organic solvent, (b) five extractions with the same total amount of organic solvent, (c) limiting case of infinite number of extractions with same total amount of solvent. 

Since a partition coefficient is involved, it is evident that, unless the coefficient is very large and favorable, not all of the solute will be transferred to the new solvent in a single extraction. The efficiency of the transfer is increased by using successive portions of the new solvent, and the amount of material C which remains in the original solvent after n extractions is given by... 

---