Instant Notes: Analytical Chemistry

possible. Readily extractable solutes, therefore, include covalent neutral mole-

cules with few polar and no ionized substituents; polar, ionized or ionic species

will remain in the aqueous phase. A wide range of solid sorbents with different

polarities are used in solid phase extraction making this a very versatile tech-

nique, but the factors affecting the distribution of solutes are essentially the

same as in solvent extraction. Two examples illustrate these points.

Example 1

If an aqueous solution containing iodine and sodium chloride is shaken with a

hydrocarbon or chlorinated hydrocarbon solvent, the iodine, being a covalent

molecule, will be extracted largely into the organic phase, whilst the completely

ionized and hydrated sodium chloride will remain in the aqueous phase.

Example 2

If an aqueous solution containing a mixture of weakly polar vitamins or drugs

and highly polar sugars is passed through a hydrocarbon-modified silica

sorbent which has a nonpolar surface, the vitamins or drugs will be retained on

the surface of the sorbent whilst the sugars pass through.

Solvent extraction is governed by the Nernst Distributionor Partition Law.

This states that at constant temperature and pressure, a solute, S, will always be

distributed in the same proportions between two particular immiscible solvents.

The ratio of the equilibrium concentrations (strictly their activities (Topic C1)) in

the two phases defines a distributionor partition coefficient, KD, given by the

expression

KD= (1)

where [] denotes concentrations (activities) of the distributing solute species, S,

and the value of KDis independent of the total solute concentration. In practice,

the solute often exists in different chemical forms due to dissociation (ioniza-

tion), protonation, complexation or polymerization, so a more practically useful

expression which defines a distributionor partition ratio, D, is

D= (2)

where (CS) represents the total concentration of all forms of the distributing

solute Sin each phase. If no interactions involving Soccur in either phase, then

Dand KDwould be identical. However, the value of Dis determined by the

experimental conditions and it can be adjusted over a wide range to suit the

requirements of the analytical procedure.

A distribution ratio can also be defined for solid phase extraction, i.e.

D (3)

where the numerator represents the solute concentration in the solid sorbent

and the denominator the solute concentration in the liquid phase.

Solutes with large values of D(e.g. 10^4 or more) will be essentially quantita-

tively extracted into the organic phase or retained by the sorbent, although the

nature of an equilibrium process means that 100% extraction or retention can

never be achieved.

(CS)sorb




(CS)liq

(CS)org




(CS)aq

[S]org




[S]aq

110 Section D – Separation techniques