Development of an Autonomous On-Site Dissolved Inorganic Carbon Analyzer using Conductometric Detection Technique

Abstract

Gradual increase of anthropogenic CO2 concentration in the Earth's atmosphere changes the CO2 uptake capacity by seawater, leading to alteration of ocean carbon chemistry and therefore resulting in ‘Ocean Acidification’. Dissolved Inorganic Carbon (DIC) is one of the key parameters among the four primary variables (i.e., pH, partial pressure of CO2 (pCO2), Total Alkalinity (TA), and DIC) along with temperature, salinity, and macronutrients to fully characterize the seawater carbonate system. To improve our quantitative and mechanistic understanding of the marine carbonate system, high-quality and high spatial-temporal resolution observations of DIC are required. To meet these expectations, an autonomous DIC analyzer is needed which is cost-effective, offers high sampling frequency, low reagent as well power consumption. Here we present the development and validation of a novel analyzer for autonomous measurements of DIC in seawater using conductometric detection technique. The presented DIC analyzer employs a gas diffusion flow injection approach in a “Tube In A Tube” configuration that facilitates diffusion of gaseous CO2 from an acidified sample through a gas permeable membrane (Teflon AF2400) into a stream of alkaline solution (NaOH). The change in conductivity in the alkaline medium is measured using a detection cell with 4-hollow brass electrodes and the change in conductivity is directly proportional to the DIC concentration of the sample. Physical and chemical optimizations of the analyzer yielded sample acidification to pH < 4, a NaOH concentration of 7 mM with a flowrate of 300 µL min-1, and an inner diameter of the gas permeable tube of 0.6 mm, allowing DIC measurements in both freshwater and marine systems between 500 and 3000 µmol kg-1. The analyzer can measure 4 samples hour-1 and it requires 0.2 mL of H3PO4, 0.75 mL of NaOH, and 2 mL of sample for each measurement. Temperature and salinity effects were characterized over the ranges 5-35°C and 0-35 in the laboratory, respectively, with the formulation of a mathematical T-S correction for accurate DIC determination. Measurements of a DIC reference material (RM) over four days yielded an analytical precision of ±4.89 µmol kg-1 (n=6) and an accuracy of +1 µmol kg-1. The operational robustness of the system has been demonstrated through a field deployment in the southwest Baltic Sea, yielding an analytical precision of ±9.69 µmol kg-1 (n=6). This study describes an autonomous, on-site, cost-effective DIC analyzer capable of measuring DIC in seawater at a high temporal resolution with an ultimate aim to develop an underwater DIC sensor. The achieved accuracy and precision offer an excellent opportunity to employ the analyzer in CO2 leakage monitoring and detection in the context of Carbon Capture and Storage.