An Introduction to Environmental Chemistry

on the border between the USA and Mexico. This very large lake was created in

1905 by floodwaters of the Colorado, forming a closed basin lake with no outflow

to the sea. The Salton Sea began as a freshwater lake with total dissolved salt con-

centrations around 3.5 g l-^1 in 1907. Evaporation since this time means that the

lakewaters are now saltier than seawater. In 1997, total dissolved salt concentra-

150 Chapter Five

Box 5.1 Ionic strength

The concentration of an electrolyte solution

can be expressed as ionic strength (I), defined

as:

eqn. 1

where cis the concentration of ion (i) in mol

l-^1 , ziis the charge of ion (i) and Srepresents

the sum of all ions in the solution.

As a measure of the concentration of a

complex electrolyte solution, ionic strength is

better than the simple sum of molar

concentrations, as it accounts for the effect

of charge of multivalent ions.

For example, the Onyx River in Antarctica

(Section 5.3), 2.5 km below its glacier source,

has the following major ion composition (in

mmol l-^1 ): Ca^2 +=55.4; Mg^2 +=44.4; Na+=125; K+

=17.6; H+= 10 -^3 ; Cl-=129; SO 42 - =32.2, HCO 3 -

=136 and OH-=10 (rivers in this region have

pH around 9, which means that HCO 3 - is the

Icz=^12 Sii^2

dominant carbonate species). Putting these

ions in equation 1 gives:

eqn. 2

Substituting the mmol l-^1 values (and

correcting to mol l-^1 with the final 10-^6 term

in eqn. 3) gives:

eqn. 3

eqn. 4

Freshwaters typically have ionic strengths

between 10-^3 and 10-^4 mol l-^1 , whereas

seawater has a fairly constant ionic strength

of 0.7 mol l-^1.

I=¥473 10. --^41 mol l

I=¥[( )+¥( )++

+++ ¥--( )++]¥

(^12)
36
55 4 4 44 4 4 125 17 6
10 129 32 2 4 136 10 10
...
.
Ic c c c
cc c
cc
=+++
++ +
++
(^12)
4
3
4411
11 4
11
SCa Mg Na K
HClSO
HCO OH
....
...
..
Box 5.2 Measuring alkalinity
Alkalinity is measured by adding acid to a
water sample until the pH falls to 4. At this
pH, HCO 3 - and CO 32 - alkalinity (known as the
carbonate alkalinity (Ac)) will have been
converted to carbon dioxide (CO 2 ), i.e.:
eqn. 1
eqn. 2
Note that twice as much H+is used up
neutralizing the CO 32 - (eqn. 2) relative to
the HCO 3 - (eqn. 1). This is expressed in the
formulation that expresses carbonate
alkalinity, by the 2 in front of the cCO 32 -
term, i.e.:
CO^23 ()-aq++ 2 H()+aqªH O 22 ()lgCO( )
HCO- 322 ()aq++H+()aqªH O()l CO()g
eqn. 3
This formulation is expressed in concentration
because the carbonate species are measured
values (see Section 2.6). The volume of acid
used is a measure of the alkalinity, which is
usually expressed as milliequivalents per
litre (see footnote to Table 4.10). These
units account for the difference in H+
neutralizing power between CO 32 - and
HCO 3 -. Note, however, that at pH around
7.5–8, monovalent HCO 3 - accounts for almost
all alkalinity (Fig. 5.5) such that at these pH
values milliequivalents are essentially the
same as millimoles.
Acc=+HCO-- 332 cCO^2