11/12/2013
The Principle of Constant Proportion
Seawater Chemistry
By: Eleonora and Kristen
Determining Salinity
Old analysis: Evaporate a known weight of seawater and weigh
the residue.
Modern analysis: Determine the Chlorinity.
Chlorinity is a measure of the total weight of chlorine, bromine
and iodine ions in seawater.
-1865, chemist Georg Forchhammer noted that the ratio of
major salts in samples of seawater from many locations
was constant.
-This constant is known as “Forchhammer Principle”.
-He was the first to observe that seawater contains fewer
silica and calcium ions than concentrated river water.
-He realized that removal of these compounds by marine
animals and plants might account for part of the
difference.
..Determine Chlorinity
Old : Nansen Bottle; it’s open at both ends, and when it
gets to the known depth it is triggered to close by a
brass weight (messenger) sent sliding down the line.
Bottles are then hauled to the surface and their
contents analyzed.
PROPORTION OF CHLORINITY TO SALINITY IS CONSTANT
Salinity in % = 1.80655 x Chlorinity in %
New : Salinometer; it is an electronic device
which measures the electrical conductivity of
seawater. Conductivity varies with the
concentration and mobility of ions present,
and with water temperature.
- Accurate to 0.001%
Refractometer; a compact optical device that
compares the degree to which light is bent by
a sample of seawater to the degree of bending
for water of known salinity.
When using a refractometer, the observer places
a drop of water on a glass plate on top of the
device and then looks into the refractometer
to determine the position of a line against a
scale.
- Not as accurate but can quickly measure
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Chemical Equilibrium and
Residence Times
Chemical Equilibrium: the proportion and
amounts of dissolved salts per unit volume of
ocean are nearly constant. Evidently, whatever
goes in must come out somewhere else.
1950, geologists developed the concept of a
steady state ocean which means ions are
added to the ocean at the same rate as they
are being removed. This explains why the
ocean isn’t getting saltier.
1952, T. F. W. Barth, devised the concept of
residence time. Which states the average
length of time an element spends in the
ocean. Residence time for an element may be
calculated by this equation:
Amount of element in the ocean
Residence time=
Rate at which the element is
added to (or removed from)
the ocean
Mixing Time
• When minerals that make up the ocean stay in
the water longer then the ocean’s Mixing Time,
they will become evenly distributed through the
ocean.
- Due to activity of currents, the mixing time of the
ocean is thought to be on the order of 1,000
years.
- The relatively long residence times of seawater’s
major constituents assure thorough mixing.
Conservative and Non-conservative
Constituent
• Conservative Constituents: an element that occurs in
constant proportion in the seawater. It has a long
residence time because they are less active and don’t
get used up.
– Chlorine, Sodium, and Magnesium.
• Non-conservative Constituent: an element whose
proportion in seawater varies with time and place,
depending on biological demand and chemical activity.
It has a short residence time because they are active
and get used up in reactions or in biological uses.
– Iron, Aluminum, Silicon, Trace Nutrients, Dissolved Oxygen,
and Carbon Dioxide.
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Dissolved Gases
• Gases in the air readily dissolve in seawater at the
ocean’s surface. Plants and animals need these
gases to survive.
– Marine animals don’t have the ability to break down
water molecules
– Marine plants can’t manufacture enough carbon
dioxide to support its own metabolism
Major gases found in the seawater
-NITROGEN
-OXYGEN
• Gases dissolve most readily in cold water.
– Nitrogen, conservative
– Oxygen and Carbon Dioxide, non-conservative
Nitrogen
-48% of the dissolved gas in seawater.
-In the upper layers of the ocean
-living organism require it to build proteins and
other important biochemicals
-CARBON DIOXIDE
Oxygen
-36% of the dissolved gas in seawater
-An average of 6 milligrams of oxygen is dissolved in
each liter of seawater.
-vital resource of animals that extract oxygen with
gills
-Photosynthetic plants are a primary source of the
ocean’s dissolved oxygen.
-Most of the available oxygen lies near the ocean’s
surface, but levels will rise again at the bottom
Carbon Dioxide
-15% of the dissolved gas in seawater
-Quickly used by marine plants
-Very soluble in water
-Carbon Dioxide combines chemically with water to
form a weak acid
-Some dissolved Carbon Dioxide forms carbonate
ions, which are locked into sediments, minerals,
and the shells and skeletons of living organisms
- [CO2] rises with depth
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ACID-BASE Balance
Water can separate to form:
-Hydrogen ions (H⁺)
-Hydroxide ions (OH⁻)
Equal concentrations of ions: Pure water
Imbalanced concentration of ions:
-Acid solution, excess of hydrogen ions
-Basic solution, excess of hydroxide ions
(alkaline solution)
The acidity or alkalinity of a solution is
measured in terms of the pH Scale.
-It measures the concentration of
hydrogen ions in a solution
The scale is logarithmic, which means that a
change of one pH unit represents a 10-fold
change in hydrogen ion concentration
Seawaters pH level
• Seawater is slightly alkaline, and its average
pH level is: 7.8.
• Seawater acts as a buffering system.
– If equilibrium is pushed to the left removing H⁺ it
would be more basic or less acidic. (at top level
where plants use up CO 2)
– If pH equilibrium pushed right it would add CO 2
making the water more acidic and less basic. (at
bottom where C02 isn’t used up)
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Sources
• http://geoclasses.tamu.edu/ocean/wormuthwork/che
mical/niskentripping.gif
• http://raysaquarium.com/images/refractometer.gif
• http://upload.wikimedia.org/wikipedia/commons/thu
mb/9/96/Oxygen_Cycle.jpg/350px-Oxygen_Cycle.jpg
• http://pubs.usgs.gov/fs/fs15099/figure2_big.gif
• http://www.skepticalscience.com/images/carbon_cycl
e_NASA.jpg
• http://staff.jccc.net/pdecell/chemistry/phscale.gif
• http://oceanexplorer.noaa.gov/okeanos/explorations/1
0index/background/edu/media/carbonate_buffer.jpg
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