2. Parameter Value
T °C 25
pH 7.0
Ca mM 3.818
Mg mM 0.864
Na mM 0.957
K mM 0.072
Alkalinity mM 4.960
Chloride mM 1.805
Sulfate mM 1.520
Nitrate mM 1.100
Start: Measured data (water analysis)
cations
anions
Example
3. The Naïve Approach (no speciation)
each measured element/parameter one ion
in aqueous solution
Element Ion z
Ca Ca+2 2
Mg Mg+2 2
Na Na+ 1
K K+ 1
Alkalinity HCO3
- -1
Chloride Cl- -1
Sulfate SO4
-2 -2
Nitrate NO3
- -1
1-to-1
correspondence
measured
charge
4. But, in reality:
Each measured concentration
comprises “free ions” and complexes.
Ca = Ca+2 + CaHCO3
+ + CaCO3(aq) + CaSO4(aq) + ...
total concentration = free ion + complexes
in the solutionmeasured
Example:
naïve approach
5. The naïve approach ignores at least
three things:
formation of
aqueous complexes
the role of alkalinity
(which is not an element)
activity corrections
(for non-ideal solutions)
2
3
1
6. Aqueous Complexation is described
by the Law-of-Mass Action (LMA):
ba
ba
}B{}A{
}BA{
K aA + bB = AaBb
equilibrium constant
curly braces denote activities as
“effective concentrations”
1
7. Alkalinity should be replaced by DIC
as dissolved inorganic carbon:
DIC = CO2 + HCO3
- + CO3
-2 + ...
The partitioning is described by LMA
(plus 3 equilibrium constants).
(bi)carbonate
complexes
2
13. DIC = CO2 + HCO3
- + CO3
-2 + ...
Note: CO2 = CO2(aq) + H2CO3
Dissolved Inorganic Carbon
CO2 0.948 mM
HCO3
- 4.762 mM
CO3
-2 0.003 mM
C complexes 0.181 mM
DIC 5.894 mM