Ocean Acidification & Acid–Base Reactions

Ocean pH Trend

• Present-day average \text{pH}=8.14 (slightly alkaline)
• Drop of 0.11 units since 1800s ⇒ \sim 30\% rise in [\mathrm{H_3O^+}]
• Projected \text{pH}\approx7.85 by 2100 (≈ 100\% more acidic than pre-industrial)
• Still alkaline (>7), but increasingly acidic (lower pH)

Anthropogenic CO₂ Input

• Fossil-fuel combustion ↑ \mathrm{CO2}, \mathrm{SO2}, \mathrm{NO_2} in air
• \tfrac13–\tfrac12 of human-emitted \mathrm{CO_2} absorbed by oceans
• Exceeds natural carbon-cycle fluctuations (ice-core record)

Dissolved CO₂ Chemistry

• \mathrm{CO2(g)} \rightleftharpoons \mathrm{CO2(aq)}
• \mathrm{CO2(aq)}+\mathrm{H2O(l)} \rightleftharpoons \mathrm{H2CO3(aq)} (weak diprotic acid)
• \mathrm{H2CO3}+\mathrm{H2O} \rightleftharpoons \mathrm{H3O^+}+\mathrm{HCO_3^-}
• \mathrm{HCO3^-}+\mathrm{H2O} \rightleftharpoons \mathrm{H3O^+}+\mathrm{CO3^{2-}}
• Rising [\mathrm{CO2}] ⇒ equilibrium shifts right ⇒ ↑ [\mathrm{H3O^+}] ⇒ ↓ pH

Calcification vs. Decalcification

• Calcification (shell/coral formation): \mathrm{Ca^{2+}}+\mathrm{CO3^{2-}}\rightarrow\mathrm{CaCO3(s)}
• Acidification consumes carbonate: \mathrm{H3O^+}+\mathrm{CO3^{2-}}\rightarrow\mathrm{HCO3^-}+\mathrm{H2O}
• Result: ↓ [\mathrm{CO_3^{2-}}], slower CaCO₃ precipitation, shell/coral dissolution (decalcification)

Biological & Ecological Impacts

• Direct: weaker shells in pteropods, shellfish, corals, crustaceans
• Indirect: disrupted food web (plankton → krill → fish → larger predators)
• Krill eggs fail to hatch at lower pH; diatoms/plankton growth declines
• Coral reef erosion ⇒ loss of coastal protection, tourism, biodiversity

Key Terminology

• Ocean acidification: pH decrease due to atmospheric \mathrm{CO_2} uptake
• Calcification: precipitation of \mathrm{CaCO_3} to build shells/skeletons
• Decalcification: dissolution of existing \mathrm{CaCO3} under elevated [\mathrm{H3O^+}]

Essential Numbers/Equations

• Present [\mathrm{H_3O^+}] increase: 30\% since 1800s
• Future projection: \text{pH}\downarrow 0.29 by 2100 relative to pre-industrial
• Core reaction summary:
  \mathrm{CO2(g)}\rightarrow\mathrm{H2CO3}\rightarrow\mathrm{HCO3^-}\rightarrow\mathrm{CO_3^{2-}} (inter-conversion governed by pH)