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EPSS 15 Spring 2017
Introduction to Oceanography
Laboratory #5
Physical and Chemical Properties of Seawater
Water (H2O) is a unique molecule because it is
highly polar, and as a result forms Hydrogen bonds
ŸPolarity means the
molecule has a positive
charge on one end and a
negative on the other. This
makes it a very good
solvent!
ŸHydrogen bonds are
intermolecular bonds
between H & O that give
water both a high melting
(0°C) and high boiling
(100°C) point!
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H-BONDS AT WORK:
•  Water has high latent heats of melting and boiling, (i.e.,
phase changes require a lot of energy).
•  Water is less dense as a solid (ice). If it weren’t, then at the
high pressures and low temperatures of the deep ocean it
would freeze solid!
•  Water has a high heat capacity, too
H-BONDS AT WORK:
•  Water becomes denser as it gets colder, until ~4°C,
after which it becomes less dense
•  Ice is less dense than liquid water. If it weren’t, then
at the high pressures and low temperatures of the
deep ocean it would freeze solid!
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How is SEAWATER different from fresh water?
•  Seawater has a high salinity, or dissolved
salt content. Around 3.47% by weight.
ŸThe Principle of Constant
Proportions: Although the
concentration may vary, the relative
proportions of these salts remain very
consistent worldwide!
ŸThus, if you know one ionic
abundance (eg. Cl-), you can
calculate the total salinity:
MAJOR IONS
% by
wt
Chloride (Cl-)
55.07
Sodium (Na+)
30.62
Sulfate
(SO42-)
7.72
Magnesium
(Mg2+)
3.68
Calcium (Ca2+)
1.17
Potassium (K+)
1.10
Salinity (‰) = 1.80655 x Chlorinity (‰)
where ‰ is parts per thousand
MEASURING SALINITY
•  Salinity is usually measured in parts per thousand
(o/oo), or grams of salt per kilograms of water. It
can also be in weight percent (%).
•  In the past, you would evaporate a carefully
weighed amount and seawater and weigh the salts
that precipitated from it
•  Now we use conductivity meters. This is because
the more ions that there are dissolved in a
solution, the more electrically conductive it is
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SOME FACTORS CAN CHANGE SALINITY
Evaporation & formation of ice
increase salinity
by removing pure water.
Precipitation and ice melting
(also groundwater flow to ocean & river runoff)
decrease salinity
by adding water.
HOW DO
DISSOLVED SALTS AFFECT WATER?
With increasing salinity…
•  Density of water increases!
•  Boiling point increases!
•  Freezing point decreases!
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WHERE DO THE DISSOLVED SALTS COME FROM?
AND WHERE DO THEY GO?
•  Sources include rivers, volcanic gases, &
hydrothermal vents at mid-ocean ridges.
•  Sinks include sedimentation and mid-ocean ridge
fluid circulation.
RESIDENCE TIME
•  The residence time (τR) is the average
amount of time an atom (or ion) of an
element remains dissolved in the ocean
–  You can also calculate residence times for other
systems, e.g. the residence time of CO2 in the
atmosphere
•  We assume the ocean is in a steady state
with its sources and sinks of different
elements or that the average amount of the
element does not change with time
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CALCULATING RESIDENCE TIME
τR =
total amount of the element dissolved in the ocean
the flux of the element in or out of the ocean
•  If we know the concentration of an element in the
ocean we can calculate the total amount of the
element (in grams) by multiplying by the volume
of the ocean (1.6 * 1021 L)
•  The main source of elements added to the ocean
are rivers so we use the river flux which is the
amount of the element added by a river each year
in g/yr
•  Residence time always has units of time!
ACIDITY AND BASICITY
•  Pure water has equal amounts of H+ and OHand is therefore neutral.
•  If you add an acid, it releases more H+
•  If you add an alkaline (a base), it releases more
OH-, or takes up H+
•  The more H+, the more acidic a solution; the
more OH-, the more alkaline or basic it is.
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PH SCALE
pH=-log10[H+]
•  The pH is the negative logarithm
of the hydrogen ion concentration,
ranging from 0 to 14, with water at
7 (neutral).
•  As [H+] increases, pH drops.
Thus, acids have lower pHs.
•  As [H+] decreases, pH goes up.
Bases have higher pHs.
OCEAN PH
ThepHoftheOCEANisfairlystableat7.5-8.4
(slightlyalkaline)
CO2+H2OçèH2CO3çèH++HCO3-çè2H++CO32-
[carbon[water][carbonic[hydrogen[bicarbonate[hydrogen[carbonate
dioxide]acid]ion]ion]ion]ion]
•  Thebicarbonateionsactasabuffer,keeping
seawaterfrombecomingtooacidicorbasic.
•  Ifyouaddmoreofanyreactant,thereacKon
willmoveawayfromthatreactant,thus
maintainingthepH!
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