Acid/Base Balance and Density Chapter 7 Seawater Chemistry Acid/Base Balance and Density © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

Acid-Base Balance What are acids and bases? An acid is a substance that releases a hydrogen ion in solution. A base is a substance that combines with a hydrogen ion in solution. A solution containing an acid is called an acidic solution. A solution containing a base is called an alkaline solution. Acidity or alkalinity is measured on the pH scale. © 2002 Brooks/Cole, a division of Thomson Learning, Inc.

The pH Scale

pH of Seawater Note that the pH of seawater varies slightly with depth, but the average pH ocean water is 8.0.

Carbonate buffering Keeps ocean pH about same (8.1) If pH gets too high (water is too basic), carbonic acid (H2CO3) releases H+ to become bicarbonate (HCO3) If pH gets too low (water is too acidic), bicarbonate (HCO3) combines with H+ to become carbonic acid (H2CO3) Precipitation/dissolution of calcium carbonate CaCO3 buffers ocean pH Oceans can absorb CO2 from atmosphere without much change in pH

Carbonate buffering

Density of seawater 1.022 to 1.030 g/cm3 surface seawater Ocean layered according to density Density seawater controlled by temperature, salinity, and pressure Most important influence is temperature Density increases with decreasing temperature (cold water is denser than warm water)

Density of seawater Polar ocean is isothermal Salinity greatest influence on density in polar oceans Salty water is denser than fresh water

Density versus depth Density differences cause a layered ocean Pycnocline, abrupt change of density with depth Thermocline, abrupt change of temperature with depth

Fig. 5.23

Layered ocean High latitude oceans (poles) are not as layered Mixed surface water Pycnocline and thermocline Deep water High latitude oceans (poles) are not as layered Isothermal Isopycnal