An Introduction to Environmental Chemistry

198 Chapter Six

Box 6.4 Ion interactions, ion pairing, ligands and chelation

Ions and water

When ionic salts dissolve in water, the

salts dissociate to release the individual ions.

The charged ions attract the polar water

molecules such that a positively charged ion

will be surrounded most closely by oxygen

atoms of the water molecules (Fig. 1). Thus,

ions are not free in solution, but interacting,

or coordinating, with water molecules. The

water molecules can be considered to be

bound to the ion by so called coordinate

bonds. For example, the hydrogen ion (H+)

is hydrated to form H 3 O+. For simplicity in

chemical equations the simple H+notation

is used.

In addition to interaction with water

molecules, individual ions may interact with

other ions to form ion pairs. For example,

dissolved sodium and sulphate ions can

interact to form a sodium sulphate ion

pair:

Na()+aq+SO^24 - ()aqªNaSO- 4 ()aq eqn. 1

The extent of this interaction varies for

different ions and is measured by an

equilibrium constant. In this case:

eqn. 2

If all the relevant equilibrium constants are

known, together with the amounts of the

ions present, the proportions of the various

ions associated with each ion pair can be

calculated. The results of such an analysis for

seawater (Table 1) show that ion pairing is

common. A full analysis of the properties of

seawater requires that these species are

taken into account.

Since anions are present at lower

concentrations than cations (except for

chloride (Cl-)), ion pairing has a

proportionately greater effect on anions

relative to cations. The extent of ion pairing

is dependent on temperature, pressure and

salinity.

K NaSO

Na SO

=

◊

• +-

a

aa

4

42

d+ H H d+

H d+

H d+

d+ H

d+ H

d–

d–

d–

O

O

O

+

Fig. 1Ionic salts dissolved in water.

(continued)