- Physical properties
of seawater
- pH of seawater is slightly above 8.0
- pH of seawater increases with phytoplankton production
- pH of lake water is mostly slightly below 7
- Six ions account for 99% of dissolved components in seawater: chloride (Cl-), sodium (Na+), sulfate (SO42-), magnesium (Mg2+), calcium (Ca2+), potassium (K+)
- Biologically important elements such as oxygen, nitrogen, iron occur in variable concentrations depending on utilization by organisms (e.g. nitrate NO3- may range from 0 to 60 mM*
- * Remark on units: 1 mM = 1 mmol l-1 = micromole per liter
- Ancient: Water vapor in Earth’s early atmosphere reacted with volcanic gases and dust; most important gases: chlorine, sulfur dioxide, carbon dioxide – formed acids (HCl, H2SO3, H2CO3) in rain water, which dissolved minerals from rocks
- Similar processes occur today, but slower, less volcanic activity
- Water / sediment interaction at seafloor – gain of dissolved material and loss of precipitates; both high-temperature reactions at mid-ocean ridges & low temperature reactions with volcanic rocks and sediments.
Polar nature of the water molecule -- hydrogen bonds -- dissolve salts into ion due to polar nature -- interaction between ions and water molecules by polarity
pH of seawater
Background
Reminder:
pH
= –log[H+]
H2O
+ H2O = H3O+ + OH-
H2O
= H+ + OH-
The
more H+ in the water, the more acidic it is. Neutral water (pH
= 7) contains 10-7 moles H+ (or H3O+)
and OH-
Chemical properties
of seawater
Where do the salts
come from?
Weathering: interaction water – rocks, dissolves new minerals & loss by precipitation