Buffering Dissociation/Formation Reaction of Biogenic Calcium Carbonate

Abstract

The oscillating stability of coral reef seawater pH has been maintained at around physiological pH values over the past 300 years (Pelejero et al., 2005). The stability mechanism of its pH has been interpreted in terms of the buffering dissolution/formation reaction of CaCO₃ as well as the proton consumption/generation reaction in CaCO₃-saturated water. Here the pH-dependent solubility product [HCO₃⁻][Ca²⁺] has been derived on the basis of the actual pH-dependent reactions for the atmospheric CO₂/CO_2 (aq.)/HCO₃⁻/CO₃²⁻/Ca²⁺/CaCO₃ system. Overbasic pH peaks appeared between pH≈8 and ≈9.5 during sodium hydroxide titration, as a result of simultaneous CaCO₃ formation and proton generation. The spontaneous and prompt water pH recovery from the acidic to the physiological range has been confirmed by the observation of acid/base time evolution, because of simultaneous CaCO₃ dissolution and proton consumption. The dissolution/formation of CaCO₃ in water at pH 7.5–9 does not take place without a proton consumption/generation reaction, or a buffering chemical reaction of HCO₃⁻+Ca²⁺⇄CaCO₃+H⁺. SEM images of the CaCO₃ fragments showed that the acid water ate away at the CaCO₃ formed at physiological pH values. Natural coral reefs can thus recover the physiological pH levels of seawater from the acidic range through partial dissolution of their own skeletons.