Seawater Chemistry: the Carbonate System and pH
Seawater's coupled carbonate equilibria resist pH change. Adding carbon dioxide lowers pH and carbonate availability. Calcium carbonate dissolution depends on saturation for the relevant mineral, which varies with water chemistry, temperature, salinity and pressure rather than a universal depth or pH threshold.
What a learner can do afterwards
- Write the carbonate equilibria and explain which species dominates at seawater pH
- Predict the effect of dissolving more carbon dioxide on pH and on carbonate ion concentration
- Explain why a shell-forming organism is affected by a pH change that sounds small
1 · Read
Seawater's carbonate system buffers pH: its linked reactions resist, but do not prevent, change. The equilibria are CO2(aq) + H2O ⇌ H2CO3, H2CO3 ⇌ H+ + HCO3^-, and HCO3^- ⇌ H+ + CO3^2-. Carbonate ions can take up added H+ to form bicarbonate; bicarbonate can react with added OH^- to form carbonate and water.
Treat dissolved CO2 and H2CO3 together as CO2*. Seawater's apparent pKa values are about 6 (CO2*/HCO3^-) and 9 (HCO3^-/CO3^2-). In pH = pKa + log(base/acid), pH above pKa favors the base; below it favors the acid. Near pH 8, HCO3^- dominates. Added CO2 shifts CO2 + H2O + CO3^2- ⇌ 2HCO3^- toward bicarbonate, reducing carbonate. pH is logarithmic: a 0.1 fall means about 26% more H+, even while seawater remains alkaline above pH 7.
Corals build calcium carbonate skeletons; many other organisms build shells. A pH fall need not make seawater acidic. Lower carbonate availability lowers mineral saturation and can hinder building. Water undersaturated with respect to a calcium carbonate mineral favors its dissolution. Saturation depends on dissolved calcium and carbonate, temperature, salinity and pressure. Depth and pH contribute, but neither sets one universal dissolution threshold.
Vinegar contains acetic acid, a weak acid, but is far more acidic than seawater. It dissolves eggshell calcium carbonate: CaCO3 + 2H+ → Ca^2+ + CO2 + H2O. This is not a model for seawater pH or dissolution speed. Undersaturated seawater can dissolve CaCO3 above pH 7. Adding carbonate to a calcium-containing solution can precipitate CaCO3 if it becomes supersaturated. Use teacher supervision and eye protection for such lab work.
Compare seawater samples at the same temperature, salinity and total alkalinity, a measure of acid-neutralizing capacity. Exposing one to more CO2 shifts its carbonate equilibria toward more H+ and less carbonate, so it has lower pH. Buffering limits this change; it does not keep pH fixed.
Saturation must be specified for the shell mineral, such as calcite or aragonite. At saturation there is no thermodynamic drive for net dissolution; below saturation, dissolution is favored. Equal depth and pH do not guarantee equal saturation: carbonate and calcium concentrations, salinity and temperature also matter. Pitting alone cannot identify the cause, since damage may also be biological or mechanical.
Extra CO2 lowers seawater pH and carbonate availability. Reduced saturation can hinder calcium carbonate growth and favor dissolution.
Original lesson and assessment text for this dataset. Graph-comparison and eggshell demonstration designs were informed by the OER Commons descriptions of High School Assessment - How Acidic Is It? Impacts of Ocean Acidification and Gravimetric Analysis_remote.
Resource summaries adapted from OER Commons catalog descriptions, including resources associated with ClimeTime educators at Graham Kapowsin High School, Institute for Systems Biology, SERC at Carleton College and Experimentium. The linked video is How do oysters make pearls? - Rob Ulrich from TED-Ed. Publisher license terms have not been verified.
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