Proton-transfer reaction dynamics and energetics in calcification and decalcification

CaCO3-saturated saline waters at pH values below 8.5 are characterized by two stationary equilibrium states: reversible chemical calcification/decalcification associated with acid dissociation, Ca2++HCO3−⇌CaCO3+H+; and reversible static physical precipitation/dissolution, Ca2++CO32−⇌CaCO3. The former reversible reaction was determined using a strong base and acid titration. The saturation state described by the pH/PCO2-independent solubility product, [Ca2+][CO32−], may not be observed at pH below 8.5 because [Ca2+][CO32−]/([Ca2+][HCO3−]) ≪1. Since proton transfer dynamics controls all reversible acid dissociation reactions in saline waters, the concentrations of calcium ion and dissolved inorganic carbon (DIC) were expressed as a function of dual variables, pH and PCO2. The negative impact of ocean acidification on marine calcifying organisms was confirmed by applying the experimental culture data of each PCO2/pH-dependent coral polyp skeleton weight (Wskel) to the proton transfer idea. The skeleton formation of each coral polyp was performed in microspaces beneath its aboral ectoderm. This resulted in a decalcification of 14 weight %, a normalized CaCO3 saturation state Λ of 1.3 at PCO2 ≈400 ppm and pH ≈8.0, and serious decalcification of 45 % and Λ 2.5 at PCO2 ≈1000 ppm and pH ≈7.8.

Suwa R., Hatta M. & Ichikawa K., in press. Proton-transfer reaction dynamics and energetics in calcification and decalcification. Chemistry – A European Journal. Article (subscription required).

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