An Introduction to Environmental Chemistry

Allowance for ion pairing indicates that about 90% of the calcium in seawater is

present as the free ion with the remainder present as CaSO 40 and CaHCO 3 +ion

pairs. For CO 32 - it is calculated that only about 10% exists as the free ion with

the remainder in ion pairs with Mg^2 +, Ca^2 +and Na+. Applying these corrections

decreases the IAP in the following way:

eqn. 6.8

Now the calculation of saturation state becomes:

eqn. 6.9

On this basis, surface seawater in the tropics is about four times supersaturated

with respect to calcite. In practice, calculating seawater Wvalues has been sim-

plified for the environmental chemist because chemical oceanographers have

measured and modelled the combined effects of ion activity and ion pairing over

a range of pressures, temperatures and salinities (ionic strength) to yield total

activity coefficients, denoted gt(Table 6.4). Using total activity coefficients for

Ca^2 +and CO 32 - from Table 6.4, the IAP calculations (eqns. 6.5 & 6.8) now sim-

plify to:

eqn. 6.10

The calculation of saturation state is now:

IAP Ca CO

Ca CO

mol l

tt

=¥

=◊¥ ◊

=¥ ¥ ¥

=¥

+-

+-

--

aa

cc

2 32

2 32

822

0 01 0 214 0 00029 0 04

248 10

gg

....

.

W=

¥

¥

=

135 10

33 10

4

8

9

.

.

IAP=¥ ¥ ¥= ¥15 10....--^78 09 01 135 10 mol l^2 -^2

200 Chapter Six

Table 6.4Total activity coefficients (gt) 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 3 - 0.570 0.574

B(OH)- 4 0.390 0.384

Mg^2 + 0.240 0.219

SO 42 - 0.104 0.102

Ca^2 + 0.203 0.214

CO 32 - 0.039 0.040

H 2 PO- 4 0.453 0.514

HPO 42 - 0.043 0.054

PO 43 - 0.00002 0.00002

* Values recommended for use in calculations.