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8/10/2019 3.Gas Terlarut
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Dissolved Gases in theOceansChemical Oceanography
By
Syarifah Hikmah JS
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OUTLINE
1. Composition of the Atmosphere2. Dissolution of Gases in Seawater
3. Air-Sea Exchange
4. Dissolved Oxygen
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Composition of the Atmosphere
The Atmosphere is made of the major dissolved gases in
seawater
The gases will vary from place to place because of different
sources and sinks
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Dissolved gases in seawater
4
Gases Molecule Percent in
atmosphere
Equilibrium concentration
in seawater (mg/kg)
N2 78% 12.5
O2 21% 7
Ar 1% 0.4CO2 0.03% 90*
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Dissolved gases in seawater
The dissolved gases can be grouped into :
Low reactivity gases or low chemical reactivity: N2, O2, Ar,
and other noble gases
high reactivity or high solubility : SO2, NO2, and NH3 The major reactive gases involve in the biological process and
the geochemical cycle such as O2, CO2 and N2 while
unreactive gases have not determined yet their role in the
geochemical process
5
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Air-Sea Exchange
The gas transferred by molecular diffusion across the
interface layers called stagnant boundary layer
This layer is only few tens of m thick. The thickness of the
boundary is not constants : it decreases with increased
turbulence in the water
The layer is almost completely transparent to solar radiation
Hydrophobic organic molecules tend to accumulate at the
interface itself, the top of layer.
Just below it, uppermost few mm of water column, live
neuston include phytoplankton
6
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Partial Pressure
PG
PG
Interface layer
Atmosphere
Liquid
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Source and Sink of Gases
The most of the gases inthe seawater originate
from three sources :
The earth atmosphere
Volcano activity
Chemical processes
occurring in the sea
8
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Dissolution of Gases in Seawater
9
PG = KG x CG
Where : PG = ng RT
V
The concentration of a
gas in solution is relatedto the partial pressure
by Henrys Law :
PG = The partial pressure
KG = Henrys Law Constanta
CG = Concentration of dissolved gases
Ng = Mole of gases
V = Volume of gases
R = Gases Constanta (8.31 joule/mol/K
T = Absolute Temperature
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Henrys Law
The solubility of a gas in a liquid isdirectly proportional to the partial
pressure the gas above the liquid.
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gas in air
> gas in water
: diffusion gases
from the air to the water
gas in air < gas in water : Gases escape
from the water to the air
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The solubility of a gas
The solubility of a gas in a
liquid depends on
Temperature, the solubility all
gases is greater in cold than
warm water
The partial pressure of the gasover the liquid, the solubility of
gases in water is increased by
pressure
The salinity, the solubility varies
inversely with salinity
The nature of the solvent
The nature of the gas
Gas exchange is more rapid
during storms than in calm
water 11
Gases 00C 240C
Solubility
(cm3/l)
Diffusion
Coeff
(x10-5
cm2/sec)
Solubility
(cm3/l)
Diffusion
Coeff
(x10-5
cm2/sec)
O2 38.7 1.2 23.7 2.3CO2 1437 1.0 666 1.9
N2 18.3 1.1 11.8 2.1
N2O 1071 1.0 476 2.0
He 7.8 2.0 7.4 4.0
Kr 85.6 0.7 46.2 1.4
Ar 42.1 0.8 26.0 1.5
Rn 406 0.7 186 1.4
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Gas Transfer Velocity
The total of gas transfer velocity is confirmed by using :R(T)W = Rw + Ra
where : R(T)W : The total of gas transfer velocity
Ra : A Gas transfer velocity in the air-water
Rw : A Gas transfer velocity in the water
If Ra > Rw : gases which have a low Henry Constanta or high
solubility in the water, ex : SO2, NH3, and HCl
If Ra < Rw : gases which have a high Henry Constanta or low
solubility in the water, ex : O2, CO2, CO, and CH4
12
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Henry Law Constanta (K)
Gases K (mole liter -1 atm-1)
O2 1.28 x 10-3
CO2 3.38 x 10-2
N2 6.48 x 10-4
CH4 1.34 x 10-3
