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.