Ion Speciation in Seawater and Gas Solubility and Exchange

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Ion Speciation in Seawater and Gas Solubility and
Exchange
Lecture 9
OEAS-306
February 17, 2009

• Outline
• Quick review of basic chemical quantities
• Thermodynamic Equilibrium
• Ideal Solutions
• Non-ideal Solutions and Activity
• Gas Solubility
• Surface Gas Exchange

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Quick Review of Basic Chemical Quantities
1 mole Number of atoms equivalent to 12 grams
of carbon-12
1 mole Avogadros constant 6.022 ? 1023
1 mole of molecules has a mass equal to the
molecular weight in grams
Example 1 mole of H2O is the number of molecules
in 18.015 grams of H2O
Molar concentration or molarity (M) usually
given as moles/dm3 of moles/L Common usage is
millimolar (mM 10-3 mol/L) or micromolar
(?M 10-6 mol/L) Molarity is moles per volume of
solution (not solvent) Molality (m) refers to
moles of solute per kilograms solvent (moles/kg)
What is the concentration in mg/L of a 2?10-4 M
solution of O2(aq)?
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Ion Speciation
Major goal of chemical oceanography is to
understand how and why different ions are
associated with other substances.

• The vast majority of ions in seawater, are not
  free, but are associated with material
• Positively or negatively charged portions of
  water molecules.
• Other ions.
• Organic matter

• How ions speciate impacts
• Their bioavailability
• Their availability to participate in chemical
  reactions
• Mechanisms of transport and fate of material.

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Example of Copper in Seawater

• Free dissolved Cu2 comprises a small fraction of
  the total Cu in seawater. If an organism
  requires free Cu2, measuring total Cu will
  seriously over-estimate amount that is available.
• The tools exits to directly measure the the total
  amount of Cu, but not the individual species.
• Therefore we must rely on our understanding of
  chemistry to determine the amount of free Cu2,
  that is available in the environment.

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Thermodynamic Equilibrium
In the absence of external forces, chemical
reactions proceed in a direction that ultimately
leave the reactants and products in their lowest
energy state.
Generalized Equilibrium Expression
reactants
products
At equilibrium, the rate of the forward reaction
is equal to the that of the reverse.
so, the concentrations of products and reactants
is constant.
Angled brackets indicate concentration of ion.
Where Keq is the equilibrium constant of the
reaction.
Lower case letters indicate number in front of
ion in chemical equation
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Example
A two liter vessel contains 0.1908 moles of CO2,
0.0908 moles of H2, 0.0092 moles of CO, and
0.0092 moles of H2O vapor. What is the
equilibrium constant?
CO2 0.1908 mol CO2/2.00 L 0.0954 M H2
0.0454 M CO 0.0046 M H2O 0.0046 M
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Activity and Activity Coefficients
Often (usually) a substance does not behave as if
it is present at a given concentration.
An ion can acts as if there is less than the
actual concentration or as if there is more than
the actual concentration.
Activity defined as the effective concentration
of a solute.
Concentration of ion i Activity of ion i
Activity of i is defined as i ai ?ii
When species i behaves less concentrated than
it really is, ?i lt 1 When species i behaves
more concentrated than it really is, ?i gt 1 An
ideal solution behave the same as their
concentration ?i 0
equilibrium constant is usually defined in terms
of activity instead of concentration
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What Causes Non-Ideal Behavior
Seawater does not behave as a thermodynamically
ideal solution because of the high concentration
of ionic solutes.
Activity is a function of 1) concentration of
cations and anions. 2) magnitude of ion charge
(i.e. /- 1 /- 2 etc ). 3) stability of
individual ion pairs
The activity coefficient approaches a value of 1
as the ionic strength of the solution is
decreased to zero. Thus, in dilute solutions,
the activity is roughly equal to the
concentration.
The activities of uncharged solutes are not as
greatly affected by increasing ionic strength
because electrostatic interactions are the source
of most of the non-ideal solution behavior of
ions.
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Interactions between ions in seawater causes
Non-Ideal Behavior
Dissolved cations and anions will most likely not
behave like free ions, but will be associated
with other ions in seawater changing their
effective concentration.
Cl-
Na
H
Cl-
Cl-
A-
Na
Na
Cl-
Na
Shielding by Sodium and Chloride ions will
prevent ideal behavior
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Activity Coefficient is a function of Ionic
Strength
m molality
Z ions charge
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How would you calculate the Ionic Strength of
Seawater?
Remember molality moles/kg
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Gases in the Ocean
Nitrogen is required by living organisms, but
most organisms cant utilize nitrogen gas in the
atmosphere or ocean. Therefore nitrogen gas
behaves conservatively. Oxygen is produced during
photosynthesis and used during respiration, so it
behaves non-conservatively. Carbon Dioxide is
produced during respiration and used in
photosynthesis, so it behaves non-conservatively
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Daltons Law of Partial Pressure
Each gas exerts a pressure, called partial
pressure, that is independent of the pressures of
the other gases.
So, ideal gas laws, which are defined in terms of
total pressure, can be used interchangeably for
partial pressure.
Gas equilibrium can be represented as
With thermodynamic equilibrium constant
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Ideal Gas Law
PVnRT
P pressure V volume n number of moles R
gas constant T temperature
Henrys Law
At a constant temperature, the amount of a given
gas dissolved in a given type and volume of
liquid is directly proportional to the partial
pressure of that gas in equilibrium with that
liquid
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Solubility of gas in water is temperature
dependent
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Profiles of O2 and CO2 in the Ocean
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Transfer of gas through the ocean surface is a
diffusive process.
Can be represented with thin-skin model
Air-sea flux
Thin skin layer thickness
Diffusion coefficient
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• Summary
• Because it is difficult to directly measure
  different ion species in seawater, chemical
  oceanographers rely on the concept of equilibrium
  to infer concentrations of various forms of ions
  in seawater.
• Seawater does not behave like an ideal solution
  because of its high ionic strength.
• As a result, most ions behave as if they had a
  lower concentration. This behavior is quantified
  in an ions activity.
• Gases in seawater also are in equilibrium between
  the gaseous and aqueous phase.
• Gases exhibit ideal behavior in seawater. As a
  result, the solubility of a gas can be easily
  predicted and is a function of temperature.
• Oxygen and carbon dioxide can be directly
  utilized in biological processes so they exhibit
  non-conservative behavior.
• Nitrogen gas cannot be used directly, but must be
  fixed, so it exhibits conservative behavior.
• Flux of gases across the air-sea interface can be
  described as a diffusive process and is
  controlled by turbulent mixing.