The Chemistry of Seawater An Introduction to the World’s Oceans Sverdrup et al. - Chapter Six - 8th Ed.

The pH of Seawater n Dissociation of water molecules (cation, H+, and anion, OH-). n Imbalance of H+ and OH-: acidic and basic solutions n pH scale n pH of seawater 7.5 to 8.5. n Buffering action of carbon dioxide

Fig. 6.1

Salts n Units of concentration –By weight (g/kg) –By volume (‰) –By molar concentration n Ocean salinities –Spatial variations – latitude, depth and proximity to rivers –Processes that affect salinity: evaporation, precipitation, runoff, freezing, and thawing n Dissolved salts –Major constituents and trace elements –Conservative vs. nonconservative constituents (local chemical and biological processes

Fig. 6.2

Seawater - Salinity = 3.3 to 3.7 % by weight - sourced by “runoff” from the land & excess volatiles (CO 2, Cl, S, H 2, Fl & N) from interior

Table 6.2

Salts n Sources of salt: –Positive ions: weathering and erosion –Negative ions: gases from volcanic eruptions n Regulating the salt balance: –Input (rivers) and output (sediments and evaporites) n Residence time: –Total amount divided by rate of input (or output) n Constant proportions: –Well-mixed solution n Determining salinity: –Conductance, evaporation, density and titration.

Forchhammer’s Principle n Principle of Constant Proportions –anywhere in the world total salinity may vary n WHY??? –... but relative proportions don’t! n WHY???

Determining Salinity Chemical Analysis problematic and slow Refractometer measures salinity as a function of light refraction because of density. Chlorinity (Cl,Br & Fl) takes advantage of Forchhammer’s Principle Salinity in % 0 = X chlorinity in % 0 Conductivity of seawater using a salinometer commonly used today.

Fig. 6.5

Gases n Distribution with depth –Photosynthesis removes CO 2 and produces O 2 at the surface –Respiration produces CO 2 and removes O 2 at all depths –Compensation depth n The carbon dioxide cycle –Ocean uptake from atmosphere n Depends on: pH, temperature, salinity, chemistry –Biological pump n The oxygen balance –Photosynthesis in ocean (net source) –Weathering and oxidation on land (net loss) n Measuring the gases –Specialized probes for dissolved oxygen –Measuring pH to deduce carbon dioxide concentration

Fig. 6.7

Other Substances n Nutrients –Ions required for plant growth –Redfield ratios –Nutrient cycles n Organics –Wide variety of substances n Some are oxidized or broken down into smaller molecules n Some accumulate in the sediment to form oil and gas deposits

Practical Considerations: Salt and Water n Chemical resources –Table salt –Magnesium and bromine –Uranium and gold (too expensive to extract) n Desalination – fresh water –Evaporation –Electrodialysis –Reverse osmosis

Fig. 6.9

Fig. 6.10

Fig. 6.11

Fig. 6.12

The End