In figure 15 the sample liquid is less dense than the extraction solvent and the refluxed liquid falls through the sample liquid, extracting the materials of interest, while falling to the solvent pool at the bottom of the extraction tube. The extract then passes up the outside of the tube and siphons through the side tube back to the boiling flask. The sample solvent is rarely completely insoluble in the extraction solvent and so some of the sample solvent will continually accumulate in the flask along with the material of interest. It follows, that the extraction time should be limited to that which allows the material to be extracted, but is accompanied by the minimum amount of sample liquid.

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Figure 16. The Continuous Liquid Extractor Where the Liquid Sample is Denser than the Extraction Solvent

In figure 16 the sample liquid is more dense than the sample solvent. In this system the sample liquid resides in the bottom of the extraction tube and a head of the lighter liquid is built up to force a flow through the sample liquid that then passes out of the side arm and back to the extract flask.

Solvent Extraction by Segmented Flow

Continuous extraction can be carried out using segmented flow. The sample is mixed with, or dissolved in, a carrier solvent that is then mixed with an immiscible extraction solvent. Hence a heterogeneous mixture is produced and, as a consequence, segmented flow is formed. The segmented flow is then passed though an extraction coil, where, due to the bolus flow that occurs within each segment of solvent, the material is extracted very efficiently. A diagram of a segmented flow system is shown in figure 17.

Figure 17. A Segmented Flow Extractor

This method of extraction is a little difficult to control in practice, and thus, has somewhat limited use. It functions well if the distribution coefficients of the solutes are strongly in favor the extracting solvent. If the distribution coefficient of a solute between the carrier and the extractant is 50, the maximum extraction efficiency is about 95% v/v.