3.21.17: Seawater Chemistry
Tuesday, March 21, 2017 10:51 AM
I. Seawater as a Solution
• Solvent: H 02
• Solutes: dissolved solids, liquids, or gases
○ NaCL (Sodium Chloride) = main
A. Dissolution
• Solid NaCL crystal
+ -
○ Attraction between Na and Cl
○ Ionic Bonds
• Dissolving NaCL
○ H 02reduces Na-Cl attraction by 80x
○ Na and Cl ions attracted to polar H 0 2
○ Dissolved: 'free' ions surrounded by H 02
○ H 02= "universal solvent" (can dissolve many solids
B. Some Properties of Solutions
• Molecular Diffusion: Dissolved substances move from areas of high concentration to areas of low
concentration, until homogenous
• Turbulent diffusion: diffusion may be aided by mixing of water (e.g. stirring)
• Advection: dissolved substances being carried by circulation
• Saturation: when on more of a given solute will dissolve
• Supersaturated: evaporation increases the concentration further (saturate a solute, then evaporate
that solution so that solids will precipitate)
II. Salinity
Total dissolved solids ~35 ppt (3.5%) by mass. (gases are treated separately)
• Most dissolved solids occur in seawater as ions
• Major ions (ppt): 7 ions account for 98% of salinity
Cl- 19
Na + 10.6
2-
SO 4 2.4
○ Mg 2+ 1.3
Ca2+ 0.4 (some elements are only
+ found as covalent
K 0.4
molecules)
HCO -3 0.1
• Minor Elements (ppm):
-, 2+ -
○ Br Sr , B(OH) , Si3OH) , F 4
• Trace Elements (ppb or less): every other non-gaseous elements on periodic table
A. Principle of Constant Proportions
• As salinity increases or decreases, the concentration of most major and minor elements increase or
decrease with constant rations to each other
• Such elements are called conservative, and only vary with depth as much as salinity does (i.e. very
little)
B. Non-conservative elements
• Nutrients and most trace elements ma change independently of salinity, usually due to biological
uptake (e.g. C, Si, P, N) uptake (e.g. C, Si, P, N)
• Such elements are often depleted in surface waters
C. Measuring Salinity
• Indirect (electrical) method: electrical conductivity increases with salinity
○ Measured with a salinometer
○ Expressed as 'practical salinity units' (equivalent to ppt)
D. Sampling Seawater
• Niskin Bottle
• CTD (
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