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Overview of Some Chemistry in Natural Waters
Gases dissolve according to Henry’s law:
KH=[X(aq)]/PX; X=O2, CO2, H2S, SO2, etc.

Oxidation-Reduction, Acid-Base,
Anaerobic digestion
According to pH, can get leaching (dissolution)
of various species from soil or rock.
Dissolved Oxygen:
At 25°C, Henry’s law gives 8.7 mg/l, or 8.7
ppm.
Note: This is small, changes greatly with
temperature; thermal pollution can lower it
significantly.
In acid: O2 + 4H+ + 4e-  2 H2O
In base: O2 + 2 H2O + 4e-  4OH-
General example for oxidation of organics:
CnH2nOm + kO2  nCO2 + nH2O
Measure of organic content in water:
Biochemical Oxygen Demand (BOD) (Microbial action)
Chemical Oxygen Demand (COD) (Na2Cr2O7/ H2SO4)
Total Organic Carbon (Typ. ~1 mg/l, 1 ppm C, in ground water)
Dissolved Organic Carbon (Typ. ~5 ppm in surface waters)
The pE scale:
Analogous to pH; measure of extent to
which waters are reducing in nature.
pE=-log[e-]effective, i.e. -log of the activity of e-.
Low pE=strongly reducing (more e-s available)
High pE=weakly reducing (fewer e-s available)
Alkalinity:
A measure of the actual concentration of the
basic anions in solution as carbonate.
total alkalinity = 2[CO32-]+[HCO3-]+[OH-]-[H+]
For total alkalinity, the titration is done to Methyl
Orange endpoint, pH = 4.
For Carbonate only (i.e., no HCO3-), the titration
is done to the phenolphthalein endpoint, pH = 8
to 9 -- “phenolphthalein alkalinity.”
Hardness:
Measures the total concentration of Ca2+
and Mg2+ ions.
Titrate with ethylenediaminetetraacetic acid
(EDTA)
Find the total number of moles of Ca2+ and
Mg2+ ions, then convert to mass per liter of
CaCO3 that would have the same number of
ions.
For example, if [Ca2+]+[Mg2+]=0.0010 M
then, since m.w. CaCO3 = 100 g/mol, the
hardness value is 0.001  100 = 100 mg /l.