Sistem karbonat

CARBONATE SYSTEM
 Know the carbon atom
 Where acid rain comes from
 What is pH and how to calculate
 Carbonate equilibrium reactions
 Why important
 Alkalinity
 Chemical weathering

CARBON
 Minimum oxidation
number is –4
 Maximum oxidation
number is +4

Carbon Isotopes
 C-12
 C-13
 C-14

Carbon forms
 Graphite
 Diamond
 Organic Matter
 DOC
 Particulate C

Types of carbon compounds
 Gas phase
 CO2, methane, volitale organic compounds
(VOCs)
 Organic
 Amino acids, DNA, etc
 Water
 Dissolved inorganic carbon (DIC)
 Dissolved organic carbon (DOC)

DOC in GROUNDWATER
 Less than 2 mg/L
 Microbial decomposition
 Adsorption
 Precipitation as solid
 > 100 mg/L in polluted ag systems
 Increases geochemical weathering

ORGANICS in WATER
 Solid phases (peat, anthracite, kerogen
 Liquid fuels (LNAPL), solvents (DNAPL)
 Gas phases
 Dissolved organics (polar and non-polar)

CARBONATE SYSTEM
 Carbonate species are necessary for all
biological systems
 Aquatic photosynthesis is affected by the
presence of dissolved carbonate species.
 Neutralization of strong acids and bases
 Effects chemistry of many reactions
 Effects global carbon dioxide content

DIPROTIC ACID SYSTEM
 Carbonic Acid (H2CO3)
 Can donate two protons (a weak acid)
 Bicarbonate (HCO3
-)
 Can donate or accept one proton (can be
either an acid or a base
 Carbonate (CO32-)
 Can accept two protons (a base)

OPEN SYSTEM
 Water is in equilibrium with the partial
pressure of CO2 in the atmosphere
 Useful for chemistry of lakes, etc
 Carbonate equilibrium reactions are thus
appropriate

PCO2 = 10–3.5 yields pH = 5.66
»What is 10–3.5? 316 ppm CO2
What is today’s PCO2? ~368 ppm = 10-3.43
»pH = 5.63

NATURAL ACIDS
 Produced from C, N, and S gases in the
atmosphere
 H2CO3 Carbonic Acid
 HNO3 Nitric Acid
 H2SO4 Sulfuric Acid
 HCl Hydrochloric Acid

http://www.motherjones.com/tom-philpott/2015/01/noaa-globes-co

OPEN SYSTEM
• Water is in equilibrium with the partial
pressure of CO2 in the atmosphere
• Useful for chemistry of lakes, etc
• Carbonate equilibrium reactions are thus
appropriate

Carbonic acid forms when CO2 dissolves
in and reacts with water:
CO2(g) + H2O = H2CO3
»Most dissolved CO2 occurs as “aqueous CO2”
rather than H2CO3, but we write it as carbonic
acid for convenience
»The equilibrium constant for the reaction is:
»Note we have a gas in the reaction and use partial
» pressure rather than activity

»First dissociation:
H2CO3 = HCO3
– + H+

»Second dissociation:
HCO3
– = CO3
2– + H+

Variables and Reactions Involved in Understanding
the Carbonate System
Gas
equilibria
Dissociation of
carbonic acid
Dissociation
of water
Cations Measure-
ments
PCO2 [H2CO3] [H+
] [Ca2+
] DIC
[HCO3
–
] [OH–
] Alkalinity
[CO3
2–
]

Activity of Carbonate Species versus pH

pH controls carbonate species
 Increased CO2 (aq) increases H+ and
decreases carbonate ion
 Thus increasing atmospheric CO2
increases CO2 (aq) and causes the water
system to become more acidic
 However, natural waters have protecting,
buffering or alkalinity

ALKALINITY refers to
water's ability, or inability, to
neutralize acids.
The terms alkalinity and total
alkalinity are often used to
define the same thing.

 Alkalinity is routinely measured in natural
water samples. By measuring only two
parameters, such as alkalinity and pH, the
remaining parameters that define the
carbonate chemistry of the solution (PCO2,
[HCO3
–], [CO3
2–], [H2CO3]) can be
determined.

Total alkalinity - sum of the bases
in equivalents that are titratable
with strong acid (the ability of a
solution to neutralize strong acids)
Bases which can neutralize acids in
natural waters: HCO3
–, CO3
2–,
B(OH)4
–, H3SiO4
–, HS–, organic
acids (e.g., acetate CH3COO–,
formate HCOO–)

Carbonate alkalinity
 Alkalinity ≈ (HCO3
–) + 2(CO3
2–)
 Reason is that in most natural waters,
ionized silicic acid and organic acids are
present in only small concentrations
 If pH around 7, then
 Alkalinity ≈ HCO3
–

CLOSED CARBONATE
SYSTEM
• Carbon dioxide is not lost or gained to the
atmosphere
• Total carbonate species (CT) is constant
regardless of the pH of the system
• Occurs when acid-base reactions much
faster than gas dissolution reactions
• Equilibrium with atmosphere ignored

How does [CO3
–2] respond to changes in Alk or DIC?
CT = [H2CO3*] + [ HCO3
–] + [CO3
–2]
~ [ HCO3
–] + [CO3
–2] (an approximation)
Alk = [OH–] + [HCO3
–] + 2[CO3
–2] + [B(OH)4
-] – [H+]
~ [HCO3
–] + 2[CO3
–2] (a.k.a. “carbonate alkalinity”)
So (roughly):
[CO3
–2] ~ Alk – CT
CT ↑ , [CO3
–2] ↓ Alk ↑ , [CO3
–2] ↑

Diurnal changes in DO and pH
What’s up?

