Liquid-Liquid Extractions

Shown here is an illustration outlining the progress of a traditional liquid–liquid extraction (LLE) using two identical extractions of a single sample with fresh portions of the extractant. The numbers give the fraction of analyte and interferent in each phase assuming equal volumes of sample and extractant and distribution ratios of 5 and 0.5 for the analyte and the interferent, respectively.

Figure12.1

Under these conditions, a single extraction using equal volumes of sample and extractant transfers 83% of the analyte and 33% of the interferent to the extracting phase. A single extraction, therefore, enriches the analyte by a factor of 0.83/0.33 or 2.5×. Interestingly, after completing a second extraction and combining the two extracting phases, the separation of the analyte and interferent is—at 0.97/0.55 or 1.8×—less efficient. As the illustration makes clear, the second extraction actually favors the interferent!

As shown here, we can improve a traditional liquid-liquid extraction by first extracting the solutes from the sample into the extracting phase, and then extracting them back into a fresh portion of solvent that matches the sample’s matrix. The numbers give the fraction of analyte and interferent in each phase assuming equal volumes of sample and extractant and distribution ratios of 5 and 0.5 for the analyte and interferent, respectively.

Figure12.2

Because the analyte has the larger distribution ratio, more of it moves into the extractant during the first extraction, and less of it moves back to the sample phase during the second extraction. In this case the concentration ratio in the extracting phase after two extractions is significantly greater. Although we recover less analyte than in the traditional LLE (69% vs. 97%), we see a significant increase in the extent of the analyte’s separation from the interferent (an improvement of 6.3× here vs. 1.8× for the traditional LLE).

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