Instant Notes: Analytical Chemistry

Section D – Separation techniques

D1 SOLVENT AND SOLID-PHASE

EXTRACTION

The purpose of an extraction technique is physically to separate components of

a mixture (solutes) by exploiting differences in their relative solubilities in two

immiscible liquids or between their affinities for a solid sorbent. Substances

reach an equilibrium distribution through intimate contact between the two

phases, which are then physically separated to enable the species in either phase

to be recovered for completion of the analysis. An equilibrium distributionof

the solutes between the two phases is established by dissolving the sample in a

suitable solvent, then shaking the solution with a second, immiscible, solvent or

by passing it through a sorbent bed or disk. Where the equilibrium distributions

of two solutes differ, a separation is possible. The principal factors that deter-

mine how a solute will distribute between two phases are its polarity and the

polarities of each phase. Degree of ionization, hydrogen bonding and other

electrostatic interactions also play a part.

Most solvent-extraction procedures involve the extraction of solutes from an

aqueous solution into a nonpolar or slightly polar organic solvent, such as

hexane, methylbenzene or trichloromethane, although the reverse is also

Extraction
techniques

Key Notes

Solvent and solid-phase extraction are two techniques for separating

mixtures of substances, either by selective transfer between two

immiscible liquid phases or between a liquid and a solid phase.

Extraction efficiency is defined as the fraction or percentage of a

substance that can be extracted in one or more steps. Selectivity is the

degree to which a substance can be separated from others in a mixture.

Procedures are based on the extraction of nonpolar, uncharged species

from an aqueous solution into an immiscible organic solvent, or the

extraction of polar or ionized species into an aqueous solution from an

organic solvent.

Sorbents are particulate materials such as silica, chemically-modified

silica, alumina and organic resins that can interact with and retain

substances from solutions. Retained substances can be subsequently

released back into a suitable solvent under controlled conditions.

Sample solutions are passed through the sorbent under conditions where

either the analyte(s) are retained and matrix components washed through

or the reverse. Retained analytes are removed with an alternative solvent

before completing the analysis.

Related topic Solution equilibria (C1)

Extraction techniques

Extraction efficiency

and selectivity

Solvent extraction

Solid-phase sorbents

Solid-phase

extraction