Instant Notes: Analytical Chemistry

D (11)

and substituting KDfor [HA]ether/[HA]aqfrom equation (9) gives

D (12)

Equation (12) shows that at low pH, when the acid is undissociated, it is

extracted with the greatest efficiency as D@KD, whereas as the pH is increased

the value of Ddecreases until at high pH the acid is completely dissociated into

the anion A-, and none will be extracted. This is shown graphically in Figure 1as

pH versus Eplots for two weak acids (curves 1 and 2) with different acid dis-

sociation constants (Kaor pKavalues), curve 1 being for the stronger of the two

acids. For a weak organic base, such as an amine, protonation occurs at low pH

according to the equation

RNH 2 +H+=RNH 3 + (13)

The relation between pH and the distribution ratio for a weak base (curve 3) is

therefore the opposite of a weak acid so that it is possible to separate acids from

bases in a mixture either by extracting the acids at low pH or the bases at high

pH. Separating mixtures of acids or mixtures of bases is possible only if their

dissociation or protonation constants differ by several pKunits.

Extraction of metals

Metal ions in aqueous solutions are not themselves extractable into organic

solvents, but many can be complexed with a variety of organic reagents to form

extractable species. Some inorganic complex ions can be extracted as neutral

ionic aggregates by association with suitable ions of opposite charge (counter

ions). There are two principal metal extraction systems:

∑ Uncharged metal chelate complexes (ring structures that satisfy the

coordination requirements of the metal) with organic reagents

e.g. 8-hydroxyquinoline (oxine)

acetylacetone (AcAc)

KD

(1Ka[H+])

[HA]ether






[HA]aq(1Ka/[H]aq)

D1 – Solvent and solid-phase extraction 113

123

100

80

60

40

20

0

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pH

Percent extracted (

E)

Fig. 1. Solvent extraction curves for two organic acids having different pKavalues (curves 1

and 2) and a base (curve 3). Curve 1 represents the acid with the larger Ka(smaller pKa).