Photosynthesis is the biochemical process in which plants and algae
harness the energy of sunlight to produce food. Photosynthesis of
aquatic plants and algae in the water occurs when sunlight acts on the
chlorophyll in the plants. Here is the general equation:
6 H20 + 6 CO2 + light energy —> C6H12O6 + 6 O2
Note that photosynthesis consumes dissolved CO2 and produces
dissolved oxygen (DO). we can see that a decrease in
dissolved CO2 results in a lower concentration of carbonic acid
(H2CO3), according to:
CO2 + H20 <=> H2CO3 (carbonic acid)
As the concentration of H2CO3 decreases so does the concentration
of H+, and thus the pH increases.

Cellular Respiration
Cellular respiration is the process in which organisms,
including plants, convert the chemical bonds of energy-rich molecules
such as glucose into energy usable for life processes.
The equation for the oxidation of glucose is:
C6H12O6 + 6 O2 —> 6 H20 + 6 CO2 + energy
As CO2 increases, so does H+, and pH decreases.
Cellular respiration occurs in plants and algae during the day and night,
whereas photosynthesis occurs only during daylight.

LITHOSPHERE
 Linkage between the atmosphere and the
crust
 Igneous rocks + acid volatiles =
sedimentary rocks + salty oceans (eq 4.1)

IMPORTANCE OF ROCK
WEATHERING
[1] Bioavailability of nutrients that have no
gaseous form:
 P, Ca, K, Fe
 Forms the basis of biological diversity,
soil fertility, and agricultural productivity
 The quality and quantity of lifeforms and
food is dependent on these nutrients

IMPORTANCE OF ROCK
WEATHERING
[2] Buffering of aquatic systems
-Maintains pH levels
-regulates availability of Al, Fe, PO4
Example: human blood.
-pH highly buffered
-similar to oceans

IMPORTANCE OF ROCK
WEATHERING
[3] Forms soil
[4] Regulates Earths climate
[5] Makes beach sand!

Sedimentary Processes
1) Weathering & erosion
2) Transport &
3) deposition
4) Lithification

Weathering:
decomposition and
disintegration of rock
Product of weathering
is regolith or soil
Regolith or soil that is
transported is called
sediment
Movement of
sediment is called
erosion

Weathering Processes
 Chemical Weathering-
Decomposition of rock as the result of
chemical attack. Chemical composition
changes.
 Mechanical Weathering -
Disintegration of rock without change in
chemical composition

Mechanical Weathering
•Frost wedging
•Alternate heating and cooling
•Decompression
causes jointing

Chemical Weathering Processes
 Hydrolysis - reaction with water (new minerals
form)
 Oxidation - reaction with oxygen (rock rusts)
 Dissolution - rock is completely dissolved
Most chemical weathering processes are
promoted by carbonic acid:
H2O +CO2 = H2CO3 (carbonic acid)

CARBONIC ACID
Carbonic acid is produced in rainwater by
Reaction of the water with carbon dioxide
Gas in the atmosphere.

CARBONATE
(DISSOLUTION)
All of the mineral is completely
Dissolved by the water.
Congruent weathering.

DEHYDRATION
Removal of water from a mineral.

HYDROLYSIS
H+ replaces an ion in the mineral.
Generally incongruent weathering.

HYDROLYSIS
 Silicate rock + acid + water = base cations
+ alkalinity + clay + reactive silicate (SiO2)

Hydrolysis
Feldspar + carbonic acid +H2O
= kaolinite (clay)
+ dissolved K (potassium) ion
+ dissolved bicarbonate ion
+ dissolved silica
Clay is a soft,
platy mineral, so
the rock
disintegrates

HYDROLYSIS
 Base cations are
 Ca2+, Mg2+, Na+, K+
 Alkalinity = HCO3
-
 Clay = kaolinite (Al2Si2O5(OH)4)
 Si = H4SiO4; no charge, dimer, trimer

OXIDATION
Reaction of minerals with oxidation.
An ion in the mineral is oxidized.

Oxidation can affect any
iron bearing mineral, for
example, ferromagnesian
silicates which react to
form hematite and limonite
Oxidation

Oxidation of pyrite and other sulfide minerals forms
sulfuric acid which acidifies surface water and rain
Pyrite + oxygen + water = sulfuric acid + goethite
(iron sulfide) (iron oxide)

Products of weathering
Clay minerals further decompose to aluminum hydroxides
and dissolved silica.

Removal of Atmospheric CO2
 Slow chemical weathering of continental rocks balances
input of CO2 to atmosphere
 Chemical weathering reactions important
 Hydrolysis and Dissolution

Atmospheric CO2 Balance
 Slow silicate rock weathering balances
long-term build-up of atmospheric CO2
 On the 1-100 million-year time scale
 Rate of chemical hydrolysis balance rate of
volcanic emissions of CO2
 Neither rate was constant with time
 Earth’s long term habitably requires only that
the two are reasonably well balanced

What Controls Weathering
Reactions?
 Chemical weathering influenced by
 Temperature
 Weathering rates double with 10°C rise
 Precipitation
 H2O is required for hydrolysis
 Increased rainfall increases soil
saturation
 H2O and CO2 form carbonic acid
 Vegetation
 Respiration in soils produces CO2
 CO2 in soils 100-1000x higher than
atmospheric CO

Climate Controls Chemical Weathering
 Precipitation closely linked with
temperature
 Warm air holds more water
than cold air
 Vegetation closely linked with
precipitation and temperature
 Plants need water
 Rates of photosynthesis
correlated with temperature

Chemical Weathering: Earth’s Thermostat?
 Chemical weathering can provide negative feedback that
reduces the intensity of climate warming

Chemical Weathering: Earth’s
Thermostat?
 Chemical weathering can provide negative feedback that
reduces the intensity of climate cooling

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