Box 64 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 H3O+. 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:

The extent of this interaction varies for different ions and is measured by an equilibrium constant. In this case:

aNaSO;

aNa+aSO4-

Na+a

NaSO

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.

Fig. 1 Ionic salts dissolved in water.

(continued)

h2c o h2C

-CH2

H2C O

H2Cv

-ch2

"CH2'

EDTA

EDTA-metal complex

Fig. 2 Chelation between a metal ion (M+) and ethylenediaminetetra-acetic acid (EDTA), a well-known laboratory ligand. Note that the six coordination bonds (white) are made between dissociated -O- groups and lone pairs of electrons (denoted by double dots) on the N atoms of the EDTA. Similar coordination bonds are made in fulvic acid chelates.

Table 1 Percentage of each major ion in seawater present in various ion pairs.

Na+

Mg2+

Ca2+

K+

Free ion

98

89

89

99

mso4

2

10

10

1

mhco3

1

1

mco3

-

Cl-

SO2-

HCO-

CO2

Free ion

100

39

81

8

NaX

37

11

16

MgX

20

7

44

CaX

4

4

21

KX

1

Mg2CO3

7

MgCaCO3

4

M, cation; X, anion; —, species present at less than 1%.

M, cation; X, anion; —, species present at less than 1%.

Ligands and chelation

Beside water, other polar molecules and some anions coordinate to metal ions; such firmly bonded molecules are known as ligands. Each ligand contains at least one atom that bears a lone pair of electrons. Lone pairs of electrons can be envisaged as non-bonded electrons that are grouped at the extremities of an atom's electron orbital, and these 'plug-in' to gaps in the electron orbital of the metal ion forming a coordinate bond. Most ligands form only one coordinate bond with a metal ion, for example the Cl- anion which helps keep metal ions in solution in mid-ocean ridge hydrothermal systems. In some cases, however, a ligand may form more than one link with the metal ion. The complex ions that form between these ligands and cations are known as chelates or chelated complexes from the Greek chelos meaning 'a crabs claw'. The ligand is envisioned to have a claw-like grip on the metal ion, binding it firmly and forming a stable complex ion (Fig. 2), which is much more soluble than the metal ion itself. Natural chelates include fulvic and humic acids formed by the degradation of soil organic matter.

Table 6.4 Total activity coefficients (g) for selected ions in surface seawater at 25°C and salinity of 35. After Millero and Pierrot (1998) with kind permission of Kluwer Academic Publishers.

Ion

Measured gt

Calculated gt*

Cl-

0.666

0.666

Na+

0.668

0.664

H+

0.590

0.581

K+

0.625

0.619

OH-

0.255

0.263

HCO-

0.570

0.574

B(OH)4

0.390

0.384

Mg2+

0.240

0.219

SO4-

0.104

0.102

Ca2+

0.203

0.214

CO2-

0.039

0.040

h2po4

0.453

0.514

HPO24-

0.043

0.054

po4-

0.00002

0.00002

*Values recommended for use in calculations.

Allowance for ion pairing indicates that about 90% of the calcium in seawater is present as the free ion with the remainder present as CaSO0 and CaHCO+ ion pairs. For CO2- it is calculated that only about 10% exists as the free ion with the remainder in ion pairs with Mg2+, Ca2+ and Na+. Applying these corrections decreases the IAP in the following way:

0 0

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