Inorganic Carbon and pH


I. Controls on CO2
    A. Photosynthesis
    B. Respiration
    C. Atmosphere
    D. Geologic inputs
vertical profile of CO2 and temperature with depth in a stratified lake

II. Forms of carbon --
     A. Forms and examples of each
 
 

PARTICULATE DISSOLVED
ORGANIC Living organisms
Dead organic material
Soluble organics:
     DOC (dissolved organic carbon)
     Amino acids
     Sugars
INORGANIC CaCO3
Carbonates of Mg, K, Na, etc.  (minerals)
DIC (dissolved inorganic carbon)
     CO2
     H2CO
     HCO3-
     CO32-

     B. Carbon cycle – how C moves between these boxes

III. Dissolved inorganic carbon, DIC
         Distribution of DIC as a function of pH – See figure 11-1, Wetzel

IV. pH
     A. Reactions and definitions
formula for dissociation of water
equilibrium constant for dissociation of water(dissociation product constant)
                                                    by definition concentration of water = 1
definition of pH
                              if [H+] = 10-7 molar, pH = 7

     B. Common pH values
        1. If distilled water reacts with CO2, get H2CO3 and pH ~5.6
        2. Rain with pH less than 5.6 is said to be ‘acid rain’
        3. Most lakes range in pH from 6-9
        4. Low pH lakes
            a. pH < 2; usually due to volcanically produced H2SO4 or mine wastes
            b. pH 3.3-4.5 – Sphagnum bogs – exchange of cations for H+ by the plants
            c. Acid deposition
        5. High pH in lakes
            a. Carbonates present  (saline lakes, pH>8)
            b. High rates of photosynthesis (decrease CO2, increase pH)

V. Carbonate buffering system

    A. Reactions
        1.   Hydration reaction CO2+H20 <-> H2CO3
                                                                [CO2]aq

        2.   Dissociation reaction H2CO3 <-> H+ + HCO3-

        3.   Dissociation reaction HCO3- <-> H+ + CO3 2-

carbonate cycle
 

    B. Solving equations
        1. Basic principles
            (a) What is distribution of C species?
            (b) What happens when one or more species is changed – when the system is perturbed?
            (c) If we know any 2 quantities, then we can determine the others
        2. System of equations

            a. Kdissociation of water; equilibrium constant

            b. K1equilibrium of hydration of CO2
                            Do not put water in the denominator – is 1
            c. K2equilibrium of dissociation of bicarbonate

            d. Ksp(CaCO3equilibrium of solubility of CaCO3
                            solids also are 1

VI. Alkalinity

    A. Definition = measure of the buffering capacity of the water; capacity of water to neutralize an acid

        Sum of the anions of weak acids

components of alkalinity

        In practice for most lakes: DIC components of alkalinity

    B. Changes in and control of alkalinity
CLOSED SYSTEMS – ALKALINITY IS CONSERVED

interaction of photosynthesis and respiration with DIC and buffering

 
        1. CASE 1
            a. respiration adds CO2
                Remember, [HCO3-] + 2[CO32-] = constant

        2. CASE 2 -- Photosynthesis removes CO2
                Opposite of CO2 addition case

alkalinity in open systems
 

        4. Buffering aspects – What happens when acid or bases are added to the system?

            1. add acid – consume alkalinity

            2. add base – increase alkalinity and increase pH
            3. carbonate system prevents pH from changing as much as expected from amount of acid or base added.

VII. Flux to atmosphere (atmospheric controls)
        A. boundary layer model

diffusive flux of CO2 to the atmosphere

              where z is the thickness of the boundary layer;
              related to wind speed

VIII. Questions that have been asked about the Carbon cycle in freshwaters
     A. Is carbon a limiting nutrient?

     B. Role of lakes, surface waters in the global C cycle

     C. Acid rain – areas with few carbonates – little alkalinity or buffering capacity
 

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