Ch7 Ocean Chemistry
Mr. Sternschein
Marine Science
Water, as a Solvent
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Water is a polar molecule (chapter 6)
Water will attract other polar substances or ions.
What is an ion?
Solutions are composed of:
 Solvent- the more abundant substance, usually a liquid
 Solute- the less abundant substance, often a solid
 Salt, NaCl is held together by ionic bonds (electrostatic
forces)
 Salt dissociates in water to Na+ and Cl Solutes can diffuse through water until homogenous.
 A saturated solution can dissolve no more solute.
Components of Seawater
 Salinity is a measure of dissolved inorganic solids in
water.
 Seawater is 96.5% water, 3.5% dissolved substances.
 Sodium and chlorine ions, from salt, are the major
constituents.
 Colligative properties:
Freezing point depression
Slower evaporation
Decreased heat capacity
Higher osmotic pressure
Measuring Salinity
 Salinity is measured in parts per 1000 (g./kg.)
 Dissolved substances = 3.5% or 35 ppt.
 The ratio of dissolved substances in the ocean is
constant. (Forchhammer’s principle)
 Chlorinity- a method of measuring salinity, uses this
principle.
 Salinometers measure salinity using conductivity with
great accuracy.
Sources of minerals and elements
 Dissolved solids originated from
the erosion of crustal rocks
 Rivers running off continental
crust
 Wave action on shorelines
 Excess volatiles: components of
ocean water that are not
accounted for by the
weathering of surface rocks.
 Outgassing of volcanoes
 Hydrothermal vents
 Decay of once-living organisms
The ocean is in a steady state
 The ocean is in chemical equilibrium.
 The proportion and amounts of dissolved solids
remain constant.
 Ions are being added to and removed from the
ocean at the same rate.
 Residence time is the average length of time an
element spends in the ocean.
Residence Time
Constituent time
 Conservative constituents of seawater are those
constituents that occur in constant proportions.
 Conservative elements have long residence times and
are the most abundant dissolved material in the
ocean.
 Non-conservative constituents have short residence
times, and are usually associated with seasonal,
biological or short geological cycles.
Dissolved gases
 Gases from the atmosphere dissolve at the ocean’s
surface.
 Nitrogen (conservative)
 Oxygen (non-conservative)
 Carbon Dioxide (non-conservative)
 Different proportions than in the atmosphere due to
rate of solubility in the ocean
Dissolved gases continued
 Unlike solids, gases dissolve best in cold water.
 Cold polar water will have more dissolved gases than
the warm tropics
 Dissolved Oxygen (DO) in the tropics may be so low
that animals cannot survive
 Tropical waters may be stressed more due to
pollutants that consume oxygen (examples: sewage
and agricultural runoff)
Temperature and Dissolution
Nitrogen & Oxygen Gas
 Nitrogen in the most abundant gas in seawater (48%)
 Upper layers are usually saturated
 Nitrogen is essential for:
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 living things to make amino acids
 amino acids then help to build proteins
 plant life
Cannot be taken directly from the atmosphere
Must first be bound, or fixed, into usable forms (by bacteria)
Dissolved oxygen is critical to marine life. (36%)
Vital for organisms that extract oxygen with their gills
Sources of oxygen:
 photosynthetic activity
 diffusion from the atmosphere
The ocean is a vast carbon reservoir
 Carbon dioxide (CO2) is low because of demand by
photosynthetic organisms (15%)
 Concentration is 60x more in ocean than the
atmosphere
 Can combine with calcium to make calcium carbonate
(CaCO3)
 CaCO3 is used to build shells and skeletons.
 Organisms die, sink and become limestone rock
Gas Concentrations Vary with Depth
 Oxygen levels are more abundant near the surface.
why?
 Photosynthesis occurs there
 Decreases with depth as marine organisms consume
it.
 CO2 levels are low near the surface (again,
photosynthesis)
 Increase with depth as organisms produce it.
Carbon dioxide vs. Oxygen
Acid and base refresher
 When water dissociates, it breaks into OH- and H3O+
 Solutions that have excess H3O+ (hydronium ions) are
acids.
 Solutions that have excess OH- (hydroxide ions) are bases.
 Solutions that have equal amounts are neutral (water)
 pH indicates whether a solution is acidic or basic
 Acids have pH less than 7
 Bases have pH less greater than 7
 Basic solutions are also called alkaline
The ocean is slowly becoming more
acidic
 CO2 and water form Carbonic acid
 The more CO2 in the atmosphere, the more there is in
the ocean.
 This is causing the oceans pH to drop and become
more acidic.
 Some marine life cannot adapt and will perish.