En-en adult card 23 hindered calcification process

Card #23: Hindered Calcification Process

Causes		Consequences
Ocean Acidification		Pteropods and Coccolithophores

When the pH drops, the formation of calcium carbonate (and more specifically, of calcified shells) becomes more difficult.

Explanation

The constitution of limestone (calcification) is the following chemical reaction: Ca⁺⁺ + 2HCO₃- ⇔ CaCO₃ + H₂O + CO₂

It requires the presence of bicarbonate HCO3- ions. However, the quantity of these ions in water depends on the pH: In water, carbon dioxide, carbonic acid, bicarbonate ion and carbonate ion are in equilibrium, depending on the pH :

CO₂ + H₂O ⇔ H₂CO₃ ⇔ H⁺ + HCO₃^- ⇔ 2 H⁺ + CO₃^2-.

The addition of an acid shifts the equilibrium to the left.

In other words, if the pH drops, the bicarbonate ions are in less quantity. Hence the difficulty of making limestone for the organisms that need it.