Calibration and evaluation of a carbonate microsensor for studies of the marine inorganic carbon system

摘要

Accurate and rapid determination of inorganic carbon constituents in ocean environments is important for understanding the carbon cycle, especially in the context of ocean-acidification research. A microsensor capable of directly measuring carbonate ion (CO_3~(2-)) concentrations would be desirable. In this study, a carbonate microsensor with a polymeric liquid membrane was fabricated, and two calibration methods were used to evaluate its performance. The first method was based on continuous titration. Small increments of HC1 were added to seawater or Na_2CO_3 solution to adjust the total alkalinity and pH values and thus obtain a series of carbonate concentrations. The second method used a series of discrete standards. Varying amounts of HC1 or NaOH were added to separate seawater aliquots, and the CO_3~(2-) concentration of each standard was calculated from the resulting total alkalinity and total dissolved inorganic carbon. Both methods were found to be adequate for achieving accurate calibration of the CO_3~(2-) sensor, and both are suitable for field work. The discrete standards method, however, is more convenient and may provide a better linear range at low CO_3~(2-) concentrations (detection range: 2-300 μmol/kg) than the continuous titration method in seawater (detection range: 10-250 μmol/kg). This CO_3~(2-) microsensor can be used for 5-7 d and detects changes in carbonate concentration as low as 2 μmol/kg in the inorganic carbon constituents of the environments where marine calcareous organisms grow. The CO_3~(2-) microelectrode was further assessed by applying it to the measurement of pore-water CO_3~(2-) concentration profiles in a marine sediment core.