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The Chemistry of Ocean Salinity and Gases: Understanding Water's Dissolving Power

This article explores the chemistry of ocean salinity and the role of various gases dissolved in seawater. It discusses the polar nature of water (H2O), which allows it to dissolve salts like sodium (Na+) and chloride (Cl-) through the reduction of ionic bonding. Learn about average ocean salinity, factors influencing it, the main constituents of seawater, and how gases like oxygen (O2) and carbon dioxide (CO2) vary with depth. Additionally, the article examines the principle of constant proportions and the importance of measuring salinity for understanding ocean chemistry.

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The Chemistry of Ocean Salinity and Gases: Understanding Water's Dissolving Power

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  1. Ocean Water Salts and Gases

  2. The Dissolving Power of Water • Dissolving power results from polar nature of H2O molecule • Na+ and Cl- are held together by ionic bonds (attraction of opposite charges) • H2O reduces the ionic bonding and pulls Na+ and Cl- apart

  3. Salinity in Oceans • Salinity • Total amount of dissolved solids expressed in grams in 1 kg of water • Average salinity in oceans = 35 ‰ • 35 parts per thousand (ppt) • 35 g/kg • Salinity variations • Due to differences in local rates of evaporation and precipitation (water budget)

  4. Constituents of Sea Water • Most abundant seawater elements are sodium (Na+) + chloride (Cl-) • Major constituents: SO42-,Mg2+, Ca2+, K+, and HCO3- • Minor and trace elements also present

  5. Seawater versus River Water

  6. Salts in the Ocean Why is the ocean salty? • Salts come from: • Rocks = cations • Gases from mantle = anions • Why is the ocean not getting saltier? • Salts going in = salts going out

  7. Salts Going Out • Sea Sra • Sea Spray • Evaporites • Biological • Fecal pellets • Shell formation • Adsorption • Mid-ocean ridge magma

  8. Principle of Constant Proportions • The amount of salt varies, but the relative proportions of ions are constant • Because of this principle, it is necessary to test for 1 salt ion (usually Cl) to determine total amount of salt present

  9. Determining Salinity 1. Calculating Salinity • Salinity=1.8065 x chlorinity (‰) 2. Salinometers • Salinity determined by the electrical conductivity produced by dissolved salts

  10. Factors Affecting Salinity • Precipitation • Evaporation • River runoff • Freezing

  11. Residence Times • Average length in time that a substance remains in solution in seawater • Ions with longer residence times are most abundant in ocean

  12. Effects of Salinity • Freezing point depression; boiling point elevation • Disrupts Hydrogen bonding (freezing point of seawater =-2°C) 2. Changes density • Pure water density = 1 kg/l • Seawater density = 1.024 kg/l 3. Changes vapor pressure • ↑ ‰, ↑ P

  13. Desalination • Fresh H20 from Salt H20 • Methods • Distillation • Electrodialysis • Reverse Osmosis

  14. Gases

  15. Gases in Seawater • Major Gases: N2, O2 and CO2 • O2 and CO2 important for life processes

  16. CO2 and O2 with depth • Surface (0m) • Oxygen (O2) abundant gas • Photosynthesis dominant • ~ 200m - ~1,000m • Oxygen (O2) ↓ • Carbon Dioxide (CO2) ↑ • Respiration dominant • Organic matter decay • 1,000m – 4,000m • O2↑ slightly • Oxygenated bottom waters • Cold salty oxygenated H2O sinks in polar region transported to depth • CO2continues to ↑ • Organic matter decay

  17. GasesSummary • Photosynthesis • CO2 consumed; O2 produced • Therefore, O2 more abundant than CO2 in photic zone • Respiration • O2 consumed; CO2 produced • Organic matter (OM) decay • O2 consumed; CO2 produced • Respiration and OM decay • = ↑ CO2 below photic zone

  18. The pH Scale • An acid has excess [H+] (Hydrogen ions) • A base has excess [OH-] (Hydroxyl ions) • CO2 controls acidity • ↑ CO2 = ↑ acidity • ↓ CO2 = ↓ acidity pH of seawater = 7.5-8.5; Average = 8.0

  19. Gases in SeawaterCarbonate Buffering System • Dissolved CO2 in water acts as a buffer • Prevents large shifts in pH • Seawater too basic: • H2CO3 HCO3- + H+ (pH drops) • Seawater too acidic: • HCO3- + H+ H2CO3 (pH rises)

  20. Gas Cycling • Photosynthesis produce O2 • Respiration & decomposition of O.M. use O2 • O.M. buried doesn’t consume all O2 • Rocks consume O2 • O2 Cycling • CO2 Cycling • Organisms use CO2 for tissue • Decay of O.M. releases CO2 back to ocean

  21. Nutrients and Organics • Required for plant growth • Cycled by plant growth and decay • Organics used by organism or buried

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