H2 7.90 x 10-4
13
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Dissolved Oxygen
Dissolved oxygen in the surface layer of seawater reach 4.5-9mg/l
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Temperature Cl- (mg/l)
0 5000 1000The solubility of Oxygen (mg/l)
26 8.2 7.8 7.4
27 8.1 7.7 7.3
28 7.9 7.5 7.1
29 7.8 7.4 7.0
30 7.6 7.3 6.9
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Distribution of Oxygen in the ocean
Vertical distribution
15
Oxygen Minima Zones
200-800 m
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Vertical distribution of O2
Near equilibration of atmosphere oxygen in the surface mixed
layer
Biological production in subsurface waters due to
photosynthesis
Biological use of O2 in respiration in all waters and oxidation
of plant material in intermediate waters
In deep water the O2 increase due to sinking of cold water
rich O2
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AOU (Apparent Oxygen Utilization) is the total of oxygen
used for organic demineralization or respiration
NAEC (Normal Atmospheric Equilibrium Concentration) is
Concentration of oxygen in seawater is in equilibrium
with its concentration in the atmosphere
NAEC is affected by temperature, salinity and partial
pressure of O2
AOU = NAEC [O2] in situ
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Horizontal distribution
The deep water masses formed at the surface in high
latitude are richest in dissolve oxygen
As the water masses sink and move away from their
source, the oxygen is progressively used by marine
organisms in respiration
The horizontal distribution of O2 is affected by
temperature, photosynthesis activity, and respiration as
well as water masses circulation
18
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Horizontal distribution
of dissolved oxygen
concentration (ml l1)
near the bottom in the
western subtropical
region of the North
Pacific.
19
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METHODS OF MEASUREMENT
1. Direct measurement in solution
(O2). Winkler Method for Oxygen
A. MnSO4 + NaOH Fixing Agent
B. Add KI, HCl and titrate with S2O32-
2. Gas Chromatography (O2,N2,Ar,CO,CH4)
3. Mass Spectrometry (low or non-reactive gases)
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Anoxic Environments in Seawater
In regions of high productivity, where amount of sinking
organic matter is greater than available oxygen supply can
cope with, so the lower water column may become
completely anoxic
In Black Sea, density stratification of the water column and
topographic barrier to mixing limit the supply of oxygen to the
bottom waters by advection
The anoxic condition can be also locally produced in coastal
water where human activities increase the supply nutrientand organic matter
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Where the water is anoxic, the oxidizing agents must be used
by bacteria to consume organic matter.
Sulphate is a major dissolved constituent in seawater
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H2S (Hydrogen Sulphide)
The source of H2S is decomposition of organic matter in
anaerobic condition by Desulphovibrio desulphuricum
This bacteria use O2 from sulphate molecule to their
metabolisms.
The process of oxidation-reduction by this bacteria is
triggered by presence of Fe ion as a catalisator
23
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24
Millero, F.J. 2002. Chemical
Oceanography. CRC Press. Boca Raton
The Open University. Ocean Chemistry
and Deep-Sea Sediments
Sanusi, H.S. dan S. Putranto. 2009. Kimia
Laut dan Pencemaran. Dept Ilmu dan
Teknologi Kelautan. IPB
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Covering the
Material
John Garratt, University Chemistry Education 2(1), 29-33 (1998)
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A New Lesson Plan Is
Needed
John Garratt, University Chemistry Education 2(1), 29-33 (1998)
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Active Learning
John Garratt, University Chemistry Education 2(1), 29-33 (1998)
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Mission Accomplished?
John Garratt, University Chemistry Education 2(1), 29-33 (1998)
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Lesson Learned!
John Garratt, University Chemistry Education 2(1), 29-33 (1998)
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