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

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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

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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

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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

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Dissolution of Gases in Seawater

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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

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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

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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

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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

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Horizontal distribution

of dissolved oxygen

concentration (ml l1)

near the bottom in the

western subtropical

region of the North

Pacific.

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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

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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



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Active Learning



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Mission Accomplished?



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Lesson Learned!



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TerimakasiH

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