Geology Reference
In-Depth Information
In addition to the charge balance expression for H
2
CO
3
, Equation
(3.64) includes appropriate expressions for NH
4
1
,H
2
S, H
4
SiO
4
, HOrg,
and H
2
PO
4
. In each case, the reference point has been selected on the
basis of the pK values for these species. For example, phosphoric acid,
H
3
PO
4
, has pK
1
¼
2.15 and pK
2
¼
7.20 and so H
2
PO
4
will be the main
'P' species in solution at the reference point, i.e. the equilibrium pH of a
solution of carbonic acid and water (pH
B
5.65). The contribution of 'P'
species to alkalinity is thus determined by using the charge balance
expression for NaH
2
PO
4
.
Another approach to determine carbonate alkalinity is by using a
charge balance expression for the major conservative ions in aqueous
solution.
Carbonate Alkalinity
¼f
HCO
3
gþ
2
f
CO
2
3
g
¼
X
ð
conservative cations
Þ
X
ð
conservative anions
Þ
¼f
Na
þ
gþf
K
þ
gþ
2
f
Ca
2
þ
gþ
2
f
Mg
2
þ
g
f
Cl
g
2
f
SO
2
4
gf
NO
3
g
ð
3
:
65
Þ
Alkalinity is an important parameter in assessing the effects of envir-
onmental change on aqueous systems (see Section 3.3.4.1). It is also
important to understand that, by definition, alkalinity (Equation (3.61)) is
independent of addition or removal of CO
2
(or H
2
CO
3
) from the system
(cf. Equation (3.62) - H
2
CO
3
does not appear in the charge balance
expression). This can be very useful in the determination of the concen-
tration of dissolved inorganic carbon species in aqueous systems that are
in equilibrium with an atmosphere containing CO
2(g)
(Example 3.7).
Example 3.7: Graphical illustration of the relationship between (i)
log
{
H
2
CO
3
*
}
, (ii) log
{
H
2
CO
3
}
, (iii) log
{
HCO
3
}
, and (iv) log
{
CO
3
2
}
and pH for an open aqueous system. Use p
CO2
¼
3.5
10
4
atm, K
H
¼
3.2
10
2
, K
1
¼
5.1
10
7
and K
2
¼
5.1
10
11
.
H
2
CO
3
* K
H
¼
{H
2
CO
3
*}/p
CO2
¼
3.2
10
2
mol L
1
atm
1
where K
H
is the Henry's Law constant for atmosphere-aqueous phase
equilibria involving gases.
CO
2(g)
þ
H
2
O
"
