The The

Chemistry Chemistry

of Seawaterof Seawater

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

The pH of SeawaterThe pH of Seawater

Dissociation of water molecules (cation, H+, Dissociation of water molecules (cation, H+,

and anion, OH-). and anion, OH-).

Imbalance of H+ and OH-: acidic and basic Imbalance of H+ and OH-: acidic and basic

solutionssolutions

pH scale pH scale

pH of seawater 7.5 to 8.5.pH of seawater 7.5 to 8.5.

Buffering action of carbon dioxideBuffering action of carbon dioxide

Fig. 6.1Fig. 6.1

SaltsSalts Units of concentrationUnits of concentration

– By weight (g/kg)By weight (g/kg)

– By volume (‰)By volume (‰)

– By molar concentrationBy molar concentration

Ocean salinitiesOcean salinities– Spatial variations – latitude, depth and proximity to Spatial variations – latitude, depth and proximity to

riversrivers

– Processes that affect salinity: evaporation, Processes that affect salinity: evaporation, precipitation, runoff, freezing, and thawingprecipitation, runoff, freezing, and thawing

Dissolved saltsDissolved salts– Major constituents and trace elementsMajor constituents and trace elements

– Conservative vs. nonconservative constituents Conservative vs. nonconservative constituents (local chemical and biological processes(local chemical and biological processes

Fig. 6.2Fig. 6.2

Seawater- Salinity = 3.3 to 3.7 % by weight- sourced by “runoff” from the land &

excess volatiles (CO2, Cl, S, H2, Fl & N) from interior

Table 6.2Table 6.2

SaltsSalts Sources of salt:Sources of salt:

– Positive ions: weathering and erosionPositive ions: weathering and erosion

– Negative ions: gases from volcanic eruptionsNegative ions: gases from volcanic eruptions

Regulating the salt balance:Regulating the salt balance:– Input (rivers) and output (sediments and Input (rivers) and output (sediments and

evaporites)evaporites)

Residence time:Residence time:– Total amount divided by rate of input (or output)Total amount divided by rate of input (or output)

Constant proportions:Constant proportions:– Well-mixed solution Well-mixed solution

Determining salinity:Determining salinity:– Conductance , evaporation, density and titration.Conductance , evaporation, density and titration.

Forchhammer’s Forchhammer’s PrinciplePrinciple

Principle of Constant Principle of Constant ProportionsProportions

– anywhere in the world total salinity anywhere in the world total salinity may varymay vary

WHY???WHY???

– . . . but relative proportions don’t! . . . but relative proportions don’t! WHY???WHY???

Determining SalinityDetermining 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 = 1.80655 X chlorinity in %0

• Conductivity of seawater using a salinometer

commonly used today.

Fig. 6.5Fig. 6.5

GasesGases Distribution with depthDistribution with depth

– Photosynthesis removes COPhotosynthesis removes CO22 and produces O and produces O22 at the at the surfacesurface

– Respiration produces CORespiration produces CO22 and removes O and removes O22 at all depths at all depths– Compensation depth Compensation depth

The carbon dioxide cycleThe carbon dioxide cycle– Ocean uptake from atmosphereOcean uptake from atmosphere

Depends on: pH, temperature, salinity, chemistryDepends on: pH, temperature, salinity, chemistry– Biological pumpBiological pump

The oxygen balanceThe oxygen balance– Photosynthesis in ocean (net source)Photosynthesis in ocean (net source)– Weathering and oxidation on land (net loss)Weathering and oxidation on land (net loss)

Measuring the gasesMeasuring the gases– Specialized probes for dissolved oxygenSpecialized probes for dissolved oxygen– Measuring pH to deduce carbon dioxide concentration Measuring pH to deduce carbon dioxide concentration

Fig. 6.7Fig. 6.7

Other SubstancesOther Substances NutrientsNutrients

– Ions required for plant growthIons required for plant growth

– Redfield ratiosRedfield ratios

– Nutrient cyclesNutrient cycles

OrganicsOrganics– Wide variety of substancesWide variety of substances

Some are oxidized or broken down into Some are oxidized or broken down into

smaller moleculessmaller molecules Some accumulate in the sediment to form Some accumulate in the sediment to form

oil and gas deposits oil and gas deposits

Practical Considerations: Practical Considerations: Salt and WaterSalt and Water

Chemical resourcesChemical resources– Table saltTable salt– Magnesium and bromineMagnesium and bromine– Uranium and gold (too expensive to Uranium and gold (too expensive to

extract)extract) Desalination – fresh waterDesalination – fresh water

– EvaporationEvaporation– ElectrodialysisElectrodialysis– Reverse osmosisReverse osmosis

Fig. 6.9Fig. 6.9

Fig. 6.10Fig. 6.10

Fig. 6.11Fig. 6.11

Fig. 6.12Fig. 6.12

The EndThe End