An Introduction to Environmental Chemistry

eqn. 5.8

the dissociation constant is,

eqn. 5.9

It is simple to demonstrate that the alkalinity in most continental waters is dom-

inated by HCO 3 - by rearranging the above equations at a typical pH value for

these waters. For example, many mature rivers have pH values around 8.

Rearranging equation 5.9 to solve for aHCO 3 - gives,

eqn. 5.10

And substituting values for pH 8 (pH=-log10aH+, see Box 3.5) and K 2 (eqn. 5.9)

gives,

eqn. 5.11

This shows that at typical pH values for continental waters the HCO 3 - anion is

200 times more abundant than the CO 32 - anion. Repeating this exercise for a range

of pH values results in the graphical relationship shown in Fig. 5.5. Note that

when pH falls below 5 on Fig. 5.5, almost all of the weak acid anions (HCO 3 - and

CO 32 - ) have disappeared and at pH of 4 only undissociated acid (H 2 CO 3 ) remains.

This relationship is used as the basis for measuring alkalinity (Box 5.2).

The pH of natural waters is in fact controlledmainly by the relative concen-

trations of dissolved inorganic carbon (DIC) species, i.e. H 2 CO 3 , HCO 3 - and

a

aa

HCO a

CO CO

3 CO

8

3

2

10 3

8

3

2

11 3

(^102)
10

110

510

- 200

--

• 
  

--

• 
  

==¥ -

¥

=

..

.

a

aa

K

HCO

HCO

3

3

2

2

• 
  

+-

=

.

K

aa

(^2) a
3
2
3

== 10 10 3

+-

• 
  

HCO -

HCO

..

HCO 3 - ()aqªH()+aq+CO^23 - ()aq

152 Chapter Five

Percentage of TDIC species

4 5 6 7 8 9 10 11 12

pH

0

10

20

30

40

50

60

70

80

90

100

H 2 CO 3

HCO 3 –

CO 3 2–

Fig. 5.5Relationship between the total dissolved inorganic carbon species (TDIC; i.e. H 2 CO 3

+HCO 3 - +CO 32 - ) and pH. Most natural waters have pH between 7 and 9 where the HCO 3 -

anion is abundant (>80% of the TDIC). In highly alkaline waters (pH>10.3) the CO 32 - anion

becomes dominant, while in acidic waters (pH<6.4) the undissociated acid (H 2 CO 3 ) is the

dominant TDIC species.