Ocean Acidification & Acid–Base Reactions

Ocean pH Trend

  • Present-day average pH=8.14\text{pH}=8.14 (slightly alkaline)
  • Drop of 0.110.11 units since 1800s ⇒ ∼30%\sim 30\% rise in [H3O+][\mathrm{H_3O^+}]
  • Projected pH≈7.85\text{pH}\approx7.85 by 2100 (≈ 100%100\% more acidic than pre-industrial)
  • Still alkaline (>7), but increasingly acidic (lower pH)

Anthropogenic CO₂ Input

  • Fossil-fuel combustion ↑ CO<em>2\mathrm{CO<em>2}, SO</em>2\mathrm{SO</em>2}, NO2\mathrm{NO_2} in air
  • 13\tfrac13–12\tfrac12 of human-emitted CO2\mathrm{CO_2} absorbed by oceans
  • Exceeds natural carbon-cycle fluctuations (ice-core record)

Dissolved CO₂ Chemistry

  • CO<em>2(g)⇌CO</em>2(aq)\mathrm{CO<em>2(g)} \rightleftharpoons \mathrm{CO</em>2(aq)}
  • CO<em>2(aq)+H</em>2O(l)⇌H<em>2CO</em>3(aq)\mathrm{CO<em>2(aq)}+\mathrm{H</em>2O(l)} \rightleftharpoons \mathrm{H<em>2CO</em>3(aq)} (weak diprotic acid)
  • H<em>2CO</em>3+H<em>2O⇌H</em>3O++HCO3−\mathrm{H<em>2CO</em>3}+\mathrm{H<em>2O} \rightleftharpoons \mathrm{H</em>3O^+}+\mathrm{HCO_3^-}
  • HCO<em>3−+H</em>2O⇌H<em>3O++CO</em>32−\mathrm{HCO<em>3^-}+\mathrm{H</em>2O} \rightleftharpoons \mathrm{H<em>3O^+}+\mathrm{CO</em>3^{2-}}
  • Rising [CO<em>2][\mathrm{CO<em>2}] ⇒ equilibrium shifts right ⇒ ↑ [H</em>3O+][\mathrm{H</em>3O^+}] ⇒ ↓ pH

Calcification vs. Decalcification

  • Calcification (shell/coral formation): Ca2++CO<em>32−→CaCO</em>3(s)\mathrm{Ca^{2+}}+\mathrm{CO<em>3^{2-}}\rightarrow\mathrm{CaCO</em>3(s)}
  • Acidification consumes carbonate: H<em>3O++CO</em>32−→HCO<em>3−+H</em>2O\mathrm{H<em>3O^+}+\mathrm{CO</em>3^{2-}}\rightarrow\mathrm{HCO<em>3^-}+\mathrm{H</em>2O}
  • Result: ↓ [CO32−][\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 CO2\mathrm{CO_2} uptake
  • Calcification: precipitation of CaCO3\mathrm{CaCO_3} to build shells/skeletons
  • Decalcification: dissolution of existing CaCO<em>3\mathrm{CaCO<em>3} under elevated [H</em>3O+][\mathrm{H</em>3O^+}]

Essential Numbers/Equations

  • Present [H3O+][\mathrm{H_3O^+}] increase: 30%30\% since 1800s
  • Future projection: pH↓0.29\text{pH}\downarrow 0.29 by 2100 relative to pre-industrial
  • Core reaction summary:
    CO<em>2(g)→H</em>2CO<em>3→HCO</em>3−→CO32−\mathrm{CO<em>2(g)}\rightarrow\mathrm{H</em>2CO<em>3}\rightarrow\mathrm{HCO</em>3^-}\rightarrow\mathrm{CO_3^{2-}} (inter-conversion governed by pH)