Seawater Chemistry: the Carbonate System and pH
Seawater is a buffered solution whose pH is held by a set of coupled carbonate equilibria, and adding carbon dioxide shifts them all. The same equilibria decide the depth below which carbonate shells dissolve, which links ocean chemistry directly to what can live where.
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 is a buffered solution: its pH is held by a set of coupled carbonate equilibria that resist change. Read three graphs side by side and the story appears. As carbon dioxide in the air climbs, dissolved carbon dioxide in the ocean climbs with it, and seawater pH slips downward.
The added gas pushes every linked equilibrium at once. The pH drop sounds small, yet carbonate ions grow scarcer as the balance shifts. That scarcity is the whole problem, because carbonate ions are the building material shell makers need.
Corals and other shell builders construct their bodies from calcium carbonate, which dissolves more easily as water turns acidic. When carbonate ions are hard to find, building or keeping a shell gets harder, and below a certain depth or pH, shells can even start to dissolve. A kitchen demo shows both directions: vinegar eats an eggshell, while added carbonate solution precipitates the solid back.
Practice the connecting skill directly. Given the equations for calcium carbonate and bicarbonate formation, ask of each new parcel of CO2: which curve rises, which falls, and which equilibrium got pushed. Equations plus graphs beat either one alone.
Extra CO2 pushes carbonate equilibria, lowers pH, and starves shell builders of carbonate.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.