Instant Notes: Analytical Chemistry

The efficiencyof an extraction depends on the value of the distribution ratio, D.

For solvent extraction, it also depends on the relative volumes of the two liquid

phases and for solid-phase extraction on the surface area of the sorbent. With

solvent extraction, the percentage of a solute extracted, E, is given by the expres-

sion

E (4)

where Vaqand Vorgare the volumes of the aqueous and organic phases, respec-

tively.

For solutes with small values of D, multiple extractions will improve the

overall efficiency, and an alternative expression enables this to calculated

(Caq)n=Caq[Vaq/(DVorgVaq)]

n

(5)

where Caqand (Caq)nare the amounts of solute in the aqueous phase initially and

remaining after nextractions, respectively.

The following example demonstrates the use of these formulae.

Example 1

A water sample contains 10 mg each of a halogenated pesticide and an ionic

herbicide which are to be separated by extraction of the pesticide into methyl-

benzene. Given that the pesticide distribution ratio, D, for methylbenzene/

water is 50, calculate the extraction efficiency for

(i) one extraction of 20 cm^3 of water with 10 cm^3 of methylbenzene

(ii) one extraction of 20 cm^3 of water with 30 cm^3 of methylbenzene

(iii) three extractions of the same 20 cm^3 of water with 10 cm^3 portions of

methylbenzene (30 cm^3 in total)

(i) Substitution of the values for D, Vaqand Vorgin equation (4) gives

E =96.15%

(ii) Substitution of the values for D, Vaqand Vorgin equation (4) gives

E =98.68%

(iii) Substitution of the values for D, Vaq, Vorg, Caqand (Caq)nin equation (5) gives

(Caq) 3 = 10 ¥[20/((50 ¥10) +20)]^3

(Caq) 3 = 10 ¥[0.03846]^3 =5.6896 ¥ 10 -^4 mg

E=99.99%

An extraction of 99.99%, as achieved in (iii), would be considered quantita-

tive, although the lower efficiencies obtained in (i) and (ii) might be acceptable

in the context of a defined analytical problem. It is clear that increasing the

volume of organic solvent, or extracting with the same volume divided into

several smaller portions, increases the overall efficiency of an extraction.

NB the ionic herbicide has a negligibly small distribution ratio, being very

polar and highly water soluble.

Selectivity in extraction procedures is the degree to which solutes in a

mixture can be separated by virtue of having different distribution ratios. For

^100 ¥^50

[ 50 (20/30)]

^100 ¥^50

[ 50 (20/10)]

100 D

[D +(Vaq/Vorg)]

Extraction
efficiency and
selectivity

D1 – Solvent and solid-phase extraction 111