Chemistry
The Acid-Base Chemistry of Carbon Dioxide in Ocean Acidification
Quick fact
The ocean absorbs about 30% of the CO₂ released by human activities, and the resulting chemical reactions have caused the ocean's average surface pH to drop by 0.1 units since the industrial revolution—that's a 30% increase in acidity.
Why this is interesting
You've heard that CO₂ is warming the planet, but did you know it's also changing the chemistry of the ocean? Every breath you take, the ocean absorbs a portion of that CO₂, and it's transforming the very water that covers most of our planet.
Read the full explanation
Understanding The Acid-Base Chemistry of Carbon Dioxide in Ocean Acidification
Seawater is naturally slightly alkaline, with a pH around 8.1. When CO₂ from the atmosphere dissolves in seawater, it reacts with water molecules to form carbonic acid (H₂CO₃). This weak acid then quickly dissociates, releasing hydrogen ions (H⁺) and forming bicarbonate ions (HCO₃⁻). The increase in H⁺ ions is what makes the water more acidic, lowering the pH. Think of it like adding a few drops of lemon juice to a glass of water—it becomes slightly more acidic. The ocean has a natural buffering system, the carbonate system, which helps resist large changes in pH, but the sheer amount of CO₂ being absorbed is overwhelming it. The key players are CO₂, H₂O, H₂CO₃, HCO₃⁻, CO₃²⁻, and H⁺, all in dynamic equilibrium.
A deeper explanation
The process hinges on a series of acid-base equilibria. First, CO₂ dissolves into seawater: CO₂(g) ⇌ CO₂(aq). Then it reacts with water: CO₂(aq) + H₂O ⇌ H₂CO₃(aq). This carbonic acid is a diprotic acid, meaning it can lose two protons. The first dissociation is: H₂CO₃ ⇌ H⁺ + HCO₃⁻; the second is: HCO₃⁻ ⇌ H⁺ + CO₃²⁻. Each step has an equilibrium constant, which defines the ratio of products to reactants. When atmospheric CO₂ levels rise, Henry's law dictates that more CO₂ dissolves into the ocean. This shifts the first equilibrium to the right, producing more H₂CO₃, and then more H⁺ and HCO₃⁻. According to Le Chatelier's principle, the system tries to counteract this by shifting the second equilibrium to the left, consuming H⁺ and CO₃²⁻ to reduce the stress. The result is a net increase in H⁺ concentration (lower pH) and a decrease in CO₃²⁻ concentration. This is crucial because many marine organisms, like corals and mollusks, rely on CO₃²⁻ to build their calcium carbonate shells and skeletons. The reduction in carbonate availability makes calcification more difficult. Simultaneously, the extra H⁺ can dissolve existing shells. The ocean's overall pH decreases, even though it remains slightly alkaline, and this is the essence of ocean acidification.