The development of pH and pCO2 microelectrodes for studying the carbonate chemistry of pore waters near the sediment‐water interface

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Abstract
We made stable and rapid‐responding pH and pCO2 microelectrodes. The pH microelectrode has a ∼100‐µm‐diameter half‐sphere bulb tip of pH glass, and the pCO2 sensor has a 200–300‐µm‐diameter silicone‐membrane covered tip. Because the pH microelectrodes have electrical resistances that are >1010 ohms, all electrical connections arc kept scrupulously clean and dry to prevent current leakage during in situ measurements. Suspension effects are negligible when measuring pH in marine sediments.To demonstrate the performance of the pH and pCO2 microelectrodes, we report in situ microprofiles of pH and pCO2 and contiguous profiles of O2 and the formation factor from coastal sediment in San Diego Bay, California. A sharp pH decrease from 7.90 to 7.15 (seawater scale) and a pCO2 increase from 578 to 3,000 µatm in the top 1 cm of sediment arc interpreted to be primarily the result of HS, Fe2+, and Mn2+ oxidation by O2 rather than metabolic CO2 production. The O2, pH, and pCO2 profiles also reveal that there was photosynthetic activity in the top 2 mm of sediment which caused an O2 maximum and a total dissolved inorganic carbon minimum.

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