Simultaneous pH and Ca2+ Imaging of Nonstoichiometric Calcite Dissolution

<jats:p> The growth and dissolution of carbonate minerals are believed to significantly impact the oceanic CO <jats:sub>2</jats:sub> cycle. However, to date, no effective method has been developed to assess the CO <jats:sub>2</jats:sub> balance at actual mineral surface reactions. This study confirmed the nonstoichiometric dissolution of calcium carbonate (CaCO <jats:sub>3</jats:sub> ) in an aqueous solution using a newly developed method to simultaneously visualize the local pH and Ca <jats:sup>2+</jats:sup> concentration distributions near the solid-liquid interface. This method used two fluorescent probes, 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) and C <jats:sub>39</jats:sub> H <jats:sub>36</jats:sub> N <jats:sub>5</jats:sub> O <jats:sub>13</jats:sub> (Rhod-5N), as the pH and Ca <jats:sup>2+</jats:sup> concentration indicators, respectively. The fluorescence intensity produced by HPTS was pH-dependent, while that produced by Rhod-5N was influenced by both the Ca <jats:sup>2+</jats:sup> concentration and pH. The temporal variations of these distributions during CaCO <jats:sub>3</jats:sub> (calcite) dissolution in aqueous solutions were visualized and quantified by first determining the pH and then quantifying the Ca <jats:sup>2+</jats:sup> concentration at that pH using pre-prepared calibration curves. The observed pH was significantly lower than that predicted from the observed Ca <jats:sup>2+</jats:sup> concentration changes, indicating that CaCO <jats:sub>3</jats:sub> does not dissolve in a 1:1 stoichiometric ratio of Ca <jats:sup>2+</jats:sup> and CO <jats:sub>3</jats:sub> <jats:sup>2-</jats:sup> but rather dissolves as HCO <jats:sub>3</jats:sub> <jats:sup>-</jats:sup> . This suggests that the pH does not increase as much as expected during the rapid dissolution of CaCO <jats:sub>3</jats:sub> , thereby enabling a precise estimation of the impact of the carbonate reaction on the global CO <jats:sub>2</jats:sub> budget. This imaging method enables direct observation of the local behavior of multiple ions, thereby revealing interfacial reaction mechanisms that are not evident from bulk measurements. </jats:p>