Seawater Chemistry

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Seawater Chemistry
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Water Salts Seawater

• Salinity the total concentration of dissolved
  inorganic solids (3.3 - 3.7)
• Remember that ions in water
• Decrease heat capacity
• Lower freezing point
• Slow evaporation
• They also increase osmotic pressure, the pressure
  of water trying to squeeze into or out of cells
• These 4 qualities are called colligative
  properties
• The higher the salinity, the more important these
  are

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So what salts are actually in the ocean?

• Most abundant Sodium and Chloride
• Also present are sulfate, magnesium, calcium,
  potassium, and bicarbonate
• All together they make up about 34 ppt or 3.4 of
  seawater
• The rest is hydrogen and oxygen

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Where do salts come from?

• The majority are excess volatiles
• Salts that come from somewhere besides erosion
• Most excess volatiles come from gasses escaping
  the mantle
• Sodium comes from weathering of the earths crust
• Chlorine comes from outgassing
• Calcium is dissolved in water at the mid-ocean
  ridges

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Does salinity change by location?

• Yes and No
• Some locations have saltier water than other
  locations (higher percentage of dissolved solids)
• All locations have the same ratio of salts
• Ex. Chloride always makes up 55.04 of the
  dissolved solids
• This is known as The Principle of Constant
  Proportions

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How do we measure salinity?

• We measure the abundance of chlorine and multiply
  by 1.80655
• This works because chlorine always makes up
  55.04 of seawater solids
• To measure chlorinity we use a salinometer, which
  measures the electrical conductivity of seawater

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Is the ocean getting saltier?

• Chemical Equilibrium The amount of solids being
  added to the ocean is the same as the amount of
  solids being removed
• The salts in the ocean usually stay in the ocean
  from 200 to 100,000,000 years
• This helps keep the salinity of the ocean
  constant
• We call substances that stay relatively constant
  in the ocean conservative constituents
• Those that change over time (ex. Associated with
  biological activity) are called nonconservative
  constituents

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What about dissolved gases?

• Three main gases
• Nitrogen conservative component
• Oxygen nonconservative component
• Carbon Dioxide nonconservative component
• Marine organisms need these gases to survive,
  because they cant use the oxygen found in the
  water molecule
• Key point gases dissolved best in cold water

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Nitrogen

• 48 of dissolved gas
• Cannot be used by organisms directly
• The nitrogen used by organisms is mainly recycled
  among themselves in the form of nitrates
• This explains why nitrogen gas is a conservative
  constituent

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Oxygen

• 36 of dissolved gas
• Sources
• Photosynthetic activity of organisms
• Diffusion of gases from the atmosphere

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Carbon Dioxide

• About 15 of dissolved gases
• Forms carbonic acid when dissolved, so the oceans
  hold 60 times as much carbon dioxide as the
  atmosphere

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Is the ocean acidic or basic?

• Usually, it is slightly basic
• CO2 H2O ? H2CO3
• H2CO3 ? HCO3- H
• HCO3- H ? CO32- 2H