Water Chemistry -- Origin of Dissolved Substances


I. Units
    A. Mass/volume
mass/volume = mg/L

    B. Mass/mass
mass/mass=mg/kg=1/10^6=ppm

    C. ppm
shows how mg/L=ppm or mg/L * 1/density when the density of water is not 1.00 g/cm^3

    D. ug-atoms
        sometimes see measurements reporting compounds vs. elements; (e.g., drinking water standards = 10 mg NO3 - N/L)

                                  molecular weight
shows example of how to calculate the amount of NO3-N from a given amount of NO3
                                   molecular weight

        oceanographers in older literature used ug-atoms/L:

         1 ug-atom/L of PO4-P = 31 ug PO4--P/L (since 31 is the atomic weight of P)

    E. molarity
shows how to calculate molarity using g/L and molecular weight

    F. molality
molarity/density=molality=mol/kg

    G. normality
defines normality and gives an example
 

II. Activity - concentration available to react with other compounds or ions

defines activity of an ion (activity coefficient*concentration)
        Debeye-Huckel Theory – you can estimate the activity coefficient by the electrostatic interactions of ions in the solution
 

III. Origins of Major Ions
    A. Major ions in river water

        World average of river water (ppm)
                   CATIONS                                                          ANIONS
           Ca2+   Mg2+   Na+   K                          HCO3-     SO42-   Cl-     SiO2
             13.4      3.4        5.2      1.3                               52         8.3         5.8     10.4
 

     B. Where do these ions come from?  Why is this the composition of surface waters?
            1. Atmospheric inputs
            2. Rock weathering
            3. Evaporative concentration
    C. Atmospheric inputs, rain

    Picks up salts from ocean spray (sodium, chloride, magnesium, sulfate) and from dust over the continents
        (calcium and potassium); also gases over volcanoes

     D. Rock weathering – interaction of rocks and water
          1. CO2+H2O yields H2CO3
          2. example: example of albite being weathered to kaolinite

          3. further weathering example of kaolinite being weathered to gibbsite

          4. analyzing weathering contributions -- 3 approaches
            a. mass balance
            b. statistical correlations
            c. thermodynamics

    D. Evaporation - can concentrate ions until you get the precipitation of minerals
          general equation; equilibrium constant = Keq = Ksp (solubility product)

equation for solubility

equation for solubility equilibrium constant

        final composition depends on amounts and ratios of ions initially present

        but if the ion product is greater than Kso, then the calculated oversaturation does not necessarily mean you
            will get precipitation

    E. Biological considerations

        Conservative ions – concentrations undergo no or minor changes due to organisms (either direct use or
            indirect conversion) [Na, K, Cl]
        Vs.
        Dynamic ions – concentrations are strongly influenced by the metabolism of organisms
 

             1. uptake of N and P by algae
             2. nitrogen fixation by bluegreen algae
             3. reactions mediated by microbes; decomposition
            4. sphagnum moss as an ion exchanger
             5. anthropogenic alterations

         ‘biogeochemistry’

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